 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






 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) .


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.

 Many WANs use a hierarchical addressing scheme that makes forwarding more efficient.
 Hierarchical address (figure 13.3) is divided into two parts
– switch#
– port#

 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 nexthop 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

 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.
– RIP
 link state routing (algorithm 13.1)
–
– aka shortest path first (SPF) (fig 13.9)
OSPF

 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)

 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

 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.

 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.

 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.

 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.

 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