Saturday, March 24, 2012

Cisco CCNA: Wide Area Networking

Cisco CCNA: Wide Area Networking


Unit 1. Introduction to WANs



Today's global society has evolved in part because of WAN technology. WANs enable computer communications across the world. This course discusses WANs in general, then delves into details about PPP, ISDN, X.25, and Frame Relay systems.
This first unit examines typical WAN devices and layouts, and identifies the WAN services supported by Cisco routers.

After completing this unit, you should be able to:
  • List WAN devices

  • Describe a typical layout for WAN services

  • Identify the WAN services that Cisco routers support


This unit does not address any specific Cisco objectives. However, it does provide background information that is essential for the CCNA exam.
In the course index, questions about background information are indicated with the abbreviation BCK and a short description of the question subject matter.

Topic 1.1: Defining WAN

*Wide Area Networks
WANs (Wide Area Networks) provide network services across large geographical areas. WANs rely heavily on serial connections and telecommunication carriers to meet the challenge of maximizing bandwidth at minimal cost, while providing adequate services to all users.
Because WANs provide resources and services that users do not own, they are called service providers. Users, who are often called subscribers, can send or receive information through a WAN. In the OSI model, WANs function primarily at Physical and Data Link layers, although they also function at the Network layer.

Topic 1.2: WAN Devices

*WAN Devices
WANs use varying combinations of devices. The following devices can be found inside a WAN:
  • WAN switches — these filter and transmit data at WAN bandwidths.
  • WAN ATM switches — these are newer devices that enable high-speed switching of cells through fixed channels. Cells are fixed-size data packets smaller than traditional packets.
  • Multiplexers — these are communications devices that combine multiple signals for transmission over a single medium. The signals can be analog or digital. Examples include T-1 and T-3 telecommunication lines.


*Interfacing a WAN
The following are used to interface a WAN:
  • Routers — these are often the last point of contact from a user's interface to a WAN service.
  • Modems — these convert digital data to the analog waves that travel over telephone lines.
  • Access servers — these are concentrators for analog WAN services and translators of protocols, such as Telnet to X.25.
  • CSU/DSUs (Channel Service Unit/Digital Service Unit) — these adapt the subscriber's equipment to the WAN interface for a switched-carrier network.
  • TAs/NT1s (Terminal Adapters/Network Termination1 devices) — these connect an ISDN (Integrated Services Digital Network) device to another interface, such as EIA/TIA-232 cable.
In the next two sections we'll put them all together.

Question 1

Topic 1.3: WAN Interfaces

*WAN Interfaces
Shown here is a generic layout for WAN interfaces. The layout can be tailored to any type of networking: circuit-switched, packet-switched, point-to-point, dial-up, public data network, and so on. The components, some of which are real and some of which are merely conceptual, will be explained on the following pages.

*Customer Premises Equipment
The CPE (Customer Premises Equipment) consists of the user's wiring and equipment. The CPE may include computers, telephones, and modems.
The demarc (demarcation point) is the physical boundary separating the user's equipment from the service provider's. It is often a twisted-pair cable that is placed in the user's telecommunications closet.
The local loop is the wiring connecting the demarc to the service provider's central office (CO).
The CO is the point where the local loop connects to the WAN cloud, which contains the service provider's high-speed trunks and switches. This point is often called a point of presence (POP).

*Data Terminal Equipment
You can also represent this layout in another way. The user's equipment is also known as the DTE (Data Terminal Equipment). The end of the DTE is usually, but not always, a router. The DTE interfaces the DCE (Data Circuit-Terminating Equipment). DCE represents the WAN service provider's equipment, including the devices that convert user data from the DTE into a form acceptable to the Wan service's facility. These devices can include modems, protocol translators, interface cards, CSU/DSU, TA/NT1(Terminal Adapter/Network Termination1), or multiplexers.
The DTE/DCE interface is the point where the responsibilities pass from the subscriber to the provider, or vice versa. The protocols of the Physical layer of the OSI reference model function at this interface.

Question 2

Topic 1.4: WAN Service Providers

*WAN Services
The WAN cloud, shown in a generic layout for WAN interfaces, represents three types of services:
  • Call setup
  • Time-division multiplexing
  • X.25 or Frame Relay


*Call Setup
Call setup service first establishes a connection between two endpoints of a WAN, then transfers data between the two endpoints.
An example of a call setup protocol is SS7 (Signaling System 7). SS7 uses telephone control messages and signals between transfer points along the way to the called destination. It is characterized by high-speed packet switching and out-of-band signaling. Out-of-band signaling establishes two links: one for signaling and another for data transfer. The signaling link contains the address information for call setup and takedown. It is a digital link. The second link, called the voice trunk, is responsible for data and voice transmission. The rate of data exchange for SS7 is 56 or 64 kbps (kilobits per second).

*Time-Division Multiplexing Service
Time-division multiplexing (TDM) transmits data from multiple sources on a single media. The data is then transferred over a fixed connection using fixed time slots. The path between the subscriber through the provider and on to the receiver is called the call route. The call route is determined by circuit-switching signaling. Examples of TDM providers include plain old telephone service (POTS) and ISDN. Transfer rates are 56 or 64 kbps.

*X.25 and Frame Relay Services
X.25 and Frame Relay services transfer information contained in packets or frames over shared, non-dedicated bandwidth.
X.25 uses packet switching and uses Network-layer routing. Because of this, X.25 is able to avoid many of the delays evidenced in call setup.
Frame Relay uses permanent virtual circuits (PVCs) and Data-Link layer identifiers.
X.25 and Frame Relay will be discussed in detail later.

Question 3

Topic 1.5: Accessing WAN with Cisco Routers

*WAN Services with Routers
Three WAN services can be accessed with Cisco routers. Two services are presented on this page, and one on the next.
  • Switched or relayed services — for example, those using X.25, Frame Relay, or ISDN protocols
  • Point-to-point or point-to-multipoint connections for remote devices to a dedicated facility — for example, those using the SDLC (Synchronous Data Link control) protocol to connect an interface front-end to an IBM enterprise data center


*Another WAN Service with Routers
Besides switched or relayed services, and point-to-point or point-to-multipoint connections, there is another WAN service accessible with Cisco routers:
  • Point-to-point links between peer devices — for example, those using HDLC (High-Level Data Link Control) or PPP (Point-to-Point) protocols
These protocols will be also discussed in the next unit. Representative frames are shown on the next few pages. As you probably remember from earlier courses, frames are the encapsulated data at the Data Link layer of the OSI reference model.

*Switched Services
Switched services use a special device to interface the WAN cloud of a service provider. A representative WAN frame format for a switched service is shown here.

*Point-to-Point or Point-to-Multipoint
Shown here is a point-to-point or point-to-multipoint service. An example of this service is the connection of an interface front-end to IBM enterprise data center computers. This example uses the SDLC protocol. The SDLC frame format, which is the same as that for the protocol LAPB, is also shown.

*Peer-to-Peer Services
This point-to-point service is peer-to-peer. This service can use Dial-on-Demand routing (DDR) to trigger the Cisco router to make the connection.
Typical protocols include HDLC or PPP for data encapsulation. The frame format for HDLC is shown here. PPP can be configured with a Cisco router on these physical interfaces: asynchronous serial, synchronous serial, HSSI (High-Speed Serial Interface), and ISDN.

Question 4

Question 5

Question 6


* Exercise 1
Try identifying the frame formats for PPP, HDLC and SDLC.


Examine the following table
Step Action
1 Write down, in order, the fields that make up PPP frames.
2 Do the same for HDLC frames.
3 Do the same for SDLC frames.


