Chapter 3. WAN Protocols and Technologies: High-Level Data Link Control (HDLC)

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Wide-area networks evolved from the need to share data across vast geographical distances. The definition of WAN is a network spanning a wide geographic area. The purpose of the WAN is to connect local-area networks (LANs) and to transport data from one LAN to another. In our studies, the WAN is used much in the same way.

WAN protocols have changed over time. At first, WAN protocols focused on error correction and were designed to operate on old, unreliable copper lines. WAN protocols today offer high-speed reliable transport on copper and fiber optic lines. Despite the vast evolution of the WAN, it still proves to be the slowest and most expensive part of any modern internetwork.

WAN protocols function at the lower three layers of the OSI model. X.25 PLP is one of the protocols that operates at Layer 3, but most WAN protocols operate at Layer 2. Table 3-1 displays some common WAN protocols and their corresponding layers in the OSI reference model.

Table 3-1. WAN Protocols and Their Corresponding OSI Layers
OSI Layers WAN Protocols
Network layer X.25 PLP
Data link layer Frame Relay PPP

HDLC

LAPB

X.25 SDLC ATM-AAL
Physical layer

ISDN-B

ISDN-D

ISDN-H

H11, H12

EIA/TIA-232

EIA/TIA-449 V.24, V.35, HSSI

G703, EIA-530

DS-1, DS-3, SONET

In the lab, using Cisco 2500 and 2600 series routers, you can readily model HDLC, PPP, Frame Relay, and X.25. Our studies start with these protocols and end with ISDN and ATM. Modeling ISDN and ATM requires the use of specialized routers and switches.

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CCIE Practical Studies, Volume I
CCIE Practical Studies, Volume I
ISBN: 1587200023
EAN: 2147483647
Year: 2001
Pages: 283
Authors: Karl Solie

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