19: Wide Area Networks (WAN)

19.1 Wide Area Network (WAN) basics

• The WAN is a network that extends over a wide geographical area.

• Generally, the WAN networks are used to connect between several LANs of a certain company.

• The company does not own the WAN, but it always takes the WAN from a ‘Service Provider’.

Image2641.JPGFigure 19.1: the WAN connection

CPE (Customer Premises Equipment)

• CPE is any circuit or equipment that is located in the customer’s side, e.g., a router, computers, etc.…

Demarcation point

• The demarcation point is the point at which the service provider responsibility ends, and the customer responsibility begins.

Local loop

• The local loop is the physical link that connects between the demarcation point and the service provider’s network edge.

DCE (Data Communication Equipment)

• The DCE is a device that provides a ‘clock signal’ for the DTE.

• The DCE may be a ‘CSU/DSU’ in case it terminates a digital circuit, e.g. a ‘T1 circuit’, or it may be a ‘modem’ in case that it terminates an analog local loop, e.g. a telephone line.

DTE (Data Terminal Equipment)

• Typically, the router’s interface that connects to the DEC is a DTE. It takes the clocking signal from the DCE.

CSU/DSU (Channel Service Unit/Data Service Unit)

• It is the DCE device that provides the clocking for the DTE and connects it to the digital circuit.


• It is the DCE device provides clocking for the DTE, and connects it to an analog line.

• The device that converts the signals that come through an analog line into digital information, and decodes the digital information and converts it into signals that can be transmitted over an analog line.

19.2 WAN connection types

There are several WAN connection types that can be used in a WAN network to connect between the customer networks.

19.2.1 Leased line

Image2649.JPGFigure 19.1: leased line

• It may be called a ‘dedicated connection’ or a ‘point-to-point connection’

• It is a dedicated connection between the customer networks, and it passes through the WAN network that is provided by the service provider.

• The leased line advantage is its high quality.

• The leased line disadvantage is that, it is very expensive, and its limited flexibility.

19.2.2 Circuit switching

Image2658.JPGFigure 19.2: circuit switching

• In the circuit switched connection, a dedicated path is established between the customer networks, no one can use this path at this time except this customer.

• This dedicated path is established only in the time when the customer networks need to communicate with each other.

• The customer is billed depending on the time of using this path, not on the amount of data sent.

• The circuit switching advantage is that, it is efficient; this is because it is a dedicated path for the customer. Therefore, the bit rate will be constant.

• The circuit-switching disadvantage is that it has a low speed bit rate.

19.2.3 Packet switching

Image2669.JPGFigure 19.3: packet switching

• In the packet switching connection, the path between the customer networks is not dedicated for this customer.

• This path, or a part of it, may be shared with other traffic from other customers.

• In the packet switching, the customer is billed depending on the amount of data that is transmitted.

• The packet switching advantage is the low compared to leased lines, and it can provide high data rates compared to the circuit switching.

• The packet switching disadvantage is that it needs more complex implementation in the service provider’s network.

19.3 HDLC and PPP protocols

• HDLC and PPP protocols are used in case that the customer is using a point-to-point link to connect between his networks.

• HDLC and PPP protocols are working in the data link layer.

19.3.1 HDLC (High Level Data Link Control) Protocol

• HDLC is the protocol that is used by default on the router’s serial interfaces.

• HDLC is a proprietary protocol, this means that unless you have all of your routers are from the same vendor, you will not be able to use this protocol.

19.3.2 PPP (Point-to-Point Protocol)

• PPP is a non-proprietary protocol. It means that if you have several routers from different vendors, you should use PPP.

• PPP uses LCP (Link Control Protocol) to establish and terminate the point-to-point connections.

• PPP uses NCP (Network Control Protocol) to allow the usage of different network layer protocols.

PPP supports the following options (which is not supported by HDLC)

Authentication: peer routers authenticate with each other before establishing a connection. Two authentication methods may be used, PAP and CHAP.

Multilink: a router can consider several physical interfaces as one logical interface. E.g. if a router has three 2Mbps links, we can configure it to consider them as one 6Mbps link.

Compression: the router compresses the sent frames in order to increase the throughput of the connection.

Error detection: the router can detect any errors during the frame transmission.

19.4 PPP configuration

To configure PPP on a router’s serial interface, we use the following commands,

Router(config)#interface interface

Router(config-if)#encapsulation ppp

PPP authentication:

Suppose that we have two routers needs to authenticate with each other. We will configure the two routers as following,

Image2680.JPGFigure 19.4: two routers connected using PPP

We must configure the same password on the two routers in order to be able to authenticate with each other.