Topic 1.6: Unit 1 Summary

In this unit you discovered that, although many variations of WAN services exist, they can be all funneled into three broad categories — call setup, time-division multiplexing, and X.25 or Frame Relay.
These three services share a basic layout. To understand this layout, you learned terms such as CPE, DTE, and DCE.
In the next unit, you will identify more terms, but this time the terms refer to the myriad of WAN protocols.

Unit 2. WAN Protocols



In order for the WANs of the world to communicate, they must conform to certain sets of standards. These standards — or protocols — are as diverse and numerous as the networks to which they apply.
In this unit, we first discuss the protocols that function in the Physical layer of the OSI reference model, then follow with the protocols that function in the Data Link layer. There is no doubt that we have omitted a few protocols, but the ones essential for routing are included.

After completing this unit, you should be able to:
  • Identify protocols for the Physical layer

  • Identify protocols for the Data Link layer


This unit provides information that is relevant to the following CCNA exam objectives:
  • Identify PPP operations to encapsulate WAN data on Cisco routers


Topic 2.1: Setting WAN Protocols

*A Multi-Organization Effort for WAN Protocols
Different types of protocols support different types of WANs. Many organizations have worked together to set standards that enable the diverse WANs to communicate.
In the following pages, we'll discuss the WAN protocols. Physical-layer protocols will be discussed first, followed by Data-Link layer protocols.

Topic 2.2: Protocols for the Physical Layer

*Protocols for the Physical Layer
The Physical layer is concerned with bits, signals, and clocking. The protocols that operate on the Physical layer of the OSI model include:
  • EIA/TIA-232 and EIA/TIA-449 (Electronics Industries Association/Telecommunications Industry Association)
  • EIA-530
  • V.24 and V.35
  • G.703
  • HSSI (High-Speed Serial Interface)


*EIA/TIA-232 and EIA/TIA-449
EIA/TIA-232, formerly called RS-232, standardizes a physical connection to voice-grade access. It applies to unbalanced circuits at signal speeds up to 64 kbps. This is very similar to the V.24 protocol. EIA/TIA-449, formerly called RS-449, is a frequently used protocol that is similar to EIA/TIA-232, but works with faster (up to 2 Mbps) connections and longer cable runs.
Incidentally, balanced transmission is characterized by equal resistance per unit length and equal capacitance and inductance between conductor and ground. Co-axial cable can be configured as balanced transmission. Anything that doesn't meet these criteria is unbalanced.

*EIA-530
EIA-530 applies to a DTE and DCE interface that transmits serial binary data and exchanges control information on separate control circuits. Specific aspects defined include signals, the mechanical interface, and the functioning of interchange circuits.
EIA-530 also applies to these two electrical implementations of EIA/TIA-449: RS-422 (balanced transmission) and RS-423 (unbalanced transmission).

*V.24 and V.35
V.24 and V.35 are ITU-T modulation protocols for modem transmission of data.
V.24 is almost equivalent to the EIA/TIA RS-232 standard. V.24 applies to the interface at the computer cable (DTE) and its attachment to the back of the modem (DCE). This standard specifies a maximum bit rate of 20 kbps per second over a maximum cable length of 15 meters.
V.35 is a standard for high-speed synchronous data exchange. It is commonly used for routers and CSU/DSUs (Channel Service Unit/Data Service Units) that interface to T-1 lines. It specifies a bit rate of 48 kbps.

*G.703
This is an ITU-T protocol that defines the electrical and mechanical specifications for connections between telephone company equipment and a DTE that uses BNC (British Naval Connector) connectors and which operate at the E1 data rate of 2.048 Mbps.
BNC connectors connect IEEE 802.3 10Base2 coaxial cable to a MAU (Media Access Unit). E1 lines are used mainly in Europe.

*HSSI
HSSI (High-Speed Serial Interface) is used for high-speed, point-to-point serial connections. HSSI defines both the electrical and physical aspects of DTE/DCE interfaces. HSSI supports a maximum signaling rate of 52 Mbps. The cable is shielded twisted pair (STP).
HSSI communication is considered peer-to-peer because HSSI assumes intelligence in both DCE and DTE devices. HSSI uses one control signal to indicate that the DTE is available and a second control signal to indicate that the DCE is available.

*Clock and Data-Signaling
HSSI's clock and data-signaling protocol makes user bandwidth allocation possible. The DCE controls the clock by changing its speed or by deleting clock pulses. Consequently, the DCE can allocate bandwidth between applications.
Network applications that can use HSSI's clock control include a router-based LAN or PBX (Private Branch Exchange). A PBX connects digital or analog telephones on a customer's premises to a public or private telephone network.

Question 7

Topic 2.3: Protocols for the Data Link Layer

*Data Link Layer
The Data Link layer is concerned with frames, specifically physical addressing, network topology, error notification, orderly delivery, and flow control. The protocols that operate on the Data Link layer of the OSI model include:
  • SDLC (Synchronous Data Link Control)
  • HDLC (High-Level Data Link Control)
  • SLIP (Serial Line Internet Protocol)
  • PPP (Point-to-Point Protocol)
  • ISDN/LAPD (Integrated Services Digital Network/Link Access Procedure on the D Channel)
  • SIP (SMDS Interface Protocol)
  • Dial on Demand
  • X.25
  • Frame Relay


*SDLC
SDLC (Synchronous Data Link Control) is the oldest WAN protocol. This is a bit-oriented, full-duplex serial protocol. It applies to point-to-point or point-to-multipoint connections from remote devices to a dedicated facility. This means that remote devices do not communicate directly to each other.
The HDLC (High-Level Data Link Control) and LAPB (Link Access Protocol Balanced) protocols are derived from SDLC.
The data rate for SDLC is 52 Mbps.

*HDLC
HDLC (High-Level Data Link Control) supports both point-to-point and multipoint configurations. This bit-oriented protocol, which specifies the method of encapsulation on synchronous serial links, uses frame characters and checksums. Checksums, which are used to verify the accuracy of data transfer, are integer values that are calculated and compared at the sending and receiving ends of a connection. One vendor's HDLC implementation may differ from another vendor's. Cisco's HDLC is the default encapsulation for routers.

Question 8

*PPP
PPP (Point-to-Point Protocol) operates on synchronous and asynchronous connections, and with such Network-layer protocols as IP (Internet Protocol), IPX (Internetwork Packet Exchange) and ARA (AppleTalk Remote Access). PPP provides error checking and security measures.

*PPP Protocols
To perform its functions, PPP uses NCP (Network Control Protocol) and LCP (Link Control Protocol). NCP and LCP are also Data Link protocols.
NCP is a family of protocols for establishing and configuring different Network-layer protocols.
LCP establishes, configures and maintains the connection.
You'll learn more about PPP in a later unit.

*SLIP
SLIP (Serial Line Internet Protocol) preceded PPP as the protocol for point-to-point serial connections. The two protocols are similar. However, two main differences exist between SLIP and PPP: SLIP does not contain error-checking mechanisms and SLIP can only transport TCP/IP.
ISPs (Internet Service Providers) typically support either SLIP or PPP, but not both.

Question 9

Question 10

*ISDN/LAPD
ISDN/LAPD (Integrated Services Digital Network/Link Access Procedure on the D Channel)) applies to data and voice transmissions on existing telephone lines. ISDN transmissions are faster than analog rates, but at the cost of increased bandwidth. ISDN is actually a collection of protocols which operate on the Physical, Data Link, Network, and Transport layers. In particular, LAPD, the Data-Link layer protocol, specifies the signaling requirements for ISDN.
ISDN is discussed in detail in a later unit.

*SIP
SIP (SMDS Interface Protocol) supports all features of SMDS (Switched Multimegabit Data Service) networks.
To understand SIP, you need to be familiar with the terms SMDS, SNI, and DQDB.

