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Chapter 10

Wide Area Networking Protocols

High-Level Data-Link Control Protocol
(HDLC)

he High-Level Data-Link Control protocol (HDLC) is a popular

ISO-standard, bit-oriented Data Link layer protocol. It specifies an encap-
sulation method for data on synchronous serial data links using frame
characters and checksums. HDLC is a point-to-point protocol used on
leased lines. No authentication can be used with HDLC.

In byte-oriented protocols, control information is encoded using entire

bytes. Bit-oriented protocols, on the other hand, may use single bits to rep-
resent control information. Bit-oriented protocols include SDLC, LLC,
HDLC, TCP, IP, etc.

HDLC is the default encapsulation used by Cisco routers over synchro-

nous serial links. Cisco's HDLC is proprietary--it won't communicate with
any other vendor's HDLC implementation--but don't give Cisco grief for it;

everyone's

HDLC implementation is proprietary. Figure 10.2 shows the

Cisco HDLC format.

One of the newer WAN services that Cisco doesn't list as a WAN service

in the CCNA objectives is a wireless connection. You can get from 1Mbps

to over 50Mbps, depending on the service, and it works too! For the

speed, it is relatively inexpensive. If you want to connect two buildings

together, then you should consider a wireless solution. Of course, Cisco

sells everything you need to do this, at a decent price (compared to an

wired solution).

You can even use a wireless solution for connecting your business to the

Internet. The problem with wireless ISPs (WISPs) is that they come and go--

they are in business one day and then gone the next. Make sure you have

a backup solution if you decide on a WISP, in case they don't answer the

phone one day. Eventually, things will mellow out and we'll find carriers

that stay in business.

If you cannot get wireless for your business, then Frame Relay is a fast,

cost-effective WAN service. ATM is nice and fast, but expensive.

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