CCNA™: Introduction to WAN protocols
Wide Area Networks
(WANs) operate over serial links. A serial
link is one that transmits and receives digitized signal one bit at a time.
Serial links using modems typically offer 56Kbps. Compare a serial link with
that of a parallel link. An example of parallel link is printer connection over
parallel cable. A printer receives several bits at a time and processes them
all. A parallel line is typically used for connecting your monitor, printer, and
CDROM drive. Parallel links run over over a very short distances.
Within serial links, there are two types:
Asynchronous serial links
Synchronous serial links
Asynchronous serial links: These are widely used for connecting
to Internet using your dial-up modem. Asynchronous link is normally used for low
Async (short for Asynchronous) links require start and stop bits for
effective communication. It can also have parity bits for error checking. When
using Async communication link, both sender and receiver need to agree on fixed
line speed (expressed in terms of bits per second), otherwise, the receiver may
not be able to receive any data at all.
2. Synchronous serial links: Synchronous links, as the
name suggests use clocking to transmit or receive data. A clock signal is
required for transmitting or receiving synchronous data.
The clock signal may be transmitted separately, or could be
derived from the received signal. In either case, the clock signals are used for
receiving the data.
Synchronous links can operate at very high speeds.
HDLC are examples of synchronous link protocols.