*SMDS Networks
SMDS networks are high-speed, packet-switched, datagram-based WAN services provided over PDNs (Public Data Networks). A subscriber is allowed to have a private virtual network excluding unwanted traffic because of some SMDS addressing functions. Some MANs can be classified as SMDS networks.
SMDS credit management is used with SMDS access classes, which regulate data for CPE (Customer Premise Equipment) devices. The group address permits a single address to refer to multiple CPE devices. Source address validation makes sure that the CPE address is legitimate.

*SNI in SMDS
In an SMDS network, the SNI (Subscriber-Network Interface) refers to the interface between the CPE and carrier equipment, which are high-speed WAN switches in a fiber- and copper-based telephone system. SIP provides connectionless service across the SNI. SIP uses the protocol DQDB (Distributed Queue Dual Bus) for cell relay.

*Physical and Data Link Layers
SIP functions at the Physical and Data Link layers of the OSI reference model: SIP Level 1 functions at the Physical layer. SIP Level 2 and SIP Level 3 both function at the MAC sublayer of the Data Link layer.

*Dial on Demand
This protocol applies to telephone lines, including ISDN lines. With Dial on Demand (DDR), a router starts and closes circuit-switched sessions.

Question 11

Question 12

Question 13

*X.25
X.25 is an ITU-T protocol that consists of a protocol stack. X.25 defines the connections between a DTE and DCE with respect to remote terminal access and computer communications in a PDN. An X.25 DCE serves as a boundary between the PDN's switch or concentrator.
The physical connections for X.25 are defined by the X.21bis protocol. X.21bis is the Physical-layer component of X.25. X.21bis is almost identical to EIA/TIA-232.

*X.25 Data-Link and Network-Layer Functions
The Data-Link layer functions for X.25 are specified by LAPB (Link Access Procedure, Balanced). This protocol was derived from HDLC and is used to detect out-of-sequence and missing frames, and to exchange, retransmit and acknowledge frames.
There are also Network-layer functions, which are defined by PLP (Packet Layer Protocol). PLP is also called the X.25 Level 3 protocol.

*Frame Relay
Frame Relay uses multiple virtual circuits, which provide a logical connection between two DTEs. Frame Relay was designed for ISDN interfaces, and often replaces X.25 because it is faster.

*Local Management Interface
Frame Relay relies on another protocol called LMI (Local Management Interface). LMI is the signaling standard between a CPE device and a FR (Frame Relay) switch. LMI is responsible for managing the connection and maintaining status between the devices.
Frame Relay is discussed in greater detail in a later unit.

Question 14

Question 15


* Exercise 1
Try identifying all of the protocols that work at different layers of the OSI reference model for PPP, X.25, and Frame Relay.

Examine the following table
Step Action
1 Draw a picture of the OSI reference model. Add, at the appropriate layers, the names of the protocols that work for PPP.
2 Repeat this procedure for X.25.
3 Repeat this procedure for Frame Relay.


Topic 2.4: Unit 2 Summary

You have just completed a whirlwind tour of WAN protocols. A little bit of information is enough for some of the protocols, but not all.
As an example, in the next unit you'll stop to take a much closer look at PPP.

Unit 3. PPP



If you polled WAN users to name the protocols that they had used, PPP would probably accumulate the most votes. In this unit, you learn the reasons why.
You also correlate PPP and its associated protocols LCP, NCP, CHAP, and PAP with their position in the OSI reference model.
After that you examine the procedure to establish a PPP session, and where the associated protocols LCP, LCP, CHAP, and PAP fit in.

After completing this unit, you should be able to:
  • List the advantages of PPP

  • Examine the frame format for PPP data encapsulation

  • Describe the options for PPP data encapsulation on Cisco routers

  • Differentiate between the CHAP and PAP protocols

  • Establish a PPP session


This unit provides information that is relevant to the following CCNA exam objectives:
  • Identify PPP operations to encapsulate WAN data on Cisco routers


Topic 3.1: The Advantages of PPP

*Advantages of PPP
The advantages of PPP (Point-to-Point Protocol) result from the following features:
  • It can be configured on synchronous and asynchronous serial networks, HSSI, and ISDN physical interfaces. The only requirement is a duplex circuit, which can be dedicated or switched.
  • It supports the simultaneous use of several Network-layer protocols, such as TCP/IP, Novell IPX and AppleTalk. In fact, it is widely regarded as the Internet standard for transmission of IP packets over serial lines.
  • It is non-proprietary, which means it works on equipment from several vendors.
  • It can operate at any transmission rate that is appropriate for the DTE/DCE interface for which it is configured.
  • It is relatively easy to configure routers for PPP encapsulation.


Topic 3.2: Protocols Used in PPP

*PPP Protocols
As we mentioned in the preceding unit, PPP is categorized as a Data-Link layer protocol. PPP functionality depends heavily on the LCP and NCP protocols.
The LCP (Link Control Protocol) establishes, configures and maintains the PPP connection.
NCP (Network Control Protocol) is a collection of protocols that establish and configure different Network-layer protocols.

Topic 3.3: Data Encapsulation

*PPP Frame Format
Shown here is the PPP frame format for data encapsulation. Unlike other frame formats, PPP frames contain a Protocol field and an LCP field. These are the frame fields:
  • Flag — One byte with the binary sequence 01111110. This indicates the beginning or end of a frame.
  • Address — One byte with the binary sequence 1111111. This sequence is the standard broadcast address because individual station addresses are not needed.
  • Control — One byte containing the binary sequence 00000011. This sets up a connectionless link service that transmits an unsequenced frame.


*More of the PPP Frame Format
These are the remaining frame fields, continued from the previous page:
  • Protocol — Two bytes that specify the Network-layer protocol that is encapsulated in the information field.
  • LCP — 0–1500 (the default maximum) bytes that contain the Code, Identifier, Length, and Data.
  • FCS (Frame Check Sequence) — Two bytes which handle error detection.


Question 16

Topic 3.4: LCP Configuration on Cisco Routers

*PPP Encapsulation
Because this series focuses on CCNA certification, we will discuss PPP encapsulation in Cisco routers. Encapsulation includes the following configuration options:
  • Authentication
  • Compression
  • Error detection
  • Multilinking
Each one is discussed separately on the following pages.

Topic 3.4.1: Authentication

*Performing Authentication
In authentication, the calling side of a link provides information that proves it is authorized to call. The authentication information is exchanged between the two peer routers — the remote or calling router, and the local or central-site router — involved in the connection.
PPP offers two protocols, and therefore two methods, to perform authentication:
  • CHAP — performs a challenge handshake
  • PAP — exchanges a password
In Cisco IOS Release 11.1, a Cisco router can function as a callback client or callback server; this new authentication option offers even greater network security.

*Password Authentication Protocol
PAP (Password Authentication Protocol) uses a password for authentication.
However, it provides only for identification of the remote device; in other words, it does not stop unauthorized access.

*PAP's Two-Way Handshake
PAP uses a two-way handshake. The remote router sends an authentication request to the local router. PAP then passes a username and password in unencrypted text. The username/password requests are sent repeatedly between the remote and local router until either an accept or reject acknowledgment is sent, or until the connection is terminated. The local end decides the frequency and timing of the username/password request. The local end also decides if the remote end is allowed access.
This protocol can decrease network security because of the repeated passing of the unencrypted username and password. It is always safer to use CHAP, which is discussed in the next section.

*Challenge Handshake Authentication Protocol
CHAP (Challenge Handshake Authentication Protocol) also provides only for identification of the remote device and does not stop unauthorized access.

