Chapter 13 WAN Technologies and Routing

1. Chapter 13 WAN Technologies and Routing

2. 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

3. 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).

4. 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 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.

5. Physical Addressing in A WAN • Many WANs use a hierarchical addressing scheme that makes forwarding more efficient. • Hierarchical address (figure 13.3)is divided into two parts • switch# • port#

6. Routing • 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

7. 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

8. Routing 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. • RIP • link state routing (algorithm 13.1) • aka shortest path first (SPF)(fig 13.9) • OSPF

9. Example 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)

10. ISDN • 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

11. 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.

12. 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.

13. 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 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.

14. 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.

15. Asynchronous Transfer Mode (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 long • Layer 2