Chapter 13 WAN Technologies and Routing

munchdrabNetworking and Communications

Oct 30, 2013 (4 years and 8 months ago)


Chapter 13 WAN
Technologies and Routing

LAN Limitations

Local Area Network (LAN) spans a single
building or campus.

Bridged LAN is not considered a Wide Area
technology because bandwidth limitations prevent
bridged LAN from serving arbitrarily many
computers at arbitrarily may sites.

Limited scalability

Wide Area Network (WAN)

spans sites in multiple cities, countries, continents.


can grow as needed to connect many sites far away with many
computers at each site.

high capacity achieved through use of many switches instead of
using a shared medium or single switch to move packets .

uses packet switching technology where complete packets are
moved from one connection to another.

Each packet switch is a dedicated computer with memory and I/O
ports to send/receive packets.

A packet switch is the basic building block of WAN. A WAN is
formed by interconnecting a set of packet switches, and then
connecting computers. Additional switch or interconnections can
be added as needed to increase the capacity of the WAN

WAN Characteristics

shared LAN that allows only one pair of computers to
exchange a frame at a given time

WAN permits many computers to send packets

switched LAN also allow many computers to
communicate simultaneously, but broadcast domain

Packet switching systems in WAN use store
switching. Incoming packets are stored in a buffer queue.
The processor is interrupted to forward (queue) the
packet to the proper outgoing port.

This technique allows a packet switch to buffer a short
burst of packets that arrive simultaneously.

Physical Addressing in A

Many WANs use a hierarchical addressing
scheme that makes forwarding more

Hierarchical address
(figure 13.3)
is divided
into two parts




aka Next
Hop Forwarding

a packet switch keeps a routing table of the next place
(hop) to send a packet so the packet will eventually reach
its destination
(figure 13.4)

When forwarding a packet, a packet switch only needs to
examine the first part of a hierarchical address.

routing table can be kept to a minimal size

Values in a routing table must guarantee

universal routing where each possible destination has a next
hop route

optimal routes where next
hop value will take the packet closer
to its destination.

Default route

Source Independence:

hp forwarding does not depend on packet’s
original source; instead the next hop to which a packet
is sent is a function of the packet’s destination address
(fig 13.6)

(fig 13.7)

Creation of routing table


routing (simple but inflexible)


routing (flexible) (RIP/OSPF).

Routing table entries

Destination network


Next hop


Routing Algorithms

distance algorithm (algorithm 13.2)

requires messages to be sent from one packet
switch to another switch that contains pairs of
values which specify a destination and a
distance to that destination.


link state routing (algorithm 13.1)

aka shortest path first (SPF)
(fig 13.9)


Example WAN Technologies


based on packet switches connected by leased 56kbps
serial data lines


popular in Europe, connection

Data link layer of X.25 (ie. LAP B) is responsible for
retransmitted bad frames


(Integrated Services Digital Network)

Frame Relay

SMDS (Switched Multi
megabit Data Service)

ATM (Asynchronous Transfer Mode)


dialed digital connection offered by telephone
companies .

Basic Rate Interface (BRI)

two 64kbps B channels, one 16kbps D (delta) channel.

Primary Rate Interface (PRI)

24 64kbps channels (23 B + 1D) over a T1 line.

TE1 (terminal equipment type 1)

eg. ISDN telephone, ISDN computer, or ISDN FAX

TE2 (terminal equipment type 2)

eg. old analog phone, fax, analog modem

ISDN (cont.)

NT1 (network Termination type 1)

provides a connection (U
interface containing 1
pair copper on RJ
11) to phone company and a
separate connection to your house’s ISDN network
(S/T interface bus containing 4wire on 8
pin RJ
operating at 192kbps to accommodate 2B +D + 48bps
overhead). NT1 requires external power supply: if
power is down, you can’t dial out; advisable to provide
UPS or install separate analog phone line.

ISDN (cont.)

TA (terminal Adapter)

aka ISDN modem. A protocol converter that contains
interfaces for connecting TE2 equipment to NT1 via
S/T interface

Eg. TE1


phone company

Eg. TE2



phone company

Eg. Ascend Pipeline 25 has Ethernet connector, 2
analog RJ
11 POTS, 1 ISDN BRI S/T or U interface

Inverse multiplexing

allows combining B
channels to get speeds greater than

Frame Relay

a link layer protocol occupying layer 2 (Data link) of
the OSI model

Bad frames are discarded by frame relay

retransmission is done by layer 4 (transport)

Frame structure

Flag ( 1 byte)

Data Link Connection ID (2 bytes)

no notion of source and destination addresses found in other

Each DLCI identifies a virtual circuit from one location to a remote

Data field(up to 4096 bytes)

may contain a Network Level Protocol ID (NLPID) header to indicate
whether data is IP or IPX or Decnet, 2 octet CRC, and a 1 octet flag.

Frame Relay (cont.)

A physical link between to physical locations may
contain multiple permanent virtual circuits (PVC)
via multiplexing

Committed Information Rate (CIR)

data rate that is guaranteed on a particutlar DLCI.

CIR is defined as a committed bust size of Bc bits over
time T .

Excess burst size Be bits are delivered on a best effort
basis. Bits over Bc + Be during time T may be
immediately discarded.

Asynchronous Transfer Mode

designed for voice, video and data services
that require low delay and low jitter
(variance in delay) and high speed.

All ATM cells are 53
octets long

Layer 2