110705-WAN Technologies-Cisco WAN Technologies Boldat Cu Rosu

8/6/2019 110705-WAN Technologies-Cisco WAN Technologies Boldat Cu Rosu

1/40

1

WAN Technologies

Cisco WAN Technologies

dr.ing. Florin Drban

Oradea, June-July 2011


2/40

2

I. Introduction1.1 Wide-Area Network (WAN)

1.2 WAN Devices1.3 WAN Standards

1.4 Circuit-switched Networks. Packet-switched Networks1.5 WAN Links Options

1.6 WAN Technologies1.6.1 Analog Dialup

1.6.2 ISDN1.6.3 Leased Lines

1.6.4 X.251.6.5 Frame Relay (FR)

1.6.6 ATM1.6.7 DSL

1.6.8 Cable

II. HDLC and PPP

2.1 Introduction2.2 HDLC2.3 PPP

2.4 PAP2.5 CHAP

2.6 Configuration of serial WAN with HDLC/PPP2.7 Verifying of serial WAN with HDLC/PPP configuration

2.8 Troubleshooting of serial WAN with HDLC/PPP configuration

III. ISDN3.1 ISDN Concepts

3.2 Configuration of ISDN3.3 Verifying ISDN configuration

3.4 Troubleshooting ISDN configuration

IV. FR4.1 FR Concepts

4.2 Bandwith and Flow Control4.3 Address mapping and topology

4.4 LMI4.5 Configuration of Frame Relay

4.6 Reachability issues with routing updates. Subinterfaces

4.7 Verifying FR configuration4.8 Troubleshooting FR configuration

Index


3/40

3

I. Introduction

Fig. 1 Local loop.

1.1. Wide-Area Network (WAN)

As the enterprise grows beyond a single location, it is necessary to interconnect the LANs in the various branches

to form a Wide-Area Network (WAN). There are many options currently available today for implementing WAN solutions.They differ in technology, speed (bandwith) and cost.

A WAN is a data communications network that operates beyond the geographic scope of a LAN. One primarydifference between a WAN and a LAN is that a company must subscribe to an outside WAN service provider in order to useWAN carrier network services. A WAN uses data links provided by WAN carrier network services to access the Internet andconnect the locations of a company to each other, to locations of other companies, to external services and to remote users.

WANs generally carry a variety of traffic types, such as voice, data and video.

Devices on the subscriber premises are called Customer Premises Equipment (CPE).The subscriber owns the CPE

or leases the CPE from the service provider.

A copper cable or fiber cable connects the CPE to the service providers nearest exchange or Central Office (CO).

This cabling is often called the local loop or "last-mile".


4/40

4Fig. 2 DTE and DCE.

Devices that put data on the local loop are called Data Circuit-terminating Equipment orData Communications

Equipment (DCE).

The customer devices that pass the data to the DCE are called Data Terminal Equipment (DTE).The DCE primarilyprovides an interface for the DTE into the communication link on the WAN cloud.

The DTE/DCE interface uses various physical layer protocols, such as HSSI (High-Speed Serial Interface), X.21,

V.35 or EIA/TIA-232. These protocols establish the codes and electrical parameters the devices use to

communicate with each other.

WAN links are provided at various speeds measured in bits per second (bps), kilobits per second (kbps or 1000 bps),megabits per second (Mbps or 1000 kbps) or gigabits per second (Gbps or 1000 Mbps). The bps values are generally fullduplex.


5/40

5

Table 1 Examples for the WAN links bandwith.

WANs are groups of LANs connected together with communications links from a service provider. Because thecommunications links cannot plug directly into the LAN, it is necessary to identify the various pieces of interfacing equipment:

-a) router,-b) switch,-c) modem (CSU=Channel Service Unit/DSU=Digital Service Unit),-d) communication server.

1.2 WAN Devices

Fig. 3 WAN Equipments.


6/40

6

Fig. 4 Router.

-a) LAN-based computers with data to transmit send data to a routerthat contains both LAN and WAN interfaces. The router will

use the L3 address information to deliver the data on the appropriate WAN interface. Routers are active and intelligent networkdevices and therefore can participate in network management. Routers manage networks by providing dynamic control overresources and supporting the tasks and goals for networks. Some of these goals are connectivity, reliable performance,management control and flexibility.


7/40

7

Fig. 5 CSU/DSU.

-b) The communications link needs signals in an appropriate format. For digital lines, a Channel Service Unit (CSU) and a DataService Unit (DSU) are required. The 2 are often combined into a single piece of equipment, called the CSU/DSU. The CSU/DSUmay also be built into the interface card in the router.

When ISDN is used as the communications link, all equipment attached to the ISDN bus must be ISDN-compatible. Compatibility is

generally built into the computer interface for direct dial connections or the router interface for LAN to WAN connections. Olderequipment without an ISDN interface requires an ISDN Terminal Adapter (TA) for ISDN compatibility.


8/40

8

Fig. 6 Modem.

