Tài liệu giảng dạy CCNA - module 06 chapter 18-Fundamentals of WANs

To get a point-topoint WAN link, you would work with a service provider to install a circuit..  PPP provides a method for encapsulating multi-protocol datagrams over a point-to-point li

Module 06 WAN

Technology

Chapter 18 Fundamentals of WANs

Table of Content

1 OSI Layer 1 for Point-to-Point WANs

2 OSI Layer 2 for Point-to-Point WANs

3 Packet-switching service

1 OSI Layer 1 for Point-to-Point

WANs

Overview WAN connections

 Point-to-point WAN links provide basic connectivity

between two points To get a point-topoint WAN link, you

would work with a service provider to install a circuit

 A point-topoint WAN connection sometimes is called a

leased circuit or leased line because you have the exclusive

right to use that circuit, as long as you keep paying for it

WAN Connections from the Customer

Viewpoint

Connecting WAN interfaces

Clock Rates, DCE, and DTE

 The terms clock rate and bandwidth both refer to the speed

of the circuit You will also hearthe speed referred to as the

link speed.

 The device that provides, clocking, typically the CSU, is

considered to be the data communications equipment

(DCE)

 The device receiving clocking, typically the router, is

referred to as data terminal equipment (DTE).

Link Speeds Offered by Telcos

2 OSI Layer 2 for Point-to-Point

WANs

HDLC

 HDLC defines framing that includes an address field, a

frame check sequence (FCS) field, and a protocol type field

 The original HDLC standards did not include a Protocol Type field

PPP Layered Architecture

 PPP uses a layered architecture A layered architecture is a

logical model, design, or blueprint that aids in

communication between interconnecting layers

 PPP provides a method for encapsulating multi-protocol

datagrams over a point-to-point link, and uses the data link layer for testing the connection

 PPP is made up of two sub-protocols:

 Link Control Protocol – Used for establishing the

point-to-point link

Synchronous vs Asynchronous

 Synchronous: A device tries to use the same speed as

another device on the other end of a serial link However,

by examining transitions between voltage states on the

link, the device can notice slight variations in the speed on

each end and can adjust its speed accordingly

 Asynchronous: Both sides agree to the same speed, but

there is no check or adjustment of the rates if they are

slightly different However, because only 1 byte per

transfer is sent, slight differences in clock speed are not an

issue A start bit is used to signal the beginning of a byte

3 Packet-Switching Services

Introducing Frame Relay

 Frame Relay is a Consultative Committee for CCITT and

 Originally Frame Relay was designed to allow ISDN

equipment to have access to a packet-switched service on

a B channel However, Frame Relay is now a stand-alone

technology

Frame Relay terminology: VC

 The connection through the Frame Relay network between

two DTEs is called a virtual circuit (VC)

 Frame Relay virtual circuits fall into two categories

switched virtual circuits (SVCs)

 May have multiple virtual circuits connecting it to various end points

on a single access line.

 Frame Relay virtual circuits are identified by data-link connection

identifiers (DLCIs) DLCI values typically are assigned by the Frame

Relay service provider (for example, the telephone company)

 Frame Relay DLCIs have local significance, which means that their

values are unique in the LAN, but not necessarily in the Frame Relay

WAN

Frame Relay bandwidth terminology:

CIR

 Usually there are several PVCs operating on the access link

with each VC having dedicated bandwidth availability This

is called the committed information rate (CIR)

 The CIR is the rate at which the service provider

agrees to accept bits on the VC.

 Individual CIRs are normally less than the port speed

 The sum of the CIRs will normally be greater than the port

speed Sometimes this is a factor of 2 or 3

SONET

 Synchronous Optical Network (SONET) defines an

alternative Layer 1 signaling and encoding mechanism The

motivation behind SONET was to allow the phone

companies of the world to connect their COs with

high-speed optical links SONET provides the Layer 1 details of

how to pass high-speed data over optical links

SONET Link Speeds

Optical Carrier Speed

ATM

 Asynchronous Transfer Mode (ATM) provides data link layer

services that run over SONET Layer 1 links

 ATM does not forward frames—it forwards cells.

 ATM cells contain 48 bytes of payload and a 5-byte header

The header contains two fields that together act like the

DLCI for Frame Relay by identifying each VC The two fields

are named Virtual Path Identifier (VPI) and Virtual Channel

Identifier (VCI).