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Chapter 13 WAN Technologies and Routing
LAN Limitations Local Area Network (LAN) spans a single building or campus. l Bridged LAN is not considered a Wide Area technology because bandwidth limitations prevent bridged LAN from serving arbitrarily many computers at arbitrarily may sites. l Limited scalability l
Wide Area Network (WAN) l l l 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 Characteristics l l l 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 WAN l Many WANs use a hierarchical addressing scheme that makes forwarding more efficient. l Hierarchical address (figure 13. 3)is divided into two parts – switch# – port#
Routing l l l 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. l Default route
l 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). l Creation of routing table – static routing (simple but inflexible) – dynamic routing (flexible) (RIP/OSPF). l Routing table entries – – Destination network Netmask Next hop Cost
Routing Algorithms l 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 l link state routing (algorithm 13. 1) – aka shortest path first (SPF)(fig 13. 9) – OSPF
Example WAN Technologies l ARPANET – based on packet switches connected by leased 56 kbps serial data lines l 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) l Frame Relay l SMDS (Switched Multi-megabit Data Service) l ATM (Asynchronous Transfer Mode) l
ISDN dialed digital connection offered by telephone companies. l Basic Rate Interface (BRI) l – two 64 kbps B channels, one 16 kbps D (delta) channel. l Primary Rate Interface (PRI) – 24 64 kbps channels (23 B + 1 D) over a T 1 line. l TE 1 (terminal equipment type 1) – eg. ISDN telephone, ISDN computer, or ISDN FAX l TE 2 (terminal equipment type 2) – eg. old analog phone, fax, analog modem
ISDN (cont. ) l NT 1 (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 4 wire on 8 -pin RJ-45 operating at 192 kbps to accommodate 2 B +D + 48 bps overhead). NT 1 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. ) l TA (terminal Adapter) – aka ISDN modem. A protocol converter that contains interfaces for connecting TE 2 equipment to NT 1 via S/T interface – Eg. TE 1 – NT 1 – phone company – Eg. TE 2 – TA – NT 1 – phone company – Eg. Ascend Pipeline 25 has Ethernet connector, 2 analog RJ-11 POTS, 1 ISDN BRI S/T or U interface l Inverse multiplexing – allows combining B-channels to get speeds greater than 64 kbps.
Frame Relay a link layer protocol occupying layer 2 (Data link) of the OSI model l Bad frames are discarded by frame relay l retransmission is done by layer 4 (transport) l Frame structure l – Flag ( 1 byte) – Data Link Connection ID (2 bytes) l l 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) l 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 l Committed Information Rate (CIR) l – 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 (ATM) l designed for voice, video and data services that require low delay and low jitter (variance in delay) and high speed. l All ATM cells are 53 -octets long l Layer 2