-c) A modem is needed if the local loop is analog rather than digital. Modems transmit data over voice-grade telephone lines bymodulating and demodulating the signal. The digital signals are superimposed on an analog voice signal that is modulated fortransmission. The modulated signal can be heard as a series of whistles by turning on the internal modem speaker. At the receivingend the analog signals are returned to their digital form, or demodulated.


9/40

9

-d) Communication servers concentrate dial-in user communication and remote access to a LAN. They may have a mixture of

analog and digital (ISDN) interfaces and support hundreds of simultaneous users.

WANs use the OSI reference model, but focus mainly on L1 and L2. WAN standards typically describe both physical layerdelivery methods and data link layer requirements, including physical addressing, flow control and encapsulation. WAN standards are

defined and managed by a number of recognized authorities: ITU (International Telecommunications Union), IETF (InternetEngineering Task Force), ISO (International Organization for Standartization), EIA (Electronic Industries Association), TIATelecommunications Industries Association.

-L1) The physical layer protocols describe how to provide electrical, mechanical, operational and functional connections to

the services provided by a communications service provider. Some of the common physical layer standards are:-EIA/TIA-232,-EIA/TIA-449,

-EIA/TIA-530

-EIA/TIA-613 (HSSI),-V.35,-X.21.

-L2) The data link layer protocols define how data is encapsulated for transmission to remote sites and the mechanisms for

transferring the resulting frames.A variety of different technologies are used, such as:-ISDN=Integraded Services Digital Network (Circuit-Switched),-X.25 (Packet-Switched),-Frame Relay (FR), (Packet-Switched),

-Asynchronous Transfer Mode (ATM) (Packet-Switched).

These protocols use the same basic framing mechanism HDLC (High-Level Data Link Control) (Cisco Default) or one of its sub-sets

or variants :

-LAPM (Link Access Procedure Modems) and PPP (Point-to-Point Protocol) (for Dial-up connections),

-LAPD (Link Access Procedure D-channel) (for ISDN connections),

-LAPB (Link Access Procedure Balanced) (for X.25 connections),

-LAPF (Link Access Procedure Frame) (for FR connections).

1.3 WAN standards


10/40

10Fig. 7 HDLC frame.

Data from the network layer (L3) is passed to the data link layer (L2) for delivery on a physical link, which is normally point-to-point ona WAN connection. The data link layer builds a frame around the network layer data so the necessary checks and controls can beapplied. Each WAN connection type uses a L2 protocol to encapsulate traffic while it is crossing the WAN link.

The choice ofencapsulation protocol depends on the WAN technology and the equipment. Most framing is based on the HDLCstandard. HDLC framing gives reliable delivery of data over unreliable lines and includes signaling mechanisms for flow and error

control.


11/40

11Fig. 8 Circuit-switched Network.

1) Circuit-switched Networks - When a subscriber makes a telephone call, the dialed number is used to set switches in theexchanges along the route of the call so that there is a continuous circuit from the originating caller to that of the called party.

Because of the switching operation used to establish the circuit, the telephone system is called a Circuit-switched Network. If thetelephones are replaced with modems, then the switched circuit is able to carry computer data.

The internal path taken by the circuit between exchanges is shared by a number of conversations. Time Division Multiplexing(TDM) is used to give each conversation a share of the connection in turn. TDM assures that a fixed capacity connection is madeavailable to the subscriber.

If the circuit carries computer data, the usage of this fixed capacity may not be efficient. For example, if the circuit is used to accessthe Internet, there will be a burst of activity on the circuit while a web page is transferred. This could be followed by no activity whilethe user reads the page and then another burst of activity while the next page is transferred. This variation in usage between none

and maximum is typical of computer network traffic. Because the subscriber has sole use of the fixed capacity allocation, switchedcircuits are generally an expensive way of moving data.

1.4 Circuit-switched Networks. Packet-switched Networks


12/40


13/40

13

The switches in a packet-switched network determine, from addressing information in each packet, which link the packet must besent on next. There are 2 approaches to this link determination:

-a) Connectionless systems (such as the Internet) carry full addressing information in each packet. Each switch must evaluate theaddress to determine where to send the packet.

-b) Connection-oriented systems (such as the FR or X.25) predetermine the route for a packet and each packet need only carry anidentifier. In the case of FR these are called Data Link Control Identifiers (DLCI). The switch determines the onward route by lookingup the identifier in tables held in memory. The set of entries in the tables identifies a particular route or circuit through the system. Ifthis circuit is only physically in existence while a packet is traveling through it, it is called a Virtual Circuit (VC).

-The table entries that constitute a VC can be established by sending a connection request through the network. In this case the

resulting circuit is called a Switched Virtual Circuit (SVC). Data that is to travel on SVCs must wait until the table entries have beenset up. Once established, the SVC may be in operation for hours, days or weeks.

-Where a circuit is required to be always available, a Permanent Virtual Circuit (PVC) will be established. Table entries are loadedby the switches at boot time so the PVC is always available.


14/40

14

-a) Circuit-switched networks establishes a dedicated physical connection for voice or data between a sender and receiver. Beforecommunication can start, it is necessary to establish the connection by setting the switches. This is done by the Plain Old Telephone

System (POTS), using the dialed number. Examples of circuit-switched connections include POTS and ISDN.

