Chapter Summary

Routers typically route traffic between different subnets. To forward traffic between subnets that aren't in the same location, the router uses connectivity that a telephone company (telco) provides. One way the telco can provide that basic connectivity is through WAN links, also called leased lines, leased circuits, and point-to-point links.

With a WAN link, two routers can send and receive traffic to and from each other. To create the physical transmission medium, the telco installs a cable between each site and a local central office (CO). The telco then somehow passes the bits over the internal telco network. The end result is that the routers believe that they have a 4-wire cable between each other, over which they can send and receive at any time.

At each customer site, a router is installed, along with either an internal or external CSU/DSU. The CSU/DSU is configured with the speed of the link, with the lower-speed options from the telco being multiples of 64 Kbps.

To pass traffic over the WAN link, a data link protocol needs to be used; HDLC and PPP are the two most popular options today. Interestingly, because any data sent out a point-to-point WAN link goes to the device on the other end of the linkand only to that devicethese data link protocols don't have a lot of work to do. They mainly provide the ability to encapsulate a packet and to check for errors. PPP does provide additional features as compared to HDLC.