WANs - Faculty of Computer Science and Information Technology

fullgorgedcutNetworking and Communications

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

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WANs

Table of Contents

WAN Technology & Devices

WANs & The OSI Model

WAN Encapsulation Formats

WAN Link Options

Go There!

Go There!

Go There!

Go There!

WAN Technology &
Devices

Table of Contents

WAN Services

A WAN is the communications network that connects LANs
through a WAN Service Provider (e.g. RBOC)

WANs operate at the first three layers of the OSI, but focus
mainly on the physical and data link layers.

Examples of WAN Services include….

Call
-
setup


SS7

used for telephone calls

ISDN


A circuit
-
switched service using
TDM
, which allocates bandwidth to different
sources on the same media

Frame Relay


A packet
-
switched service using
statistical multiplexing

WAN Physical Structure

Toll Network



collection of
switches/trunks in WAN cloud

CO Switch



nearest point of
presence for the WSP’s service

Local loop



extends from the
CPE (at the demarc) to the CO

CPE



devices located on
premises, either owned or leased

DTE/DCE Interface

The transfer of responsibility for service between the
ISP and the customer occurs at the DTE/DCE
interface.

The DTE generates the data and passes them, along
with any necessary control characters to a DCE.

The DCE converts the signal to a format appropriate to
the transmission medium and introduces it into the
network link.

(Reverse process at the receiving end)

WAN Virtual Circuits

Switched Virtual Circuits (SVCs) are WAN paths to the
destination established and terminated on demand.

Three phases:


Circuit establishment


creates the virtual circuit


Data transfer


sending and receiving user data


Circuit termination


tearing down the virtual circuit

Telephone service and ATM use SVCs

Increased use of bandwidth but decreased cost

Permanent Virtual Circuits (PVCs) are permanently
established circuits with one mode: data transfer

X.25 and Frame Relay use PVCs

Decreased use of bandwidth but increased cost

Link Types & Bandwidth

WAN Devices

In order to connect to a leased line, the customer
must have…

Access to the service provider’s circuit

An appropriate router port available

An a CSU/DSU, modem or ISDN Terminal Adapter

Routers

Routers offer many services including:

Internetworking

WAN serial interfaces

Routers can operate as...

Internal Routers

Backbone Routers

Area Border Routers

Autonomous System Boundary Routers


Modems

Also called CSU/DSUs (channel service
units/digital service units)

Interface with voice
-
grade connection in order to
convert analog signal to digital.

Represents the DCE side of the DTE/DCE
connection.

Leased line

Router

Router

CSU/DSU

CSU/DSU

WANs & The OSI Model

Table of Contents

WAN Standards

What layers of the OSI model do WAN standards
describe?

Physical and Data Link Layers

WAN Physical Layer

Protocols that describe how to provide electrical,
mechanical, operational, and functional
connections for WAN services.

These services are most often obtained from
WAN service providers such as RBOCs, alternate
carriers, post, telephone and telegraph (PTT)
agencies.

Describes the interface between the data terminal
equipment (DTE) and the data circuit
-
terminating
equipment (DCE).

WAN Physical Layer

Typically, the DCE is the service provider and the
DTE is the attached device.

In this model, the services offered to the DTE are
made available through a modem or a CSU/DSU.

WAN Physical Layer

Several physical layer standards specifying this
interface between the DTE & DCE are...

EIA/TIA
-
232

EIA/TIA
-
449

V.24

V.35

X.21

G.703

EIA
-
530

WAN Data
-
Link Layer

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

They include protocols designed to operate over
dedicated point
-
to
-
point, multipoint, and multi
-
access switched services.

WAN standards are defined and managed by a
number of recognized authorities, including the
following agencies: ITU
-
T, ISO, IETF, & EIA

WAN Encapsulation
Formats

Table of Contents

WAN Data
-
Link Encapsulations

PPP (Point
-
to
-
Point Protocol)

Developed by IETF; replacing SLIP

Contains a field to identify the network layer protocol

PPP can check for link quality during connection
establishment

Supports PAP (Password Authentication Protocol) & CHAP
(Challenge Handshake Authentication Protocol)

WAN Data
-
Link Encapsulations

High
-
Level Data Link Control (HDLC)

Cisco default encapsulation; typically used between routers
running Cisco IOS; replacing SDLC

Streamlined: no windowing or flow control

may not be compatible with different vendors because of
the way each vendor has chosen to implement it.