*CHAP's Three-Way Handshake
Unlike PAP, CHAP uses a three-way handshake. The local router sends a challenge (i.e., message) to the remote device, and the remote device sends back an encrypted ID number, a secret password, and a random number. If this response from the remote device matches what the local router expects, the local router sends an authentication acknowledgement of accept or reject. Challenges are issued on session establishment and then repeated randomly.
Network security is greater with CHAP because the challenge values are unique and unpredictable, and because an authentication server, such as TACACS (Terminal Access Controller Access Control System) or RADIUS (Remote Authentication Dial In User Service), can control the frequency and timing of the challenges.

Question 17

Topic 3.4.2: Compression

*PPP Encapsulation Options
Moving back to the PPP encapsulation options, we just discussed authentication and now it's time to discuss compression. Compression decreases the data that must cross the connection. Data is compressed at the source, then decompressed at the destination of the PPP connection.
PPP offers compression with these two protocols:
  • Stacker
  • Predictor


*Predictor and Stacker Algorithms
The Predictor algorithm (also called the RAND algorithm) should be used when a bottleneck occurs on a router or server. The Predictor uses a compression dictionary to predict what the next character in a frame will be.
The Stacker algorithm (called LZS for Limpel, Zif, and Stac) should be used when a bottleneck occurs because of a lack of bandwidth.

Topic 3.4.3: Error Detection

*Error Detection
Error detection monitors and identifies fault conditions with these two protocols:
  • Quality — monitors connection for dropped data

  • Magic Number — prevents frame looping
Error detection provides more reliable data transfer.

Topic 3.4.4: Multilinking

*Multilinking
Multilinking is a relatively new option that balances loads across multiple links.
The Multilink Protocol (MP) uses packet fragmentation and sequencing, and sends the fragments over multilink, parallel PPP circuits. These multilink circuits act like one logical link; this improves throughput between the peer PPP routers.

Question 18


* Exercise 1
Try describing all the LCP configuration options available to Cisco routers using PPP encapsulation.

Examine the following table
Step Action
1 List the four configuration options available for Cisco routers using PPP encapsulation.
2 List the protocols associated with each option, and describe their functions.


Topic 3.5: PPP Session Establishment

*Establishing a PPP Session
To establish a PPP session, the connection must carry out these three phases:
1. Link Establishment
2. Optional Authentication
3. Application of Network Layer
    Protocol

*Link Establishment
Link establishment depends on the LCP protocol. LCP opens the connection, then negotiates the configuration. To do this, each PPP device transmits LCP packets. The LCP packets typically include a Configuration Option field. The Configuration Option field determines the following:
  • The maximum size of receive unit
  • Which PPP fields are compressed
  • Which link authentication protocol(s) and sequence (CHAP, PAP, CHAP PAP, or PAP CHAP) are used
If all Configuration Options are not specified, then default values are used for each missing option.
This phase ends when a configuration-acknowledgment frame is sent and received.

*Optional Authentication
Optional authentication uses whichever protocol was agreed upon in the Configuration Option field negotiations between PPP devices.
The authentication protocols were discussed earlier in this unit.

*Applying the Network-Layer Protocols
Once authentication has been completed, each PPP device sends NCP packets to negotiate and configure whatever Network-layer protocols are needed.
Each needed Network-layer protocol is configured separately. After the configuration for each protocol is completed, the protocol-specific packets are sent, and the appropriate NCP protocol can be taken down.

Topic 3.6: Ending the PPP Session

*LCP Protocol
The LCP protocol has the ability to end a PPP session at any time. If LCP terminates a link, it usually notifies the Network-layer protocol.
The usual reason to terminate a link is that the user requests it, but the LCP will terminate the link if physical problems occur, such as the carrier is lost or the idle-period timer expires.

Topic 3.7: Unit 3 Summary

In this unit, you learned a little more about PPP. You took a closer look at the LCP and NCP protocols, and examined their part in data encapsulation and session establishment. You also discovered that several protocols are involved in configuring Cisco routers for PPP encapsulation.
In the next unit, you'll take a close look at another popular WAN protocol — ISDN.

Unit 4. ISDN



ISDN makes use of existing telephone networks to provide digital, high-speed transmission of voice and data information. This makes it valuable for telecommuters, physicians, and even small business owners.
However, like so many WAN services, there is no one ISDN service and no one set of ISDN devices.
In this unit, you look at the combinations of devices that make up an ISDN network. You learn the names of the interfaces between components. You also examine the two ISDN services — BRI and PRI.

After completing this unit, you should be able to:
  • List the advantages and uses of ISDN

  • Identify ISDN components

  • Name the interfaces between ISDN components

  • Define function groups, reference points, and channels

  • Differentiate between BRI and PRI

  • Identify the protocols used in ISDN


This unit provides information that is relevant to the following CCNA exam objectives:
  • State a relevant use and context for ISDN networking

  • Identify ISDN protocols, function groups, reference points, and channels


Topic 4.1: About ISDN

*ISDN Protocols
ISDN (Integrated Services Digital Network), as you learned in an earlier unit, is a collection of protocols that carries digital data and voice transmissions on existing telephone lines.

*ISDN and TDM
It is an example of WAN time-division multiplexing (TDM). TDM takes information from multiple sources and allocates bandwidth on a single media. Circuit-switched signaling determines the call route, which is a dedicated path between the sender and the receiver.

*Advantages of ISDN
The following are some advantages of ISDN:
  • Operates on switched digital connections — this eliminates digital-to-analog conversions and leased lines (a permanent connection between two points that is set up through a telecommunications carrier), while providing high speed, voice clarity, and a lower ratio of signal to voice.
  • Simultaneously supports all types of information  — voice, data, sound, video, still graphics.
  • Manages multiple devices and multiple telephone numbers on one ISDN line.
  • Manages a minimum of three calls at the same time. For example, two voice, fax or PC "conversations", and one data "conversation" can occur simultaneously on a BRI (Basic Rate Interface) connection.
  • Can increase bandwidth even more by bonding channels for specific applications.
  • Decreases the time needed for call setup and latency in transmitting data.


Topic 4.2: Uses for ISDN

*Uses for ISDN
ISDN can be used wherever more bandwidth is required: multiple phone links at one location; videoconferencing; high-speed file transfers; high-speed image applications (e.g., Group IV facsimile); high speed Internet access; transmission of data, text, voice, music, video, and graphics. Consequently, ISDN is useful for:
  • Telecommuters who can make use of the multiple phone links and videoconferencing.
  • Engineers at remote locations who can simultaneously talk to peers and exchange data on a particular computer application.
  • Hospitals, where radiologists can quickly transmit X-rays, which constitute high file sizes.
  • Businesses in remote locations, which can utilize the capability for voice and data transmission to simultaneously connect security alarms and relay sound and movement.


Topic 4.3: Components of an ISDN Network

*ISDN Components
ISDN components are known as function groups. They consist of the following:
  • Terminals — such as TE1 and TE2
  • Terminal adapters (TAs) — these depend on whether the terminal is TE1 or TE2
  • Network terminating devices — such as NT1 or NT2
  • Line-terminating equipment — the local loop connection to the carrier equipment
  • Exchange-terminating devices — these are the switches in the carrier equipment
In the next few pages, we'll show you how these form an ISDN network.

*TE1 and TE2 Devices
TE1 devices are specific to ISDN. Examples include a computer, videoconferencing equipment, ISDN telephones, ISDN FAX machines, and ISDN bridges/routers. TE1 devices use the subscriber's four-wire, twisted-pair digital wiring to connect to the ISDN.
TE2 devices are not specific to ISDN. For example, standard analog telephones and modems are TE2s. TE2 devices require a TA. The TA can be either a standalone device or board inside the TE2. If the TA is a board inside the TE2, then the TE2 needs to connect to a Physical-layer interface, such as EIA/TIA-232-C, V.24, or V.35.