We use the following commands to configure the authentication of the routers,

Router(config)#username peer hostname password password

Router(config)#interface interface

Router(config)#ppp authentication {PAP | CHAP}

PAP: Password is sent in clear text format.

CHAP: More secure than PAP, It is a three way handshaking authentication method.

In figure (19.4), to configure the routers to authenticate with each other,

Router1’s configureation,

Router1(config)#username Router2 password abcd

Router1(config)#interface ser 0/0

Router1(config-if)#ppp authentication CHAP

Router2’s configuration,

Router2(config)#username Router1 password abcd

Router2(config)#interface ser 0/0

Router2(config-if)#ppp authentication CHAP

19.5 Frame Relay (FR)

• The frame relay (FR) is a packet switching technology. It is used in the service provider’s network.

• The frame relay (FR) is a cost efficient technology. It reduces the costs of the connection.

19.5.1 Virtual Cuircuits

• The frame relay (FR) can provide a VC (Virtual Circuit) between the customer networks, so that the customer feels like he has a dedicated connection or a leased line between his networks.

Image2687.JPGFigure 19.5: frame relay virtual circuits

• Two types of VC (Virtual Circuit) exist, PVC (Permanent Virtual Circuit), and SVC (Switched Virtual Circuit).

PVC (Permanent Virtual Circuit) is a virtual circuit that exists between the customer networks and always used in the communication between the customer networks.

SVC (Switched Virtual Circuit) is a virtual circuit that is not permanent. A new VC is established for every new session between the customer networks.

19.5.2 DLCI

• DLCIs (data Link connection identifiers) identify the PVCs to the customer’s DTE interfaces.

Image2695.JPGFigure 19.6: identifying PVCs using the DLCI

• As seen in figure (19.6), the customer’s HQ router serial interface can be divided into two sub-interfaces; every sub interface will use one PVC to connect to one of the branches.

• Every sub-interface will be assigned a DLCI that is mapped to the PVC it uses.

• The service provider provides the DLCI number of the PVC to the customer.

19.5.3 LMI

• LMI (local management interface) is a protocol that exchanges the information about the status of the virtual circuit between the customer’s router (DTE), and the service provider’s device (DCE).

19.6 FR congestion control

19.6.1 CIR vs. access rate

• When a customer deals with a service provider to connect between the customer networks, the deal contains two important values, the CIR, and the access rate.

• CIR (committed information rate) is the bit rate that is guaranteed by the service provider to be delivered to the customer’s connection.

• Access rate is the maximum bit rate that can be used by the customer.

19.6.2 DE (discard eligibility)

• DE (discard eligibility) is a bit in the FR header.

• When the customer consumes the CIR of his connection and starts sending frames above his CIR, the DE bit in those frames will be set.

• When there is congestion in the network of the service provider, the FR switches in the service provider’s network will start discarding the frames that has its DE bit set.

19.6.3 FECN and BECN

• FECN (forward explicit congestion notification) and BECN (backward explicit congestion notification) are two bits in the FR header.

• When there is congestion in the service provider’s network, the FR switch will set the FECN bit in the frames it forwards to the destination DTE, this is done to inform the destination that there is congestion in the network.

• When there is congestion in the service provider’s network, the FR switch will set the BECN bit in the frames it forwards to the source DTE, this is done in order to tell the source that there is congestion in the network, the source DTE in response should slow down its sending rate.

19.7 FR configuration

Image2702.JPGFigure 19.6: identifying PVCs using the DLCI

• In figure (19.7), we need to configure the router1’s serial interface to connect to its network branch routers through the service provider’s frame relay (FR) network.

• First, we divide (ser0) physical interface into two logical sub-interfaces (ser0.1, ser0.2).

• Then, we configure ser0.1 sub-interface to use PVC_1 (DLCI=100) to connect to the ‘branch 1’ router.

• Then, we configure ser0.2 sub-interface to use PVC_2 (DLCI=200) to connect to the ‘branch 2’ router.

• The following commands are used to make this configuration,

Router1(config)#interface ser0

Router1(config-if)#encapsulation frame-relay

Router1(config-if)#no shutdown

(To bring the interface up)

Router1(config-if)#interface ser0.1 point-to-point

(creates ser0.1 sub interface and determine its operation in point-to-point mode)

Router1(config-subif)#ip address

Router1(config-subif)#frame-relay interface-dlci 100

(To make ser0.1 use PVC_1 to connect to branch 1 router)

Router1(config-subif)#interface ser0.2 point-to-point

(creates ser0.2 sub interface and determine its operation in point-to-point mode)

Router1(config-subif)#ip address

Router1(config-subif)#frame-relay interface-dlci 200

(To make ser0.1 use PVC_2 to connect to branch 2 router)