-b) To avoid the delays associated with setting up a connection, telephone service providers also offer permanent circuits. Thesededicated lines or leased lines offer higher bandwidth than is available with a switched circuit.

-c) Many WAN users do not make efficient use of the fixed bandwidth that is available with dedicated lines or switched circuitsbecause the data flow fluctuates. Communications providers have packet-switched networks available to more appropriatelyservice these users. In these networks, the data is transmitted in labeled cells, frames or packets through a packet-switched network.

Because the internal links between the switches are shared between many users, the costs of packet-switching are lower than thoseof circuit-switching. Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks.This is because the links are shared and packets must be entirely received at one switch before moving to the next. Despite thelatency and jitter inherent in shared networks, modern technology allows satisfactory transport of voice and even videocommunications on these networks.

Packet-switched networks may establish routes through the switches for particular end-to-end connections.-Routes established when the switches are started are PVCs.-Routes established on demand are SVCs.-If the routing is not pre-established and is worked out by each switch for each packet, the network is called connectionless.-If the routing is pre-established the network is called connection-oriented.

Examples of packet-switched or cell-switched connections include X.25, Frame Relay, ATM, SMDS (Switched Multimegabit DataService).

1.5 WAN link options


15/40

15

Fig. 10 WAN link options.


16/40

16

1.6 WAN Technologies

1.6.1 Analog DialupWhen intermittent, low-volume data transfers are needed, modems and analog dialed telephone lines provide low capacity anddedicated switched connections.

Traditional telephony uses a copper cable, called the local loop, to connect the telephone handset in the subscriber premises to thePublic Switched Telephone Network (PSTN). The signal on the local loop during a call is a continuously varying electronic signal

that is a translation of the subscriber voice.

The local loop is not suitable for direct transport of binary computer data, but a modem can send computer data through the voice

telephone network. The modem modulates the binary data into an analog signal at the source and demodulates the analog signal atthe destination to binary data.

The physical characteristics of the local loop and its connection to the PSTN limit the rate of the signal. The upper limit is around 33kbps. The rate can be increased to around 56 kbps if the signal is coming directly through a digital connection.

For small businesses, this can be adequate for the exchange of sales figures, prices, routine reports and email. Using automaticdialup at night or on weekends for large file transfers and data backup can take advantage of lower off-peak tariffs (line charges).Tariffs are based on the distance between the endpoints, time of day and the duration of the call.

The advantages of modem and analog lines are simplicity, availability and low implementation cost. The disadvantages are the lowdata rates and a relatively long connection time. The dedicated circuit provided by dialup will have little delay or jitter for point-to-pointtraffic, but voice or video traffic will not operate adequately at relatively low bit rates.

Fig. 11 Analog Dialup.


17/40

17

The internal connections (or trunks) of the PSTN have changed from carrying analog Frequency-Division Multiplexed (FDM) signalsto Time-Division Multiplexed (TDM) digital signals. An obvious next step is to enable the local loop to carry digital signals that result inhigher capacity switched connections. Integrated Services Digital Network (ISDN) turns the local loop into a TDM digitalconnection. The connection uses 64 kbps Bearer channels (B) for carrying voice or data and a signaling Delta channel (D) for callset-up and other purposes.

Basic Rate Interface (BRI) ISDN is intended for the home and small enterprise and provides two 64 kbps B channels and one 16kbps D channel (2B+D). For larger installations, Primary Rate Interface (PRI) ISDN is available. PRI delivers twentythree 64 kbps B

channels and one 64 kbps D channel (23B+D) in North America, for a total bit rate of up to 1.544 Mbps (including synchronizationoverhead). In Europe, Australia and other parts of the world, ISDN PRI provides thirty B channels and one D channel (30B+D) for atotal bit rate of up to 2.048 Mbps (including synchronization overhead). In North America the rate of ISDN PRI corresponds to a T1connection. The rate of international ISDN PRI corresponds to an E1 connection.

The BRI D channel is underutilized, as it has only two B channels to control. Some providers allow the D channel to carry data at lowbit rates such as X.25 connections at 9.6 kbps.

a) For small WANs, the BRI ISDN can provide an ideal connection mechanism. BRI has a call setup time that is less than a secondand its 64 kbps B channel provide greater capacity than an analog modem link. If greater capacity is required, a second B channelcan be activated to provide a total of 128 kbps. Although inadequate for video, this would permit several simultaneous voice

conversations in addition to data traffic.Another common application of ISDN is to provide additional capacity as needed on a leased line connection. The leased line is sizedto carry average traffic loads while ISDN is added during peak demand periods. ISDN is also used as a backup in the case of afailure of the leased line. ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice connections.b) With PRI ISDN, multiple B channels can be connected between two endpoints. This allows for video conferencing and highbandwidth data connections with no latency or jitter. Multiple connections can become very expensive over long distances.

1.6.2 ISDN

Fig. 12 ISDN.


18/40

18

When permanent dedicated connections are required, Leased Lines (LL) are used with capacities ranging from 56 kbps up to 2.5Gbps.

