General Networking Tutorial: WAN Protocols and Design

Point-to-Point Leased Lines:

WAN protocols used on point-to-point serial links provide the basic function of delivery of data across that one link. As a CCNA, you will be required to understand and configure a variety of protocols used on point-to-point links, including Link Access Procedure Balanced (LAPB),High-Level Data Link Control (HDLC), and Point-to-Point Protocol (PPP). Each of these WAN protocols has the following functions in common:

1)LAPB, HDLC, and PPP provide for delivery of data across a single point-to-point serial link.

2)LAPB, HDLC, and PPP deliver data on synchronous serial links. (PPP supports asynchronous functions as well.)

FRAMING:
Framing is one core feature of any synchronous serial data link protocol. Each of these protocols defines framing so that receiving stations know where the beginning of the frame is, what address is in the header, and the point at which the packet begins. By doing so, the router receiving data can distinguish between idle frames and data frames. Synchronous links, rather than asynchronous links, are typically used between routers.

Synchronous simply means that there is an imposed time ordering at the sending and receiving ends of the link. Essentially, the sides agree to a certain speed, but because it is very expensive to build devices that can truly operate at exactly the same speed,the devices adjust their rates to match a clock source. The process works almost like the scenes in spy novels, when the spies synchronize their watches; in this case, the watches or clocks are synchronized automatically multiple times per minute. Unlike asynchronous links, in which no bits are sent during idle times, synchronous data links define idle frames. These frames do nothing more than provide plenty of signal transitions so that clocks can be adjusted on the receiving end, consequently maintaining synchronization.

4-wire circuit A line from the telco with four wires, comprised of two twisted-pair wires. Each pair is used to send in one direction, so a 4-wire circuit allows full-duplex communication.

2-wire circuit A line from the telco with two wires, comprised of one twisted-pair wire. The pair is used to send in only one direction at a time, so a 2-wire circuit allows only half-duplex communication.