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FOR THE EXAM: You will have to be familiar with how WANs work, what equipment is necessary to connect over a WAN and which protocols are available.

The main difference between the way we connect out to the telco and they way they connect to us is the type of equipment used. We have to get something called a clock from the telco so the speed we want to send data out at matches the speed they are expecting to receive the data at.

The interface we use to connect to the telco with is known as Data Terminal Equipment or DTE. The telco uses a different interface which is used to provide the clock know as Data Communication Equipment or DCE.

DCE equipment also includes something known as channel service unit / data service unit. This device provides conversion from your LAN data format to one compatible with your telcos requirements.


FIG 8.1 WAN Equipment

In any WAN diagram we normally use a cloud to refer to a telcos network. We are not normally able to draw every switch and router inside there and it is usually out of our control anyway.

Other Networking Terms

There are many other terms you will hear when discussing WAN technologies with other engineers.

Customer Premise Equipment (CPE)

CPE is basically any equipment you own and maintain and which are located on your premises. If CPE breaks normally it will be your responsibility to resolve the problem unless you have outsourced your IT support.

Data Terminal Equipment (DTE)

This is normally the interface on your side of the WAN link which connects to the telco. The DTE interface uses clocking signals generated by the DCE interface.

Data Communications Equipment (DCE)

DCE interface provide connection to the telcos network, here traffic is forwarded, data is synchronised and clocking signals provided. When practising networking with routers at home you will have to configure your own DCE interface because one end of the cable will be DTE and one end DCE. On the DCE end of the cable you simply add the clock rate command and give a clocking number.

You can normally tell which end of the cable is DCE because it will have DCE stamped on it. A DCE interface is normally defined by the cable and not the actual interface. If you have a DCE cable attached to the interface you need to add the clock rate command.

RouterA(config-interface)#clock rate 56000 <- sets the speed to 56000bps

Local Loop

This is the cabling which extends from the demarcation point to the local telco switch or exchange.

Demarcation Point (demarc)

Normally inside a switching closet in the communications room. This is where the CPE meets the local loop.

Central Office (CO)

This is the main point of presence for the for the telcos WAN service to the end user.

Modems

A modem converts digital signals to analogue and back again.


FIG 8.2 Modem Converts Digital Signals to Analogue

The correct name for a modem is modulator/demodulator. Modems are a cheap and easy way to connect over a WAN, especially in places like North America where local calls are free.


FIG 8.3 An End to End Modem Connection