A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider networkto a remote destination. Point-to-point lines are usually leased from a carrier and are called leased lines. Leased lines are available in

different capacities. These dedicated circuits are generally priced based on bandwidth required and distance between the twoconnected endpoints. The cost of leased-line solutions can become significant when they are used to connect many sites. There aretimes when cost of the leased line is outweighed by the benefits. The dedicated capacity gives no latency or jitter between the

endpoints. Constant availability is essential for some applications such as electronic commerce.

A router serial port is required for each leased-line connection. A CSU/DSU and the actual circuit from the service provider are alsorequired.

Leased lines are used extensively for building WANs and give permanent dedicated capacity. They have been the traditional

connection of choice but have a number of disadvantages. WAN traffic is often variable and leased lines have a fixed capacity. Thisresults in the bandwidth of the line seldom being exactly what is needed. In addition, each endpoint would need an interface on therouter which would increase equipment costs. Any changes to the leased line generally require a site visit by the carrier to changecapacity.

Leased lines provide direct point-to-point connections between enterprise LANs and connect individual branches to a packet-switched network. Several connections can be multiplexed over a leased line, resulting in shorter links and fewer required interfaces.

Fig. 13 Leased Line.

1.6.3 Leases Lines


19/40

19

In response to the expense of leased lines, telecommunications providers introduced packet-switched networks using shared lines toreduce costs. The first of these packet-switched networks was standardized as the X.25 group of protocols. X.25 provides a low bitrate shared variable capacity, with a maximum of 48 kbps, that may be either switched or permanent.

X.25 is a network-layer (L3) protocol and subscribers are provided with a network address. Virtual Circuits (VC) can be establishedthrough the network with call request packets to the target address. The resulting SVC is identified by a channel number. Data

packets labeled with the channel number are delivered to the corresponding address. Multiple channels can be active on a singleconnection. X.25 networks can also have pre-established channels between subscribers that provide a PVC.

Subscribers connect to the X.25 network with either leased lines or dialup connections.

X.25 can be very cost effective because tariffs are based on the amount of data delivered rather than connection time or distance.Data can be delivered at any rate up to the connection capacity. This provides some flexibility. X.25 networks are usually lowcapacity, with a maximum of 48 kbps. In addition, the data packets are subject to the delays typical of shared networks.

Typical X.25 applications are Point-Of-Sale (POS) card readers. These readers use X.25 in dialup mode to validate transactions on acentral computer. Some enterprises also use X.25 based Value-Added Networks (VAN) to transfer Electronic Data Interchange (EDI)invoices, bills of lading and other commercial documents. For these applications, the low bandwidth and high latency are not a

concern, because the low cost makes the use of X.25 affordable.

In US, X.25 technology is no longer widely available as a WAN technology. Frame Relay has replaced X.25 at many service providerlocations.

Fig. 14 X.25.

1.6.4 X.25


20/40

20

1.6.5 Frame Relay

With increasing demand for higher bandwidth and lower latency packet switching, communications providers introduced FrameRelay (FR). Although the network layout appears similar to that for X.25, available data rates are commonly up to 4 Mbps, with someproviders offering even higher rates.

FR differs from X.25 in several aspects. Most importantly, it is a much simpler protocol that works at the data link layer (L2) rather

than the network layer (L3).

FR implements no error or flow control. The simplified handling of frames leads to reduced latency and measures taken to avoidframe build-up at intermediate switches help reduce jitter.

Most FR connections are PVCs rather than SVCs. The connection to the network edge is often a leased line but dialup connectionsare available from some providers using ISDN lines. The ISDN D channel is used to set up an SVC on one or more B channels. FRtariffs are based on the capacity of the connecting port at the network edge. Additional factors are the agreed capacity and

Committed Information Rate (CIR) of the various PVCs through the port.

FR provides permanent shared medium bandwidth connectivity that carries both voice and data traffic. FR is ideal for connectingenterprise LANs. The router on the LAN needs only a single interface, even when multiple VCs are used. The short-leased line to theFrame Relay network edge allows cost-effective connections between widely scattered LANs.

Fig. 15 FR.


21/40

21

1.6.6 ATM

Communications providers saw a need for a permanent shared network technology that offered very low latency and jitter at muchhigher bandwidths. Their solution was Asynchronous Transfer Mode (ATM). ATM has data rates beyond 155 Mbps. As with theother shared technologies, such as X.25 and Frame Relay, diagrams for ATM WANs look the same.

ATM is a technology that is capable of transferring voice, video and data through private and public networks. It is built on a cell-

based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The 53 byte ATM cellcontains a 5 byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice andvideo traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be

transmitted.

The 53 byte ATM cell is less efficient than the bigger frames and packets of FR and X.25. Furthermore, the ATM cell has at least 5bytes of overhead for each 48-byte payload. When the cell is carrying segmented network layer packets, the overhead will be higherbecause the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20% greaterbandwidth than FR to carry the same volume of network layer data.

ATM offers both PVCs and SVCs, although PVCs are more common with WANs. As with other shared technologies, ATM allowsmultiple VCs on a single leased line connection to the network edge.

Fig. 16 ATM.


