Semester 4 - Chapter 2 - WANs

uptightexampleNetworking and Communications

Oct 24, 2013 (3 years and 9 months ago)

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Semester 4
-

Chapter 2
-

WANs

Introducing the various protocols and technologies used in wide
-
area
network (WAN) environments


You will learn about the basics of WANs, including


common WAN technologies,


types of wide
-
area services,


encapsulation formats, and link options.


In this chapter, you also will learn about


point
-
to
-
point links,


circuit switching,


packet switching,


virtual circuits,


dialup services, and


WAN devices.

WAN = Data
Communications

Network

operating beyond a LAN's
geographic scope.


You must
subscribe

to an outside
WAN

service

provider

to use WAN
carrier network services.


WANs use data links such as


Integrated Services Digital Network (
ISDN
) and


Frame

Relay


A WAN
connects

the locations of an organisation to:



each other



other organizations



external services (such as databases), and to



remote users

WANs

WANs generally carry a variety of
traffic

types
, such as


voice
,


data
, and


video
.

WAN technologies function at the three lowest layers of the OSI reference
model:


the
physical

layer,


the
data link

layer, and


the
network

layer.

Telephone

and
data

services

are the most commonly used WAN
services.

An overview of the WAN cloud organises WAN provider services into
three main types:


Call setup (also called
signalling
)


Time

Division

Multiplexing (TDM)


Frame Relay

WANs

The most commonly used terms associated with the main parts of
WAN services:



Customer premises equipment

(
CPE
)
--

Devices physically located on
the subscriber's premises.



Demarcation
(or
demarc
)
--

The point at which the CPE ends and the
local loop portion of the service begins. Often occurs at the
POP

of a
building.



Local

loop

(or "last
-
mile")
--

Cabling (usually copper wiring) that extends
from the demarc into the WAN service provider's central office.



CO

switch

--

A switching facility that provides the nearest point of
presence for the provider's WAN service.



Toll

network

--

The collective switches and facilities (called trunks) inside
the WAN provider's cloud.

WAN TERMINOLOGY

A key interface is between the data terminal equipment (
DTE
) and the data
circuit
-
terminating equipment (
DCE
).


Typically, the
DTE

is the
router
, and the
DCE

is the device used to convert
the user data from the DTE into a form acceptable to the WAN service's
facility (eg a
MODEM
).


The WAN path between the DTEs is called the


link


circuit


channel


line


The DTE/DCE interface uses various protocols (such as
HSSI

and
V.35
)
that establish the codes that the devices use to communicate with each
other. This communication determines how call setup operates and how
user traffic crosses the WAN.

DTE / DCE

A virtual circuit is a logical circuit created to ensure reliable communication
between two network devices. There are two types:


switched

virtual circuits (SVCs) and


permanent

virtual circuits (PVCs).


SVCs are
dynamically

established on demand and terminated when
transmission is complete. There are three phases:


circuit

establishment


data

transfer


circuit

termination
.

SVCs are used in situations where data transmission between devices is sporadic.


Disadvantage

= increased bandwidth due to the circuit establishment and
termination phases


Advantage

= decreased costs compare with constant virtual
-
circuit availability.

A PVC is a
permanently

established virtual circuit that consists of one mode:


data transfer.

PVCs are used in situations where data transfer between devices is constant.


Advantage

= PVCs decreased bandwidth use because there are no establishment
and termination phases


Disadvantage

= increased costs due to constant availability.

SVC / PVC

Routers are devices that implement the network service.

They provide interfaces for a wide range of links and subnetworks at a wide range
of speeds.

Routers are active and intelligent network devices and can participate in managing
the network by:


providing dynamic control over resources


supporting the tasks and goals for networks.

These goals are


Connectivity


reliable

performance


management

control


Flexibility

A WAN switch is a multiport networking device, which typically switches such
traffic as


Frame

Relay


X.25


Switched Multimegabit Data Service (
SMDS
).

This switching function operates at the
data link layer

of the OSI reference
model.

WAN DEVICES
-

ROUTER



A
modem

is a device that interprets digital and analogue signals by
modulating

and
demodulating

the signal


This enables
data

to be transmitted over
voice
-
grade

telephone lines.


At the source,
digital

signals are
converted

to

a
form

suitable for
transmission over
analogue

communication facilities.


At the destination, these
analogue

signals are
returned

to

their
digital

form
.

WAN DEVICES
-

MODEM





A CSU/DSU is a
digital
-
interface

device


or sometimes two separate digital devices


A CSU/DSU
adapts

the
physical

interface

on a
DTE

device to the
interface of a
DCE

device in a
switched
-
carrier

network.

WAN DEVICES
-

CSU / DSU







An ISDN
Terminal

Adapter

(TA) is a device used to connect ISDN Basic
Rate Interface (BRI) connections to other interfaces.


WAN DEVICES
-

TA

WANs use the OSI reference model layered approach to encapsulation,
just as LANs do, but they are mainly focused on the
physical

and
data

link

layers.


WAN standards typically describe both physical
-
layer delivery methods
and data link
-
layer requirements, including
addressing
,
flow

control

and
encapsulation
.


WAN standards are defined and managed by a number of recognized
authorities, including the following agencies:



International Telecommunication Union
-
Telecommunication
Standardization Sector (
ITU
-
T
), formerly the Consultative Committee for
International Telegraph and Telephone (CCITT)



International Organization for Standardization (
ISO
)



Internet Engineering Task Force (
IETF
)



Electronic Industries Association (
EIA
)



Telecommunications Industries Association (
TIA
)


WAN ENCAPSULATION

The WAN data link layer defines how data is encapsulated for transmission to
remote sites.

