Devices extend the reach of LANs (Local Area Networks). WANS typically span over a wide area, such over multiple cities / countries. WANS are connected over serial lines that typically operate at lower speeds than LANs.
Some of the WAN devices
1. Routers: Routers are responsible for routing the packets in an internetwork.
2. Modems: Modems connect to public telephone circuits through dial-up.
3. CSU/DSU: Stands for Channel Service Unit / Data Service Unit. CSU/DSUs are used for connecting to Central Office of a Telephone switching company and provides serial WAN connections.
4.Communication Servers: These are used for dial in/out to remote users. Provides RAS (Remote Access Server) functionality.
5. Multiplexers (mux): Multiplexers combine two or more signals before transmitting on a single channel. Multiplexing can be done by sharing "time" or "frequency".
6.1. DTE (Data Terminal Equipment) and DCE (Data Communication Equipment) :
A DTE device is expected to communicated with a DCE device. In such event, the connections between the DCE and DTE devices is straight. However, if the communication need to happen between two DTEs or two DCEs, the signals need to be rolled. Note that rolling the signals twice is same as using straight through cables or adapters without any rolling at all. The purpose of DCE device is to provide clocking and switching services in a network. Clock rate doesn't have a default value so we have to configure it manually (while bandwidth does have a default value). Cisco routers can be configured as DTE or DCE devices. This is achieved by rolling the pins in the cable or in the DB-25 adapter.
Typically, a DTE to DTE communication can be achieved by:
1. By connecting DTE to a rolled RJ-45 cable to a straight DB-25 adapter to DTE,
2. By connecting a straight RJ-45 cable to a rolled DB-25 adapter to DTE.
Similarly, a DTE to DCE communication can be achieved by:
1. Connecting a DTE to a straight RJ-45 cable to a straight DB-25 adapter to DCE,
2. Connecting a rolled RJ-45 cable to a rolled DB-25 adapter to DCE.