1. Wide Area Networks School of Business Eastern Illinois University © Abdou Illia, Spring 2007 (Week 11, Thursday 3/22/2007)

2. 2 Learning Objectives n Distinguish between LAN and WAN n Distinguish between – Circuit Switching Networks – Datagram Packet Switching Networks – Virtual Packet Switching Networks n Understand routing decisions

3. 3 LAN and WAN n LAN = a communication network that interconnects networking devices within a small geographic area using broadcast system. n WAN = a communication network that interconnects networks and networking devices within a wide geographic area using Point-to-Point system. n A Point-to-Point system: – Many connections between individual pairs of stations – A packet may have to visit one or more intermediate stations – Multiple routes are possible; so routing decisions have to be made Q: What are the main differences between a LAN and a WAN?

4. 4 Wide Area Networks basics n WANs follows a mesh topology n Only neighbor devices are connected to each other n To transmit across the mesh, data has to be passed along a route from node to node Subnet LAN

5. 5 Wide Area Networks basics n A station is a device that interfaces a user to a network n A station could be: – a computer (for Data networks) – a telephone (For Voice networks)

6. 6 Wide Area Networks basics n Data is transferred from node to node through the network n A Node is a transfer point for passing data through the network n A Node is often a computer, a router, or a telephone switch Q: What is the difference between a station and a node?

7. 7 Wide Area Networks basics n The subnet is the underlying physical connection of nodes and communication lines that transfer data from one location to another. n A Subnet is a collection of nodes and different types of transmission media Q: How does the subnet differ from the network?

8. 8 Types of Subnets n Based on the way data is transferred from one end of the subnet to the other: – Circuit Switching Subnet – Packet Switching Subnet

9. 9 Circuit Switching Subnet n Traditionally used for Voice networks n A subnet in which a dedicated circuit is established between sender and receiver and all data passes over this circuit.

10. 10 Reserved Capacity (Circuit Switching) – Circuit capacity is reserved during duration of each communication – At each switch – On each trunk line Circuit Reserved Capacity Reserved Capacity

11. 11 Reserved Capacity n Nothing like congestion on the Internet n Reserved Circuit Capacity is Expensive – Pay for it whether you use it or not – Good for voice, because conversations are fairly constant – Bad for data, because most data transmission is bursty; e.g., in World Wide Web, download, then stare at screen for a long time until next download Q: What are the main characteristics of Circuit Switching networks?

12. 12 Packet Switching Subnet n Usually used for Data networks n A subnet in which all data messages are transmitted using fixed-sized packages, called packets. n Two types of Packet Switching Subnets: – Datagram Packet Switching Subnets – Virtual-Circuit Packet Switching Subnets

13. 13 Datagram Packet Switching Subnet n In all types of Packet Switching Subnets, large messages are broken into small pieces called packets – Packets are short (averaging a few hundred bytes) because most nodes (routers) handle short messages more efficiently Multiplexing on Trunk Line Packet Switching n Multiplexing – Packets from many conversations are mixed (multiplexed) over each trunk line Message Packets

14. 14 Datagram Packet Switching Subnets n As each packet arrives at a node a routing decision is made: – Which route the packet will follow next ? n This dynamic routing decision allows flexibility should the network experience congestion or failure n But, a node has to examine each individual packet and determine the next path Router A B? D? C? B C D Packet

15. 15 Virtual-Circuit Packet Switching Subnets n Amount of data broken into n packets n A virtual circuit (temporary path through the network) is determined n Note: The virtual circuit is not dedicated (not reserved) n All n packets transmitted through the virtual circuit n When transfer completed, virtual circuit dissolved Q:

16. 16 Routing decisions n Many possible routes exist for forwarding a packet. n How does a router decide which line to transmit on?

17. 17 Routing decisions n A subnet can be viewed as weighted network graph n A weight is associated to each line n Weight can be: – Cost of using the transmission line – Time delay for transmitting data – Size of the queue

18. 18 Routing decisions n Different algorithms (or techniques) for selecting a route through a network n Common algorithms/techniques: – Dijkstra’s least cost algorithm – Flooding technique n Dijkstra’s least cost algorithm – Calculate the costs for using each route for a given node – Determine the route that minimize the sum of the costs – Calculation performed on a periodic basis or when changes happened (connection or node failure for example)

19. 19 Routing decisions n Flooding Routing – When a packet arrives at a node, the node sends a copy of the packet out every link except the link the packet arrived on – Traffic grows very quickly when every node floods the packet – To limit uncontrolled growth, each packet has a hop count. Every time a packet hops, its hop count is incremented. When a packet’s hop count equals a global hop limit, the packet is discarded

20. 20 Summary Questions n How does the subnet differ from the network? Answer: The network include: the subnet(s), the stations, the OS & other application software, and the other networking devices & transmission medium needed to connect the stations to the subnet n What is the difference between a station and a node? Answer: A station is the device that interfaces a user to the network. A node is a transfer point for passing data through the network. A node can be a computer, a router, or a telephone switch. n What are the main characteristics of Circuit Switching networks? Answer: In Circuit Switching networks: (1) a dedicated circuit is established between sender and receiver, (2) circuit capacity is reserved during the duration of each communication, at each node (switch) and on each transmission line; (3) no routing decisions are necessary since circuit is dedicated.

21. 21 Summary Questions n What are the main characteristics of Virtual-Circuit Packet Switching networks? Answer: (1) Data sent in packets, (2) all packets follow the same virtual circuit, (3) the virtual circuit may be shared with packets from other conversions, (4) no routing decisions except the first one that creates the circuit. n Name some criteria that routing decisions are based on Answer: see slide #17 n Name two routing algorithms/techniques. Answer: see slide #18

22. 22 Bridge versus Router Layers Bridge2-layer device Role Interconnecting LANs or segments of same LAN Router3-layer device -Interconnecting LANs to WANs, and nodes in WANs -Making routing decisions Ethernet Token Ring Transparent bridge Source-routing bridge