Chapter 13 WAN Chapter 13 WAN Technologies and RoutingTechnologies and Routing

LAN LimitationsLAN 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)Wide Area Network (WAN) spans sites in multiple cities, countries, continents. Scalable

– 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 (figure 13.2).

WAN CharacteristicsWAN 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

simultaneously switched LAN also allow many computers to communicate

simultaneously, but broadcast domain differ)  Packet switching systems in WAN use store-and-forward

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 Physical Addressing in A WANWAN

Many WANs use a hierarchical addressing scheme that makes forwarding more efficient.

Hierarchical address (figure 13.3)is divided into two parts – switch# – port#

RoutingRouting 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: – next-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 only (fig 13.6) (fig 13.7).

Creation of routing table – static routing (simple but inflexible) – dynamic routing (flexible) (RIP/OSPF). 

Routing table entries– Destination network– Netmask– Next hop– Cost

Routing AlgorithmsRouting Algorithms

vector-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.

– RIPlink state routing (algorithm 13.1)

– aka shortest path first (SPF)(fig 13.9) – OSPF

Example WAN TechnologiesExample WAN Technologies ARPANET

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

X.25 – popular in Europe, connection-oriented – Data link layer of X.25 (ie. LAP B) is responsible for

retransmitted bad frames ISDN (Integrated Services Digital Network) Frame Relay SMDS (Switched Multi-megabit Data Service) ATM (Asynchronous Transfer Mode)

ISDNISDN

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.)ISDN (cont.)NT1 (network Termination type 1)

– provides a connection (U-interface containing 1 twisted-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-45 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.)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 – NT1 – phone company– Eg. TE2 – TA – NT1 – 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

64kbps.

Frame RelayFrame 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 protocols.

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

– 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.)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 Asynchronous Transfer Mode (ATM)(ATM)

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 longLayer 2