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Fundamentals Of WANs

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Fundamentals Of WANs

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Cisco CCNA Study Guide

By Cbrzana

Table Of Contents

Fundamentals of WANs

Physical cabling owned, installed, and managed by, typically, a telephone company and is leased out. Also called Public Telephone and Telegraph (PTT) companies or service provider.

- > Routers connect to CSU/DSU, which connects the Telco on the other end

- > Demarcation Point indicates where the Telco is responsible and where the customer is responsible (usually at CSU/DSU side)

- > CPE (Customer Premises Equipment) indicates equipment at customer's site

WAN Cabling Standards

Serial interfaces use a variety of proprietary connectors

Standard Connectors Standards Body # of Pins
EIA/TIA-232 TIA 25
EIA/TIA-449 TIA 37
EIA/TIA-530 TIA 25
V.35 ITU 34
X.21 ITU 15
  • These connectors connect to the CSU/DSU
  • Cable between CSU/DSU and Telco is typically RJ-48
  • As a rule of thumb, the shorter the cable from CSU/DSU, the faster

Maximum Speeds for Cables

Data (bps) Distance (Meters) EIA/TIA-232 Distance (Meters) EIA/TIA-449, V.35, X.21, EIA-530
2400 60 1250
4800 30 625
9600 15 312
19,200 15 156
38,400 15 78
115,200 3.7 --
T1 (1.544 Mbps) -- 15

  • CSU/DSU provide clocking signal to routers so they transmit at correct speed
  • Considered to be clocking the unit

DCE (Data Communications Equipment): Device that provides the clocking (typically CSU/DSU)

DTE (Data Terminal Equipment): device that receives the clocking (typically router)

*Note: Need DTE cable for router, unless router is acting as clocking unit.

Link Speeds

Pulse Code Modulation (PCM): Original standard for converting analog voice to digital signal.

  • Analog voice signaled 8000 times/sec, with 8-bit code (total of 64,000 bits/sec)
  • Known as Digital Signal Level 0 (DS0)
  • 56-kbps originally offered since 8 bits needed for management overhead

Speed Summary

Type of Line Signaling Type Bit Rate
56 DS0 56-kbps
64 DS0 64-kbps
T1 DS1 1.544 Mbps (24 DS0s, 8 kbps overhead)
T3 DS3 44.736 Mbps (28 DS1s, management overhead)
E1 ZM 2.048 Mbps (32 DS0s)
E3 M3 34.064 Mbps (16 E1s, management overhead)
J1 Y1 2.048 Mbps (32 DSOs)

Two most popular Data-link protocols on point-to-point links:

1. High-Level Data Link Control (HDLC)

a. 1 byte address field (routers put decimal value of 3 in field)

b. FCS field in HDLC trailer

c. 2-byte Protocol Type field identifies type of packet

d. Cisco's HDLC is proprietary (added Protocol Type field), won't work w/ other vendor's router

2. Point-to-Point Protocol (PPP)

a. Link Control Protocol (LCP) apply regardless of Layer 3 Protocol

b. Control Protocols (CP) special purposes for particular L3 Protocol

- > IP Control Protocol (IPCP) assigns IP address over PPP link

PPP LCP Features

Function LCP Feature Description
Error Detection Link quality monitoring (LQM) PPP can take down a link based on % of errors
Looped link detection Magic Number PPP uses "Magic Number" to detect looped link and shuts link down.
Multilink support Multilink PPP Allows multiple parallel serial links to be connected between same routers (balance traffic)
Authentication PAP and CHAP Verifies identity of device

The other PPP WAN Data-Link Protocols

Protocol Error Correction Type Field Comments
Synchronous Data Link Control (SDLC) Yes No Supports multipoint links
Link Access Procedure Balanced (LAPB) Yes No (Cisco proprietary) LAPB is used by X.25 primarily
Link Access Procedure on the D Channel No No LAPD is used by ISDN lines
Link Access Procedure for Frame Mode Bearer Services (LAPF) No Yes Used over frame relay
High-level Data Link Control No No (Cisco proprietary) HDLC is Cisco's default
Point to Point Protocol (PPP) Supported, not enabled by default Yes Meant for multiple protocols from its inception

Synchronization: Imposed time ordering at the link's sending/receiving ends (agrees to certain speed. Set up by having master/slave CSU/DSU.

Packet-Switching Services

Frame Relay: Multi-access networks that can have more than two devices attached to the network.


  • Router examines frames, header is Data-link connection identifier (DLCI).
  • Virtual circuits exist between routers, appears to be a physical link, but not.
  • Committed information rate (CIR) is the minimum bandwidth provided.

SONET: Synchronous Optical Network, alternative to layer 1 signaling (uses fiber optics over electrical signals)

- > Note: Outside U.S., term Synchronous Digital Hierarchy (SDH) represents same standards as SONET

ATM (Asynchronous Transfer Mode): Data-link protocol to provide voice and data over same infrastructure. Breaks data into 53-byte cells (48 bytes data, 5 byte header)

  • Header consists of 2 fields, the Virtual Path Identifier and Virtual Channel Identifier (VPI/VCI) - > Devices forward cells based on these headers
  • Router breaks up data into 48-byte segments, attaches 5-byte header.
  • The entire segmentation process is called segmentation and reassembly (SAR)

Dedicated Circuit Summary
Packet Switching DTE connects to telco using single physical line, telco makes forwarding decision.
Frame Switching In concept, identical to packet switching.
Cell Switching In concept, identical to packet switching but uses 53-byte cells instead.
Circuit Switching Process of dialing, setting up a circuit, and then hanging up (circuit switched on and then off).

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