22/40

22

1.6.7 DSLDigital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport

high-bandwidth data to service subscribers. DSL service is considered broadband, as opposed to the baseband service for typicalLANs. Broadband refers to a technique which uses multiple frequencies within the same physical medium to transmit data. The termxDSL covers a number of similar yet competing forms of DSL technologies: Asymmetric DSL (ADSL), Symmetric DSL (SDSL), HighBit Rate DSL (HDSL), ISDN (like) DSL (IDSL), Consumer DSL (CDSL, also called DSL-lite or G.lite).DSL technology allows the service provider to offer high-speed network services to customers, utilizing installed local loop copperlines. DSL technology allows the local loop line to be used for normal telephone voice connection and an always-on connection forinstant network connectivity. Multiple DSL subscriber lines are multiplexed into a single, high capacity link by the use of a DigitalSubscriber Line (DSLAM) at the provider location.The voice channel of a standard consumer telephone covers the frequency range of 330 Hz to 3.3 KHz. A frequency range or windowof 4 KHz is regarded as the requirements for any voice transmission on the local loop. DSL technologies place upload (upstream)

and download (downstream) data transmissions at frequencies above this 4 KHz window. This technique is what allows both voiceand data transmissions to occur simultaneously on a DSL service.The 2 basic types of DSL technologies are asymmetric (ADSL) and symmetric (SDSL). All forms of DSL service are categorized as

ADSL or SDSL and there are several varieties of each type. Asymmetric service provides higher download bandwidth to the userthan upload bandwidth. Symmetric service provides the same capacity in both directions. Not all DSL technologies allow the use of a

telephone. SDSL is called dry copper because it does not have a ring tone and does not offer telephone service on the same line.Therefore a separate line is required for the SDSL service.Current DSL technologies are using sophisticated coding and modulation techniques to achieve data rates up to 52 Mbps. Thetransfer rates are dependent on the actual length of the local loop and the type and condition of its cabling. For satisfactory service,the loop must be less than 5.5 kilometers (3.5 miles).

It is not a popular choice for enterprise computer departments to support home workers. Generally, a subscriber cannot choose toconnect to the enterprise network directly, but must first connect to an Internet Service Provider (ISP). From here, an IP connection ismade through the Internet to the enterprise. Thus, security risks are incurred. To address security concerns, DSL services providecapabilities for using Virtual Private Network (VPN) connections to a VPN server, which is typically located at the corporate site.

Fig. 17 DSL.


23/40

23

1.6.8 Cable

Coaxial Cable is widely used in urban areas to distribute television signals. Network access is available from some cable televisionnetworks. This allows for greater bandwidth than the conventional telephone local loop.

Enhanced cable modems enable two-way, high-speed data transmissions using the same coaxial lines that transmit cable television.Some cable service providers are promising data speeds up to 6.5 times that of T1 leased lines. This speed makes cable anattractive medium for transferring large amounts of digital information quickly, including video clips, audio files and large amounts of

data. Information that would take 2 minutes to download using ISDN BRI can be downloaded in 2 seconds through a cable modemconnection.

Cable modems provide an always-on connection and a simple installation. An always-on cable connection means that connectedcomputers are vulnerable to a security breach at all times and need to be suitably secured with firewalls. To address securityconcerns, cable modem services provide capabilities for using Virtual Private Network (VPN) connections to a VPN server, which istypically located at the corporate site.

A cable modem is capable of delivering up to 30 to 40 Mbps of data on one 6 MHz cable channel. This is almost 500 times faster

than a 56 Kbps modem.

With a cable modem, a subscriber can continue to receive cable television service while simultaneously receiving data to a personal

computer. This is accomplished with the help of a simple one-to-two splitter.

Cable modem subscribers must use the ISP associated with the service provider. All the local subscribers share the same cablebandwidth. As more users join the service, available bandwidth may be below the expected rate.

Fig. 18 Cable.


24/40


25/40

25

II. HDLC and PPP

2.1 Introduction

WAN technologies are based on serial transmission at the physical layer.This means that the bits of a frame are transmittedone at a time over the physical medium.

The bits that make up the L2 frame are signaled one at a time by physical layer processes onto the physical medium. The signalingmethods include NonReturn to Zero Level (NRZ-L), High Density Binary 3 (HDB3) and Alternative Mark Inversion (AMI). Theseare examples of physical layer encoding standards, similar to Manchester encoding for Ethernet. Among other things, thesesignaling methods differentiate between one serial communication method and another.

Fig. 1 Serial transmission at the physical layer.


26/40

26

Time-Division Multiplexing (TDM) is the transmission of several sources of information using one common channel, or

signal, and then the reconstruction of the original streams at the remote end.

In the example shown in Figure 2, there are 3 sources of information carried in turn down the output channel. First, a chunk ofinformation is taken from each input channel. The size of this chunk may vary, but typically it is either a bit or a byte at a time.

Depending on whether bits or bytes are used, this type of TDM is called bit-interleaving orbyte-interleaving.

Each of the 3 input channels has its own capacity. For the output channel to be able to accommodate all the information from the 3inputs, the capacity of the output channel must be no less than the sum of the inputs.

