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

1
Module 06 WAN
Technology
Chapter 18
Fundamentals of WANs
2
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
3
1. OSI Layer 1 for Point-to-Point
WANs
4
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.
5
WAN Connections from the Customer
Viewpoint
6
Connecting WAN interfaces
7
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).
8
Link Speeds Offered by Telcos
9
2. OSI Layer 2 for Point-to-Point
WANs
10
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
11
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.

Network Control Protocol – Used for configuring the
various network layer protocols.
12
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.
13
3. Packet-Switching Services
14
Introducing Frame Relay


Frame Relay is a Consultative Committee for CCITT and
ANSI standard.

Frame Relay is a packet-switched, connection-oriented,
WAN service.

It operates at the data link layers of the OSI reference
model.

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.
15
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)

permanent virtual circuits (PVCs).

Frame Relay provides no error recovery mechanism
16
Frame Relay terminology: DLCI


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.
17
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.
18
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.
19
SONET Link Speeds
Optical Carrier Speed
OC-1 52Mbps
OC-3 155Mbps
OC-12 622Mbps
OC-48 2.4Gbps
OC-192 9.6Gbps
OC-768 40Gbps
20
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).
21
Summary

WAN connections.


HDLC.

PPP.

FRAME-RELAY.

SONET.

ATM