WAN data
-
link protocols describe how frames are carried between systems on a
single data path.

Common data
-
link
encapsulations

associated with WAN lines are:



Frame Relay

--

Simplified encapsulation

with
no error correction

mechanisms
over
high
-
quality digital

facilities, Frame Relay can transmit data very
rapidly

compared to the other WAN protocols.


Point
-
to
-
Point Protocol

(PPP)
--

Described by RFC 1661, PPP was developed by
the IETF. PPP contains a protocol field to identify the network
-
layer protocol.


ISDN

--

A set of digital services that transmits
voice

and
data

over existing
phone

lines
.


Link Access Procedure, Balanced

(LAPB)
--

For
packet
-
switched

networks,
LAPB is used to encapsulate packets at Layer 2 of the X.25 stack. LAPB provides
reliability

and
flow

control

on a point
-
to
-
point basis.


Cisco/IETF

--

Used to encapsulate
Frame Relay

traffic. The Cisco option is
proprietary and can be used
only

between
Cisco

routers
.


High
-
Level Data Link Control

(HDLC)
--

An ISO standard, HDLC might not be
compatible

between different vendors because of the way each vendor has chosen
to implement it. HDLC supports both
point
-
to
-
point

and
multipoint

configurations.

DATA LINK ENCAPSULATION

The two most common point
-
to
-
point WAN encapsulations are HDLC and PPP. All
the serial line encapsulations share a common frame format, which has the
following fields:




Flag


-

Indicates the beginning of the frame and is set to
the hexadecimal pattern 7E.




Address

-

A 1
-

or 2
-
byte field to address the end station in multidrop
environments.




Control

-

Indicates whether the frame is an information, a supervisory,
or an unnumbered type frame. It also contains specific function codes.




Data


-

The encapsulated data.




FCS


-

The frame check sequence (FCS).




Flag


-

The trailing 7E flag identifier.


Each WAN connection type uses a Layer 2 protocol to encapsulate traffic while it
is crossing the WAN link. To ensure that the correct encapsulation protocol is
used, you need to configure the Layer 2 encapsulation type to use for each
serial interface on a router. The choice of encapsulation protocol depends on
the WAN technology and the communicating equipment. Encapsulation
protocols that can be used with the WAN connection types covered in this
chapter are PPP and HDLC.

HDLC & PPP


The two most common point
-
to
-
point WAN encapsulations are




HDLC

and
PPP
.




PPP

can check for link quality during connection establishment.


In addition, there is support for authentication through



Password Authentication

Protocol (PAP)

and



Challenge Handshake Authentication Protocol (CHAP).




HDLC

-

a data link
-
layer protocol derived from the Synchronous Data
Link Control (SDLC) encapsulation protocol.



HDLC is Cisco's
default

encapsulation for serial lines.

HDLC & PPP


Dedicated lines, also called leased lines, provide
full
-
time service.



Dedicated lines typically are used to carry



data



voice



video



In data network design, dedicated lines generally provide core or backbone
connectivity between major
sites

or campuses, as well as
LAN
-
to
-
LAN

connectivity.



Dedicated lines generally are considered reasonable design options for WANs.




The following are required for dedicated line connections.



a
router

port



CSU

/
DSU



actual
circuit

from the service provider.


DEDICATED LINES



Packet switching is a WAN switching method in which network devices
share a
permanent

virtual circuit



The following are examples of packet
-
switched WAN technologies.


Frame relay


SMDS
(
Switched Multimegabit Data Service
)


X.25


PACKET SWITCHING

Frame Relay was designed to be used over
high
-
speed
,
high

quality

digital

facilities.


As a result, Frame Relay does not offer much
error checking

or
reliability


Frame Relay expects upper
-
layer protocols to attend to these issues.


A data
-
link connection identifier (
DLCI
) identifies a
PVC
.


The DLCI number is a
local

identifier

between the DTE and the DCE


The DLCI identifies the
logical

circuit

between the source and
destination devices.


FRAME RELAY




Circuit switching is a WAN switching method in which a
dedicated

physical circuit is
established
,
maintained
,
and
terminated

through a carrier network for each
communication session.


ISDN

is an example of a circuit
-
switched WAN technology.


CIRCUIT SWITCHING

Dial
-
on
-
demand routing (DDR) is a technique in which a router can
dynamically

initiate

and
close

circuit
-
switched sessions when
transmitting end stations need them.


When the router receives traffic destined for a remote network, a circuit is
established
, and the traffic is transmitted normally.


DDR

enables you to make a
standard

telephone

connection or an
ISDN

connection only when required by the volume of network traffic.


DDR can be used to provide
backup

load

sharing

and interface backup.


Compared to LAN or campus
-
based networking, the traffic that uses DDR
is typically
low

volume

and
periodic
.


DIAL ON DEMAND ROUTING

ISDN BRI operates mostly over the copper twisted
-
pair
telephone wiring in place today.


ISDN BRI delivers a total bandwidth of a
144 kbps

line into
three

separate channels.


Two of the channels, called
B (bearer) channels
, operate at
64

kbps

and are used to carry
voice

or
data

traffic.


The third channel, the
D (delta) channel
, is a
16
-
kbps

signalling channel used to carry instructions that tell the
telephone network how to handle each of the B channels.


ISDN BRI often is referred to as
2B+D
.


ISDN







Good luck on the test !!!