*NT1 and NT2
The NT1 terminating device is part of the subscriber's CPE, which is also known as the DTE. It is used in small businesses or homes. (Note: this is true only for North America; otherwise the NT1 is classified as carrier equipment.) The NT1 connects the CPE to the CO (Central Office) switching equipment, and converts a four-wire interface into a two-wire interface. The two-wire interface is on the CPE side. It is a Physical-layer device.
The NT2 is a device that supplies multiple ISDN interfaces on the ISDN line. For example, it could be a simple bridging device connected to an NT1, or it could be a complicated PBX that is used in larger enterprises. The NT2 applies Layer 2 and Layer 3 protocols, which will be discussed later in this unit.

Question 19

Topic 4.4: Reference Points

*Reference Points
The logical interfaces between ISDN components are called reference points. These are the four reference points and their location in the ISDN network:
  • R — between non-ISDN equipment and a TA
  • S — between the subscriber's terminals and the NT2
  • T — between the NT1 and NT2 (S and T are electrically equivalent)
  • U — between the NT1 and the carrier's line-termination equipment


*Another Look at NT1
To look at this another way, the NT1 converts the two-wire U interface into the four-wire S/T interface. The S/T interface supports multiple devices (up to 7 devices) because it provides two wires to receive data and two to transmit data.

Topic 4.5: Channels

*ISDN Line
Now it's time to look at the ISDN line itself. The ISDN line connects the subscriber to the standard, circuit-switched telephone network that is found everywhere. The ISDN line, or pipe as it is often called, is composed of channels. Channels are communication paths. Depending on the type of ISDN service, there may be three channels or 24 channels.
There are two types of channels:
  • B, which stands for Bearer
  • D, which stands for Data


*ISDN Channels
The B channel carries voice, data, and B channel packets at the high speed of 64 kbps. The D channel carries the signaling and D channel packets at either 16 or 64 kbps. The signaling is out-of-band and is transmitted through the SS7 network, a separate network just for call-signaling. The D channel functions on the Physical, Data Link, and Network layers of the OSI.

Topic 4.6: ISDN Services

*Types of ISDN Services
As we mentioned in the previous section, there are two types of ISDN services.
These are BRI and PRI.

Topic 4.6.1: BRI

*Basic Rate Interface
BRI (Basic Rate Interface) Service consists of two 64 kbps B channels and one 16 kbps D channel. BRI is also known as a 2B+D connection. The fastest rate possible is 144 kbps. The two B channels are called B1 and B2, even though they share the same characteristics. These combinations of transmissions are possible with BRI:
 
  • Two simultaneous voice or data transmissions to the same or different locations
  • Simultaneous D packet transmissions to yet another location


Topic 4.6.2: PRI

*Primary Rate Interface
In North America and Japan, PRI (Primary Rate Interface) Service consists of twenty-three 64-kbps B channels, and one 64-kbps D channel. This requires a T1 line, which can transmit at a speed of 1.544 Mbps through the telephone network. The PRI channels operate as in BRI, but the total bandwidth is 1.544 Mbps.
In other places in the world, the PRI consists of 30 B channels and one 64-kbps D channel, for a total speed of 2.048 Mbps. PRIs are typically dedicated lines. They support all ISDN devices: PBXs, LANs and WANs, multiplexers, and so on.

Question 20

Topic 4.7: ISDN Protocols

*ISDN Protocols
The ISDN protocols operate on the Physical, Data Link, Network, and Transport layers. ISDN supports most Network-layer protocols such as IP, IPX, AppleTalk, and the encapsulation protocols PPP, HDLC, and LAPD. ISDN protocols are grouped and named according to their function. The names begin with either E, I, or Q. The functions of each are identified below:
  • E — protocols that apply to ISDN on existing telephone networks
  • I — protocols that deal with concepts, terminology, and services
  • Q — protocols that deal with switching and signaling


*Q and I Protocols
The protocols can therefore be grouped according to function and position on the OSI reference model.
These I protocols function on the Physical layer:
  • ITU-T I.430 for BRI
  • ITU-T I.431 for PRI
The Network-layer protocols establish, maintain and clear the ISDN network connections. These Q protocols function on the Network layer:
  • ITU-T I.450— also known as ITU-T Q.930
  • ITU-T I.451 — also known as  ITU-T Q.931


*Another Q Protocol
This Q protocol is another signaling protocol that functions on the Data Link layer:
  • Q.921, also known as LAPD
Shown here is the protocol hierarchy for the D channel.

Question 21


* Exercise 1
Try classifying the ISDN protocols according to function and placement in the OSI reference model.

Examine the following table
Step Action
1 Make a list of ISDN protocols.
2 Classify them into their respective E, I, or Q function. Distinguish the functions that separate E, I, and Q.
3 Draw a model of the OSI reference model. Place the protocols at the proper layer of functionality.


Topic 4.8: Unit 4 Summary

By now you have seen how ISDN lines can make efficient use of existing telephone lines.
More importantly, you have examined what makes up the ISDN network — from user devices through an ISDN link and on to the existing telephone PDN. As always, you discovered a slew of terms for interfaces, types of ISDN services, and protocols.
In the next unit, you will examine another WAN service that relies on packet switching rather than circuit switching in its involvement with a PDN — X.25.

Unit 5. X.25



X.25 is the parent of WAN networks. It's older than most, overprotects its users, is dependable and stable, and it's probably been everywhere you want to go. Because it's so ubiquitous and stable, you may not know you're using it. Nonetheless, you certainly need to know about it.
In this unit, you discover that virtual circuits are a major player in X.25 networks. You also investigate the protocols that enable X.25 to send data from one Network-layer protocol to any number of other Network-layer protocols.

After completing this unit, you should be able to:
  • Describe the layout of an X.25 network

  • Define virtual circuits

  • Identify the X.25 suite of protocols

  • Examine X.25 data encapsulation


This unit does not address any specific Cisco objectives. However, it does provide background information that is essential for the CCNA exam.
In the course index, questions about background information are indicated with the abbreviation BCK and a short description of the question subject matter.

Topic 5.1: The Role of X.25 in WAN

*X.25
X.25 is an older standard that was developed in the 1970s to work in analog phone networks. X.25 uses packet-switching to enhance, not replace, the existing phone PDN. Some of the enhancements include flow control and error checking.
Although X.25 is still widely used, ISDN and Frame Relay are either replacing it or extending its capabilities. By design, both ISDN and Frame Relay do not include the amount of flow control and error checking that is found in X.25.

*X.25 Protocols
X.25 transmits different Network-layer protocols by tunneling datagrams within X.25 packets. The X.25 protocols support the following protocols:
  • Apollo
  • AppleTalk
  • Banyan VINES
  • Compressed TCP
  • DECnet
  • IP
  • Novell IPX
  • ISO-CLNS — ISO Connectionless Network Service — a service on the Network layer that does not need to establish a circuit before transmitting data
  • XNS — Xerox Network Systems


Topic 5.2: X.25 Network Layout

*X.25 Network
At the minimum, an X.25 network consists of DTEs, DCEs, and a PSE (Packet Switching Exchange).

*DTEs
The DTEs are located on the users' — or subscribers' — premises. They can be any of these devices:
  • Dumb terminals, computers, or network hosts

  • Routers


*DCEs, and PSE
The DCEs are the interfaces between the DTE and the PSE. The DCE is usually located in the carrier network. DCEs consist of these devices:
  • Modems
  • Concentrators
The PSE are the switches that make up most of the carrier network, or PDN.

