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Chapter 21
WAN Basics, Remote Access Technologies, and Serial PPP
- Organizations often extend their LANs to WANs, Wide Area Networks for connections to
remote sites. WANs allow the information exchange, communication and collaboration
between customers, suppliers, and among employees effectively.
- Cisco supports many types of WAN protocols. CCNA covers Serial PPP (leased lines), ISDN,
and Frame Relay.
- Below lists some common WAN terminologies:
i) Customer Premises Equipment (CPE) is the equipment that is owned by the service
subscriber and is located at the subscriber’s premise, eg: router.
ii) Demarcation Point is the spot where the responsibility of the service provider ends and
the CPE begins. The demarc is not a device or cable – it is a concept of where each party
responsibility starts and ends. When someone reported a WAN problem to the Telco and
the Telco replied that they have performed tests and are fine up to the demarc, the
problem must be caused by the CPE and is not the responsibility of the Telco.
iii) Local Loop is the connection from the demarc to the Telco switch in the closest service
provider switching office – the local CO (central office).
iv) Toll network is a collection of trunk links inside a service provider’s network.
- Below lists the common WAN connection types in the order of costing (from higher to lower):
Leased Lines Also referred to as PPP or dedicated connections. They are pre-established
connections which allow communication at any time (hence a circuit does not
need to be established before data transmission). Their cost is very high.
HDLC and PPP encapsulation protocols are frequently used on them.
They provide high bandwidth and constant data rate for data transfer.
Packet
Switching
Circuit
Switching
This WAN service allows the sharing of bandwidth with other companies to
save money. It only works well for data transfer in bursty nature; hence leased
lines would be the better choice if constant data transfer is required.
Ex: X.25, Frame Relay, and ATM.
Circuit switching operates much like a normal telephone call. The advantage
of this WAN service is low cost, where subscribers only pay for the duration of
the usage. A dedicated circuit is established, maintained, and terminated for
each communication session (hence a circuit needs to be established before
data communication). In circuit-switched networks, the resources along the
path are reserved for the duration of the communication session.
It normally provides low bandwidth for data transfer.
Ex: Modem dial-ups and ISDN.
- Data Terminal Equipment (DTE) and Data Communications Equipment (DCE):
DTEs are mostly router interfaces that connect to DCEs, eg: Channel Service Unit / Data
Service Unit (CSU/DSU), which are connects into the demarcation point (the start of the Telco
responsibility). CSUs/DSUs provide signal timing (clocking) for communication between DTEs
and DCE devices (Telco switch). CSUs/DSUs reside in the physical layer of the OSI model.
- A WAN network normally consists of 2 DTE networks connect through a DCE network.
The DCE network includes the CSU/DSU at both ends, the Telco wiring, and Telco switches.
DCE devices provide clocking to DTE interfaces, eg: router serial interfaces.
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