In TDM, the output timeslot is always present whether or not the input channel has any information to transmit.

TDM is a physical layer concept, it has no regard for the nature of the information that is being multiplexed onto the output channel.TDM is independent of the L2 protocol that has been used by the input channels.

Fig. 2 TDM.


27/40

27

The demarcation point (or "demarc" as it is commonly known) is the point in the network where the responsibility of the

service provider (or "telco") ends.

In the US, a telco provides the local loop into the customer premises and the customer provides the active equipment such as theCSU/DSU on which the local loop is terminated. This termination often occurs in a telecommunications closet and the customer isresponsible for maintaining, replacing or repairing the equipment.

In other countries around the world, the NTU (Network Termination Unit) is provided and managed by the telco. This allows thetelco to actively manage and troubleshoot the local loop with the demarcation point occurring after the NTU. The customer connects

a CPE device, such as a router or Frame Relay Access Device (FRAD), into the NTU using a V.35 or RS-232 serial interface.

Fig. 3 The demarcation point.


28/40

28

A serial connection has a Data Terminal Equipment (DTE) device at one end of the connection and a Data

Communications Equipment (DCE) device at the other end.The connection between the two DCEs is the WAN serviceprovider transmission network.

-The CPE, which is generally a router, is the DTE. Other DTE examples could be a terminal, computer, printer or fax machine.

-The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to

the WAN service provider transmission link. This signal is received at the remote DCE, which decodes the signal back into asequence of bits. This sequence is then signaled to the remote DTE.

Fig. 4 WAN connection.


29/40


30/40


31/40

31

Initially, serial communications were based on character-oriented protocols. Bit-oriented protocols were more efficient but theywere also proprietary.

In 1979, the ISO agreed on HDLC (High-Level Data Link Control) as a standard bit-oriented data link layer protocol thatencapsulates data on synchronous serial data links. This standardization led to other committees adopting it and extendingthe protocol.

Since 1981, ITU-T has developed a series of HDLC derivative protocols. The following examples of derivative protocols are calledLink Access Protocols:-Link Access Procedure for Modems (LAPM) and PPP for modems,-Link Access Procedure on the D channel (LAPD) for ISDN,-Link Access Procedure Balanced (LAPB) for X.25 ,-Link Access Procedure for Frame Relay (LAPF) for Frame Relay .

HDLC uses synchronous serial transmission providing error-free communication between 2 points. HDLC defines a L2

framing structure that allows for flow control and error control using acknowledgments and a windowing scheme . Eachframe has the same format, whether it is a data frame or a control frame.

Fig. 6 HDLC frame.

2.2 HDLC


32/40

32

-The frame always starts and ends with an 8-bit flag field, the bit pattern is 01111110. Because there is a likelihood that this patternwill occur in the actual data, the sending HDLC system always inserts a 0 bit after every five 1s in the data field, so in practice theflag sequence can only occur at the frame ends. The receiving system strips out the inserted bits. When frames are transmitted

consecutively the end flag of the first frame is used as the start flag of the next frame.

-The address field is not needed for WAN links, which are almost always point-to-point. The address field is still present and may be1 or 2 bytes long.

-The control field indicates the frame type, which may be Information, Supervisory, Unnumbered: The control field is normally 1byte, but will be 2 bytes for extended sliding windows systems. Together the address and control fields are called the frame header.-a) Information frames (I-frames) carry network layer data.-b) Supervisory frames (S-frames) control the flow of information frames and request data retransmission in the event of an error.-c) Unnumbered frames (U-frames) carry line setup messages. The code field identifies the U-frame type.

The first one or two bits of the control field serve to identify the frame type. In the control field of an Information frame (I-frame), thesend-sequence number N(S) refers to the number of the frame to be sent next. The receive-sequence number N(R) provides the

number of the frame to be received next. Both sender and receiver maintain send and receive sequence numbers.

Poll/Final (P/F) is a single bit with 2 names. It is called Poll when set by the primary station to obtain a response from a secondarystation and Final when set by the secondary station to indicate a response or the end of transmission. In all other cases, the bit isclear. The bit is used as a token that is passed back and forth between the stations. Only one token should exist at a time. Thesecondary only sends a Final when it has received a Poll from the primary. The primary only sends a Poll when it has received aFinal back from the secondary or after a timeout indicating that the bit has been lost.

S field indicates S-frames type: 00=RR (Receive Ready), 01=RNR(Receive Not Ready), 10=REJ (REJect), 11=SREJ (Selective

REJect).

Standard HDLC does not inherently support multiple protocols on a single link, as it does not have a way to indicate which protocol isbeing carried. Both PPP and the Cisco version of HDLC have an extra field, called Protocol field, in the frame header to identify thenetwork layer protocol of the encapsulated data. This Protocol field enables multiple network layer protocols to share the same seriallink.

-The encapsulated data follows the control field.

-The Frame Check Sequence (FCS) field uses the Cyclic Redundancy Check (CRC) mechanism to establish a 2 or 4 byte field.