*Packet Assembler/Disassembler
Between the DCE and DTE, there may be a PAD (Packet Assembler/Disassembler). A PAD connects to end devices for the purpose of buffering data, and assembling and disassembling packets for end devices that do not offer total support of a specific protocol.

Question 22

Topic 5.3: Virtual Circuits

*X.25 Layout
The logical X.25 layout includes SVCs (Switched Virtual Circuits) and PVCs (Permanent Virtual Circuits). Virtual circuits are bi-directional (or full-duplex) paths that connect DTEs to other DTEs. The physical connection may pass through many nodes between the source and destination.
One X.25 interface can be configured for a maximum of 4,095 virtual circuits. Many virtual circuits can be multiplexed onto one physical connection.
We will discuss SVCs and PVCs in the following sections.

*Configuring a Router
When configuring a router for virtual circuits, the following parameters need to be considered:
  • The data transfer across a virtual circuit must be configured in this order: incoming, two-way, and outgoing.
  • The default packet sizes for input and output. The default is 128 bytes, although these byte sizes are also supported: 16, 32, 64, 128, 256, 512, 1024, 2048, 4096. However, if the stations of an X.25 attachment do not share the same maximum packet size, communication is unlikely.
  • The default window sizes and window moduli (number of windows).


*Switched Virtual Circuits
SVCs are established on a demand basis, and terminated when data transfer is complete. SVCs are appropriate for connections where data flow is inconsistent.
One X.25 interface can support a maximum of 4,095 SVCs. Up to eight SVCs can be multiplexed for a single protocol going to a single destination.
One SVC–host connection can carry multiple protocols, but only a maximum of nine protocols can be mapped to a single host.

*Advantage of PVCs
PVCs are always available for data transfer. PVCs, although more expensive, work well when there is a constant data flow.

Question 23

Question 24

Topic 5.4: X.25 Protocols

*X.25 Protocols
There is no "one" X.25 protocol. X.25 is a collection of protocols that function on the Network, Data Link, and Physical layers.
We will discuss the protocols, beginning with the Network-layer protocol PLP.

Topic 5.4.1: Network-Layer Protocols

*PLP
PLP (Packet Level Protocol) is responsible for the exchange of packets between DTE devices.
Specifically it manages network routing functions and multiplexing of logical connections that occur simultaneously over a single physical connection.
It works on PVC, SVC, LAN Logical Link Control 2, and ISDN/LAPD connections.

*PLP Operation
PLP supports five modes of operation:
  • Call setup — establishes the SVC connection between DTEs. This is not applicable to PVCs.
  • Data transfer — transfers data between the DTE devices over SVC or PVC connections. This includes segmentation, reassembly, bit padding, error control, and flow control.
  • Idle — maintains logical connection when an SVC has been established but no data is being transferred. This is not applicable to PVCs.
  • Call clearing — terminates the SVC connection.
  • Restarting — synchronizes transmission between a DTE and a local DCE device.


*PLP Address
During call setup, PLP defines an address that conforms to the X.121 protocol, which defines two fields for the address. The PDN service provider must provide information for the X.121 address. This is the first field of the address:
  • DNIC — Data Network ID Code — this number is assigned by the ITU and identifies the country and PSN where the destination DTE is located. This field is four digits.


*PLP Addresses, Continued
This is the second field of the address:
  • NTN — Network Terminal Number — identifies the actual destination DTE device. The first eight digits are defined by the PSN provider. The last two or three digits are assigned locally to a particular application or device. A private X.25 network may assign addresses that fit their network.
Combined together, the DNIC and NTN can range from 1–15 digits. The first eight digits of the NTN and the last digit of the DNIC combine to form a unique address. These digits must be decimal digits.

*X.25 Routing
When the source DTE is using a Network-layer protocol that differs from the destination DTE or the next-hop router, the local router must be manually configured for every protocol on the route. Incidentally, a hop is the passing of a data packet from one network node, such as a router, to another.
As shown here, this manual mapping follows the same logical format as the LAN ARP (Address Resolution Protocol).

Topic 5.4.2: Data-Link Layer Protocols

*LAPB
LAPB (Link Access Procedure, Balanced) initializes the link between the DTE and the local DCE, and frames the X.25 data packets before they are transmitted from the DTE to the DCE. LAPB is a derivative of HDLC.
When the X.25 data packet reaches the local DCE, LAPB removes the LAPB frame and extracts the datagram from the packet. LAPB re-encapsulates the datagram in the frame appropriate for its next hop and sends it on its way.

Topic 5.4.3: Physical-Layer Protocols

*X.21bis
X.21bis is the X.25-specific protocol at the Physical layer, although other Physical-layer protocols, such as V.35, EIA/TIA-232, EAI/TIA-449, EIA-530, G.703, may be involved. X.21bis is almost identical to EIA/TIA-232.
X.21bis supports bi-directional (full-duplex) transmission over four-wire media at a maximum speed of 19.2 kbps.

Question 25

Topic 5.4.4: Data Encapsulation

*PLP Packet
The PLP packet is encapsulated in the LAPB frame. Shown here is the PLP packet. The packet generally consists of four fields:
  • GFI — General Format Identifier — specifies if packet is user data or control information, the type of windowing, and the need for delivery confirmation
  • LCI — Logical Channel Identifier — specifies the virtual circuit across the local DTE/DCE interface
  • PTI — Packet Type Identifier — specifies the PLP packet type
  • User Data — present only in data packets


*X.25 and Windowing
Packet structure varies, depending on whether it is following modulo 8 or modulo 128 windowing. As you learned in an earlier course, windows are the number of data packets a source can send without receiving any acknowledgment messages. Modulo 128 is rare because it is typically used for satellite instead of virtual circuit transmissions.
Modulo 8, shown here, allows a maximum of 8 packets (numbered 0–7), and modulo 128 allows a maximum of 128 packets (numbered 0–127). The X.25 modulo command is set when configuring your router for X.25.

*LAPB Frame
LAPB frames the X.25 data packets before they are transmitted from the DTE to the DCE. The LAPB frame is shown here. This frame consists of the following fields:
  • Flag — marks the beginning or end of the LAPB frame
  • Address — specifies whether frame is a command or response
  • Control — specifies whether the frame is information (I-frame), supervisory (S-frame) or unnumbered (U-frame)
  • Data — carries the PLP packet
  • FCS — manages error checking



* Exercise 1
Try setting up X.25 data encapsulation for your own network.

Examine the following table
Step Action
1 Simulate sending data to a real or mock site across an X.25 network. Begin by writing down the information you need for encapsulation: Network-layer protocols at your site, Physical-layer protocols, the PDN address for X.121 addressing.
2 Diagram the PLP packet, then diagram the LAPB frame.
3 Also indicate the X.121 address.


Topic 5.5: Unit 5 Summary

If you didn't know about X.25 before, you certainly do now. In this unit, we discussed the X.25 protocols, and how the X.25 network relies on packet-switching and virtual circuits. You also examined the data encapsulation used in X.25.
In the next — and last — unit, you will learn about the WAN service that is sometimes enhancing and sometimes replacing X.25. It is the very robust Frame Relay.

Unit 6. Frame Relay



As far as WAN services go, Frame Relay is the new kid on the block. It's similar to X.25 in that it applies virtual circuits, packet-switching, and multiplexing in a PDN, but it's significantly faster because it was created to meet the WAN requirements of the 1990s.
In this unit, you learn how Frame Relay uses virtual circuits. You examine the format of the Frame Relay frame. To understand how all these things work, you look at the Frame Relay protocol.