HDLC supports both point
-
to
-
point and multipoint
configurations with minimal overhead

WAN Data
-
Link Encapsulations

Frame Relay

uses high
-
quality digital facilities;

uses simplified framing with no error correction
mechanisms (connectionless!!);

it can send Layer 2 information much more rapidly than
other WAN protocols

WAN Link Options

Table of Contents


T1, E1, T3, E3


xDSL


SONET

Switched



POTS



ISDN



X.25



Frame Relay



ATM (cell)



SMDS (cell)

WAN Link Options

Circuit
Switched

Packet/Cell
Switched

Dedicated


T1, E1, T3, E3


xDSL


SONET

Switched



POTS



ISDN



X.25



Frame Relay



ATM (cell)



SMDS (cell)

WAN Link Options

Circuit
Switched

Packet/Cell
Switched

Dedicated

Dedicated Digital Services

T series

in U.S. and
E series

in Europe

Uses
time division multiplexing

to “slice up” data and
assign time slots for transmissions


T1 = 1.544 Mbps


T3 = 44.736 Mbps


E1 = 2.048 Mbps


E3 = 34.368 Mbps


Uses twisted pair & fiber


Extremely popular


Moderate cost

Dedicated Digital Services provide full
-
time
connectivity through a point
-
to
-
point link

Dedicated Digital Services

Digital Subscriber Lines (xDSL)
;

the x stands for
a family of technologies

WAN Technology for home use; decreasing bandwidth
with increasing distance from the phone companies CO.

Data rates as high as 51.84 Mbps but more common to
be in the 100s of Kbps

Varieties include
HDSL, SDSL, ADSL, & VDSL

Moderate expense and getting cheaper

Dedicated Digital Services

Synchronous Optical Network

(
SONET
)

Specialized high bandwidth technology for use at various
Optical Carrier speeds (OC) ranging from 51.84 Mbps
(OC
-
1) to 9,952 Mbps (OC
-
192)

Uses lasers to divide the wavelength of the light into
sections that can carry large amounts of data (Wave
Division Multiplexing)

Very expensive; used by large ISPs and other Internet
backbone entities.


T1, E1, T3, E3


xDSL


SONET

Switched



POTS



ISDN



X.25



Frame Relay



ATM (cell)



SMDS (cell)

WAN Link Options

Circuit
Switched

Packet/Cell
Switched

Dedicated

Circuit Switched Services

Plain Old Telephone System

(POTS)

Not

a computer data service but...


POTS is an important component of our communication
infrastructure and


It is still the standard for designing reliable networks

Circuit Switched Services

Integrated Services Digital Network

(
ISDN
)

Historically important
--
first dial
-
up digital service

Provides both voice & data transmission over existing
phone lines

Cost is moderate; max. bandwidth = 128 kbps for BRI
(Basic Rate Interface)


2 B channels @ 64kps and 1 D channel @ 16kps


B channels are voice/data channels; D for signaling

B

B

D


T1, E1, T3, E3


xDSL


SONET

Switched



POTS



ISDN



X.25



Frame Relay



ATM (cell)



SMDS (cell)

WAN Link Options

Circuit
Switched

Packet/Cell
Switched

Dedicated

Packet Switched Services

X.25

(Connection
-
oriented)

Older WAN technology developed in 1970s

Reliable
--
X.25 has been extensively debugged and is now
very stable
--
literally no errors in modern X.25 networks

Store & Forward
--
Since X.25 stores the whole frame to
error check it before forwarding it on to the destination, it
has an inherent delay (unlike Frame Relay) and requires
large, expensive memory buffering capabilities.

Packet Switched Services

Frame Relay

(Connectionless)

More efficient and much faster than X.25

Packet switched version of ISDN (which is circuit
switched); data rates up to 44.736Mbps with 56kbps and
384kbps being the most popular

Used mostly to forward LAN IP and IPX packets but can
be used to forward other types of traffic

Primary competitive advantage is its low cost

Cell Switched Services

Asynchronous Transfer Mode

(
ATM
)

Relatively new WAN Technology related to broadband
ISDN; max. bandwidth = 622 Mbps

Developed in order to provide one technology for both
WANs and LANs to transport data, video, and voice.
(High Cost)

Key Benefits:


One network for all traffic
--
voice, data, video


Compatible with current wiring infrastructure (cable plant)


Very flexible and scalable


Simplifies network management

Cell Switched Services

Switched Multimegabit Data Service

(
SMDS
)

Closely related to ATM; SMDS is the MAN (Metropolitan
Area Network) implementation of ATM

High Cost with max. bandwidth 44.736 Mbps


WAN Link Options Review

Table of Contents

End Slide Show