33/40

33

PPP (Point-to-Point Protocol) uses a layered architecture and provides a method for encapsulating multi-protocol

datagrams over a point-to-point link and uses the data link layer for testing the connection. Therefore PPP is made up of 2sub-protocols:1) Link Control Protocol (LCP) - Used for establishing the point-to-point link.2) Network Control Protocol (NCP) - Used for configuring the various network layer protocols.

PPP can be configured on the following types of physical interfaces:Asynchronous serial, Synchronous serial, High-SpeedSerial Interface (HSSI), Integrated Services Digital Network (ISDN).

2.3 PPP

Fig. 7 PPP layered architecture.


34/40

34

1) PPP uses Link Control Protocol (LCP) to establish, configure and test the WAN data link connection. PPP also uses LCP toautomatically agree upon encapsulation format options such as:

-Authentication - Authentication options require that the calling side of the link enter information to help ensure the user has thenetwork administrator's permission to make the call. Peer routers exchange authentication messages. Two authentication choices are

Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP) (in general CHAP is thepreferred protocol) (ppp authentication pap orchap).

-Compression - Compression options increase the effective throughput on PPP connections by reducing the amount of data in theframe that must travel across the link. The protocol decompresses the frame at its destination. Two compression protocols availablein Cisco routers are Stackerand Predictor(ppp compress stackerorpredictor).

-Error detection - Error detection mechanisms with PPP enable a process to identify fault conditions. The Quality and MagicNumber options help ensure a reliable, loop-free data link (ppp quality ).

-Multilink - Cisco IOS Release 11.1 and later supports multilink PPP. This alternative provides load balancing over the routerinterfaces that PPP uses (ppp multilink).

-PPP Callback - To further enhance security, Cisco IOS Release 11.1 offers callback over PPP. With this LCP option, a Cisco routercan act as a callback client or as a callback server. The callback client makes the initial call, requests that it be called back andterminates its initial call. The callback server answers the initial call and makes the return call to the client based on its configurationstatements.

LCP will also do the following: handle varying limits on packet size, detect common misconfiguration errors, determine when a link isfunctioning properly or when it is failing, terminate the link.

2) PPP uses the Network Control Protocol (NCP) component to encapsulate and negotiate options for multiple network layerprotocols. PPP permits multiple network layer protocols to operate on the same communications link. For every network layer

protocol used, a separate Network Control Protocol (NCP) is provided. For example, Internet Protocol (IP) uses the IP ControlProtocol (IPCP), Internetwork Packet Exchange (IPX) uses the Novell IPX Control Protocol (IPXCP), Appletalk uses Appletalk ControlProtocol (ACP). NCPs include functional fields containing standardized codes to indicate the network layer protocol type that PPPencapsulates.


35/40

35

The fields of a PPP frame are as follows:

-Flag - Indicates the beginning or end of a frame and consists of the binary sequence 01111110.-Address - Consists of the standard broadcast address, which is the binary sequence 11111111. PPP does not assign individualstation addresses.-Control - 1 byte that consists of the binary sequence 00000011, which calls for transmission of user data in an unsequencedframe.-Protocol - 2 bytes that identify the protocol encapsulated in the data field of the frame (8021=IPCP, 8023=NCP, 8029=ACP,

802b=IPXCP, c021=LCP, c023=PAP, c223=CHAP)-Data - 0 or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the data field is foundby locating the closing flag sequence and allowing 2 bytes for the FCS field. The default maximum length of the data field is 1500bytes.-FCS - Normally 2 bytes added to a frame for error control purposes.

PPP session establishment progresses through 3 phases:-1) Link-establishment phase - In this phase each PPP device sends LCP frames to configure and test the data link. LCP framescontain a configuration option field that allows devices to negotiate the use of options such as the Maximum Transmission Unit

(MTU), compression of certain PPP fields and the link-authentication protocol. If a configuration option is not included in an LCPpacket, the default value for that configuration option is assumed. Before any network layer packets can be exchanged, LCP mustfirst open the connection and negotiate the configuration parameters. This phase is complete when a configurationacknowledgment frame has been sent and received.-2) Authentication phase (optional) - After the link has been established and the authentication protocol decided on, the peer maybe authenticated. Authentication, if used, takes place before the network layer protocol phase is entered. As part of this phase, LCPalso allows for an optional link-quality determination test. The link is tested to determine whether the link quality is good enough tobring up network layer protocols.-3) Network layer protocol phase - In this phase the PPP devices send NCP frames to choose and configure one or morenetwork layer protocols, such as IP. Once each of the chosen network layer protocols has been configured, packets from each

network layer protocol can be sent over the link. If LCP closes the link, it informs the network layer protocols so that they can takeappropriate action. The show interfaces command reveals the LCP and NCP states under PPP configuration.

The following 3 classes of LCP frames are used in a PPP session:

-Link-establishment frames are used to establish and configure a link.-Link-termination frames are used to terminate a link.-Link-maintenance frames are used to manage and debug a link.The PPP link remains configured for communications until either of the following:

-LCP or NCP frames close the link,

-An inactivity timer expires,-A user intervenes.