After completing this unit, you should be able to:
  • Define key terms used in Frame Relay

  • Describe Frame Relay layouts

  • Specify fields in Frame Relay frames

  • Explain the Frame Relay protocol LMI

  • Understand how routers work in Frame Relay


This unit provides information that is relevant to the following CCNA exam objectives:
  • Recognize key Frame Relay terms and features

  • List commands to configure Frame Relay LMIs, maps, and subinterfaces


Topic 6.1: Details of Frame Relay

*Frame Relay
Although the concept of Frame Relay was around in the 1980s, it wasn't until 1990 that Frame Relay truly became a WAN alternative. In 1990 the "gang of four" (Cisco, Digital Equipment, Northern Telecom, and StrataCom) specified the LMI (Local Management Interface) protocol, which tailors X.25 to Frame Relay.
Unlike the previous WAN services we have discussed, Frame Relay relies on LMI, which functions on the Data Link layer of the OSI reference model.

*Benefits of Frame Relay
Speed is a big benefit of Frame Relay. Because Frame Relay tries to statistically distribute available bandwidth, it is especially suited to bursty traffic, which is traffic characterized by sporadic increases in bandwidth over time. Frame Relay inherently increases transmission rates because it doesn't include error correction; in fact, its method of "correcting" frame errors and network congestion is to simply discard the frame.
Frame Relay can do this because the upper-layer OSI protocols on DTE devices can detect and recover the loss of data.

Topic 6.2: The Frame Relay Network

*A Typical Frame Relay Network
Shown here is a typical Frame Relay network. On the next page we will describe the devices.

Topic 6.2.1: Devices

*DTE Devices
Frame Relay connects DTE devices to DCE devices. DTE devices consist of the following:
  • Routers
  • Bridges
  • Network hosts
  • Personal computers or terminals
  • Frame Relay Access Devices — FRADs — FRADs are optional devices that frame outgoing data with header and trailer control information before sending it to the Frame Relay switch. They also strip away the header and trailer information at the receiving end. They may or not be present, and they may be standalone devices, or part of another device, such as a router, switch or multiplexer.


*DCE Devices
The DCE devices are the carrier equipment that supply clocking and switching functions to transfer data through the network. These include:
  • packet switches
  • network routers
  • T1/E1 multiplexers
The cabling from the DTE to the Central Office of the PDN is the local loop. Frame Relay operation includes the local loop.

Topic 6.2.2: Topologies

*Frame Relay Topologies
Three types of topologies exist for connecting to a remote site using Frame Relay:
  • Full mesh
  • Partial mesh
  • Star


*Full Mesh Topology
Each router in the full mesh topology connects to all the others. This is the most expensive, but the fastest and most reliable.

*Partial Mesh Topology
In the partial mesh topology, each router generally connects to another router with very little or no redundancy in connections. Not all routers connect to a central site, and no router connects directly to all the routers. This is not the most reliable topology.

*Star Topology
The star is also known as a hub-and-spoke. As you can see, remote sites connect to a central site. The router at the central site forms a single interface.
This is the most popular and the cheapest topology.

Topic 6.2.3: The Logical Layout

*Frame Relay Circuits
Frame Relay relies on virtual switching to define the logical path. The Frame Relay virtual circuit defines the connection between two DTE devices across a Frame Relay packet-switching network. Before data is transmitted, the logical path is assigned a specific bandwidth. Then, when actual data needs to be transmitted, bandwidth is allocated on a per-packet basis.
Like X.25, Frame Relay can use either SVCs and PVCs, and multiple virtual circuits can be multiplexed over one physical connection.
The FR (Frame Relay) circuit is assigned a DLCI (Data Link Connection Identifier). We will discuss DLCI in the next section.

Topic 6.2.4: The Data Link Connection Identifier

*DLCI
The DLCI is a number that identifies the virtual circuit between the DTE device (usually a router) and the FR (Frame Relay) switch. The FR switch maps the DLCI between each set of two routers along the Frame Relay connection. In essence, this creates a PVC. The DLCI is included in the header of the Frame Relay frame, which we will discuss in a later section.

*Sending Data
The DLCI employs these three steps to send data from the FR switch across the network:
1.   The DLCI uses the FCS (Frame Check Sequence) to check the integrity
      of the frame. If there is an error, the frame is discarded. You'll learn more
      about FCS a little later in this unit.
2.   The DLCI references a table for the DLCI. If the DLCI is not defined for
      the link, the frame is discarded. 
3.   The DLCI relays the frame towards its destination through the connection
      indicated in the table.
As you've already learned, the DLCI doesn't spend much time with problematic frames. If a frame has a problem, it is discarded. Frame Relay can do this because it assumes that protocols on DTE devices are smart enough to detect and recover the lost data.

Topic 6.3: Reasons for Discarding Frames

*Discarding Frames
There exist two main reasons for discarding frames:
  • Bit errors
  • Network congestion
Bit errors, which usually occur because of line noise, are detected with the FCS (Frame Check Sequence).

*Types of Network Congestion
There are two types of network congestion:
  • Receiver congestion — when a network node cannot process the quantity of frames which are sent to it
  • Line congestion — when a network node needs to send frames at a rate faster than the line speed
Congestion management mechanisms are represented in Frame Relay frames by the FECN (Forward Explicit Congestion Notification) and BECN (Backward Explicit Congestion Notification) fields, which will be discussed later.

Question 26

Topic 6.4: LMI Protocol

*Local Management Interface
The LMI (Local Management Interface) is the signaling standard between the DTE device and Frame Relay switch. Generally, it is responsible for managing the connection and maintaining status between the devices.
LMI is actually a set of extensions which apply to specific areas, such as global addressing, the status of virtual circuits, the status of data flow, and the assignment of multicasting groups. We'll briefly discuss each of these extensions.

*LMI in Action
With the global addressing extension, the DLCI is assigned a global value instead of a local one. This means that the DLCI contains a unique address.
The extension for virtual circuit status provides the communication and synchronization between DTEs and DCEs. One important status issue is determining when PVCs no longer exist.
Multicasting is the sending of one message to many destinations. The extension for multicasting groups reduces bandwidth because it specifies that routing updates and address resolution messages will be sent to specific groups of routers.

*LMI Classifications
LMI can be classified into three different types:
  • Cisco — this is the default, and is the LMI defined by the "gang of four"
  • ANSI — Annex D, defined by the ANSI standard T1.617
  • Q933a — Annex A, defined by the Q933a standard
These types are configured on your router. For Cisco IOS Release 10.3 and earlier, you have to configure the LMI; for Cisco IOS Release 11.2 and later, Cisco routers try to auto-sense the LMI type that the FR switch is using. If you need to configure the LMI yourself, you may need to contact your Frame Relay provider for advice.

Question 27

Topic 6.5: Data Encapsulation

*Format of a Frame Relay Frame
Unlike other WAN services, Frame Relay does not alter user data packets. Here is the format of a Frame Relay frame. It includes the following fields:
  • Flag — marks the beginning of a frame. The value of this field never changes, only the numbering system. The value can be either 7E (hexadecimal) or 01111110 (binary)
  • Frame Relay Header — this is listed on the next page
  • Information Field — also called the Data field — this contains the upper-layer data. This is a variable-length field that transports the upper-layer protocol packet through the Frame Relay network
  • FCS — checks the integrity of transmitted data by performing a Cyclic Redundancy Check (CRC)
  • Flag — marks the end of a frame. The value of this field never changes, only the numbering system. The value can be either 7E (hexadecimal) or 01111110 (binary)


*Frame Relay Header
The Frame Relay header contains the following fields:
  • DLCI — a ten-bit number which has been discussed earlier in this unit
  • C/R —Command/Response — a one-bit number that is application-specific and is not modified by the Frame Relay network
  • FECN — Forward Explicit Congestion Notification — a one-bit field that notifies the destination of network congestion
  • BECN — Backward Explicit congestion Notification — a one-bit field that notifies the source of network congestion
  • DE — Discard Eligibility — a one-bit number that indicates the frame carries lower importance than other frames. If the DE is set, the frame is discarded when network congestion, i.e., oversubscribed traffic, occurs.
  • EA —Extension Bit —a one-bit number that indicates a longer header that is either three or four bytes


Question 28

Topic 6.6: Frame Relay Operation

*Workings of Frame Relay
Now that you know what makes up Frame Relay, it's time to see how it works. However, when referring to a working Frame Relay network, several terms must be defined.