36/40

36

PAP provides a simple method for a remote node to establish its identity, using a two-way handshake. After the PPP linkestablishment phase is complete, a username/password pair is repeatedly sent by the remote node across the link untilauthentication is acknowledged or the connection is terminated.

PAP is not a strong authentication protocol. Passwords are sent across the link in clear text and there is no protection from playback

or repeated trial-and-error attacks. The remote node is in control of the frequency and timing of the login attempts.

The hostname on one router must match the username the other router has configured. The passwords must also match.

Fig. 8 PAP.

Fig. 9 Configuring of PAP.

2.4 PAP


37/40

37

CHAP is used at the startup of a link and periodically verifies the identity of the remote node using a three-way handshake. CHAP isperformed upon initial PPP link establishment and is repeated during the time the link is established.

After the PPP link establishment phase is complete, the local router sends a "challenge" message to the remote node. The remotenode responds with a value calculated using a one-way hash function, which is typically Message Digest 5 (MD5). This response isbased on the password and challenge message. The local router checks the response against its own calculation of the expected

hash value. If the values match, the authentication is acknowledged, otherwise the connection is immediately terminated.

CHAP provides protection against playback attack through the use of a variable challenge value that is unique and unpredictable.Since the challenge is unique and random, the resulting hash value will also be unique and random. The use of repeated challengesis intended to limit the time of exposure to any single attack. The local router or a third-party authentication server is in control of thefrequency and timing of the challenges.

The hostname on one router must match the username the other router has configured. The passwords must also match.

Fig. 11 Configuration of CHAP.

Fig. 10 CHAP a.

2.5 CHAP


38/40

38

1. Challenge

2. Response3.

Accept/Reject

Calculations

Fig. 10 CHAP b.


39/40

39

The default encapsulation method used by Cisco devices on synchronous serial lines is Cisco HDLC. If the serial interface isconfigured with another encapsulation protocol, and the encapsulation must be changed back to HDLC, enter the interfaceconfiguration mode of the serial interface. Then enter the encapsulation hdlc command to specify the encapsulation protocol on the

interface. Cisco HDLC is a point-to-point protocol that can be used on leased lines between 2 Cisco devices. When communicatingwith a non-Cisco device, synchronous PPP is a more viable option.

Router(config)#interface Router(config-if)#encapsulation hdlc

Router(config-if)#exit

When the encapsulation ppp command is used, either PAP or CHAP authentication can be optionally added. If no authentication isspecified the PPP session starts immediately. If authentication is required the process proceeds through the following steps:-The method of authentication is determined.

-The local database or security server, which has a username and password database, is checked to see if the given username andpassword pair matches.-The process checks the authentication response sent back from the local database. If it is a positive response, the PPP session isstarted. If negative, the session is terminated.

Router(config)#interface Router(config-if)#encapsulation ppp

Router(config-if)#exit

Fig. 12 Starting the PPP session.

2.6 Configuration of serial WAN with HDLC/PPP


40/40

40

-1) The output of the show interfaces serial command displays information specific to serial interfaces. When HDLC isconfigured, "Encapsulation HDLC" should be reflected in the output. When PPP is configured, "Encapsulation PPP" should be

seen in the output.

Five possible problem states can be identified in the interface status line of the show interfaces serial display:Serial x is down, line protocol is downSerial x is up, line protocol is downSerial x is up, line protocol is up (looped)Serial x is up, line protocol is down (disabled)Serial x is administratively down, line protocol is down.

-2) The show controllers command is another important diagnostic tool when troubleshooting serial lines. The showcontrollers output indicates the state of the interface channels and whether a cable is attached to the interface (for example,DTE V.35 TX and RX clocks detected). The command syntax varies, depending on platform. For serial interfaces on Cisco7000 series routers, use the show controllers cbus command.

If the electrical interface output is shown as UNKNOWN, instead of V.35, EIA/TIA-449 or some other electrical interface type,an improperly connected cable is the likely problem. A problem with the internal wiring of the card is also possible. If theelectrical interface is UNKNOWN, the corresponding display for the show interfaces serial command will show that theinterface and line protocol are down.

2.7 Verifying of serial WAN with HDLC/PPP configuration

The following are some debug commands that are useful when troubleshooting serial and WAN problems:-debug serial interface - Verifies whether HDLC keepalive packets are incrementing. If they are not, a possible timing problem

exists on the interface card or in the network.-debug arp - Indicates whether the router is sending information about or learning about routers (with ARP packets) on the other side

of the WAN cloud. Use this command when some nodes on a TCP/IP network are responding, but others are not.-debug ppp negotiation - Shows PPP packets transmitted during PPP startup where PPP options are negotiated.-debug ppp packet - Shows PPP packets being sent and received. This command displays low-level packet dumps.-debug ppp error- Shows PPP errors, such as illegal or malformed frames, associated with PPP connection negotiation andoperation.-debug ppp authentication - Shows PPP CHAP and PAP packet exchanges.-debug ppp chap - Shows PPP CHAP packet exchanges.

2.8 Troubleshooting of serial WAN with HDLC/PPP configuration

Documents

110705-WAN Technologies-Cisco WAN Technologies Boldat Cu Rosu