Topic 6.6.1: Terminology

*Speed
As we stated in an earlier unit, local access rate is the rate at which data travels across the local loop. It is also known as port speed or clocking.
Tc is the minimum interval of time for committed rate measurement. In other words, it is the time during which a user can send Bc data, which is data the Frame Relay network (i.e., Frame Relay switch) commits to transfer (see the next paragraph). Tc begins when the network receives incoming data, and then continues for the predetermined interval.

*Burst
Committed burst (Bc) is the maximum amount of data that the network commits to transfer in the time interval Tc. This assumes normal conditions. The amount is measured in bits.
By the way, excess burst (Be) is the maximum amount of uncommitted data, i.e., the amount of data above the Bc, that a Frame Relay network can deliver during the time interval Tc. The amount is measured in bits. The network places a lower priority on delivering this type of data and therefore rates it as discard-eligible in the DE field.

*Committed Information Rate
CIR (Committed Information Rate) is the agreed-upon (or committed) rate at which frames travel from the FRAD through the Frame Relay network and to the destination device. The rate is measured in bits per second under normal conditions.
Usually the CIR is averaged over the Tc.

*Oversubscription
This means that Tc is proportional to the burstiness of network traffic. Also, a higher Bc-to-CIR ratio indicates that the network can handle a sustained burst of traffic for a longer period of time.
Oversubscription occurs when the sum of CIRs on all virtual circuits coming into a device exceeds the speed of the access line. This can result when the access line cannot handle the CIRs and the bursting capacities of the virtual circuits, even though the access line can handle all of the CIRs. Packets get dropped when oversubscription occurs.

Topic 6.6.2: Frame Relay in Action

*Subscribing to Frame Relay, Steps 1-6
This section describes what happens when you subscribe to Frame Relay.
1.   Each router connects to the FR switch, where it is assigned a DLCI.
      The router can connect directly or through a FRAD.
2.   Your local CPE router, after it is enabled, sends a Status Inquiry
      message to the FR switch.
3.   Your router sends an Inverse ARP (Address Resolution Protocol)
      request packet for each active DLCI the router can reach. The
      request packet identifies itself, then, in return, asks each remote
      router to identify itself. This identification consists of each remote
      router's Network-layer address.
4.   Next, for each DLCI sending an Inverse ARP message, your CPE
      router creates a map entry in its Frame Relay map table. The map
      entry consists of your router's DLCI, each remote router's Network-
      layer address, and the connection state, which can be active,
      inactive, or deleted.
5.   If the remote router does not support Inverse ARP, or if Inverse ARP
      is not functioning, you need to configure a static map that contains
      DLCI and IP addresses of the remote routers.
6.   When the mapping is finished, the routers exchange Inverse ARP
      messages every 60 seconds. Every 10 seconds, your CPE router sends
      a keepalive message to the FR switch to ensure that the FR switch is
      active. If the messages indicate a change, your router will change
      the DLCI status of a remote router.

Question 29

Topic 6.6.3: Issues in Frame Relay Operation

*Reachability Problems
Sometimes when using Frame Relay, you may encounter what is called a reachability problem when you use a single interface to connect to multiple sites. Reachability refers to whether all information needed for a site reaches that site. Reachability problems occur because of the following Frame Relay defaults:
  • Frame Relay exhibits NBMA (Nonbroadcast Multi-access Nature). This means that routing update broadcasts may not be forwarded to all sites.
  • Frame Relay uses split horizon routing to minimize routing loops. Split horizon will be covered in a later course; for now, you should just know that this routing technique never sends information about a route backward to the source of the information; that is, it prevents update information from returning to the interface from which it came.


*Solutions to Reachability Problems
The solutions are not easy. For NBMA, a router could replicate a broadcast for each active connection. This is a resource-heavy solution, however.
A router could also disable split-horizon. However, only IP allows this disabling. Also, this removes the protection against routing loops.

*Using Subinterfaces
A solution that does work is to create logical subinterfaces. Subinterfaces are logically assigned subdivisions of the interface. To configure a subinterface, every virtual circuit is considered a point-to-point connection. In essence, then, the connection is similar to a leased line, or a PVC. There are two types of configuration for subinterfaces:
  • Point-to-point
  • Multipoint
We will describes these two types of configuration in the next two sections. However, the actual configuration commands will be discussed in a later course.

*Point-to-Point Subinterfaces
A point-to-point subinterface consists of a single subinterface that makes a PVC connection to another interface, which can be either a subinterface or physical interface. Each interface is assigned a single DLCI, and each point-to-point connection forms its own subnet (look at the DLCIs shown here). A subnet is a network that is arbitrarily segmented.
Use a point-to-point interface in the following situations:
  • You do not want your router to transmit broadcasts and routing updates
  • You want each point-to-point connection to have its own subnet
  • You want your subinterface to operate like a leased line
  • You have a partial mesh or star topology


*Multipoint Subinterface
A multipoint subinterface consists of a single subinterface that uses multiple PVCs to connect to multiple interfaces (physical interfaces or subinterfaces) on remote routers. These multipoint subinterfaces share the same subnet, but each interface receives its own DLCI (look at the DLCIs shown here). Use this type of interface in the following situations:
  • You want your router to transmit broadcasts and routing updates
  • You can use a single subnet
  • You want to use IP routing
  • You have a full mesh topology


Topic 6.7: Configuring Frame Relay

*Conditions Assumed
When configuring Frame Relay on a router, Frame Relay assumes certain conditions:
  • Frame Relay will be configured on one or more physical interfaces

  • Remote routers support LMI and Inverse ARP (Address Resolution Protocol)


*Required Configuration
The following must be configured:
  • Interface
  • Network-layer address
  • Encapsulation type
  • LMI type used by FR switch (this is needed just for Cisco IOS Release 11.1 and earlier)
  • Bandwidth for the link in kbps
  • Inverse ARP (if the default enabled state has been disabled)


*Optional Configuration
The following are optional:
  • Configuring the static address-to-DLCI map — do this for situations in which Inverse ARP is not supported by the remote router, configuring OSPF (Open Shortest Path First) over Frame Relay, or when trying to use routing to control broadcast traffic.
  • Configuring the percentage of configured bandwidth for EIGRP (Enhanced Interior Gateway Routing Protocol) routing traffic.
  • Configuring the router keepalive interval rate to something other than the default 10 seconds.
  • Configuring the DLCI for each local interface. Do this when your network does not use one of the three LMI types, or if you are doing back-to-back testing between routers.


Question 30


* Exercise 1
Try searching the Internet for Frame Relay information.

Examine the following table
Step Action
1 The Internet is an excellent source of information for Frame Relay. Use the browser of your choice to search for Frame Relay Web sites.


Topic 6.8: Unit 6 Summary

You've now explored the last WAN service discussed in this course. Frame Relay is the youngest and arguably the best performer in WAN networking. It combines virtual circuits and packet-switching over a PDN in a true push toward cheaper, faster services.
You now have the tools to decide what is best for you: you've explored a great many of the WAN protocols, the basic layouts, and how these come together in the major WAN service performers such as PPP, ISDN, X.25, and Frame Relay.
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