Computer Networks: Perspectives, Definitions, and Evaluation Criteria

1. Introduction Computer networks: definition computer networks from the perspectives of users and designers Evaluation criteria Some concepts: packet switching & circuit switching point-to-point networks & broadcast networks multiplexing performance metrics: delay & throughput LAN & WAN

2. Definition : • Something that interconnects computers, allowing computers to exchange digital information (bits). • Hardware or software? • What do computer networks offer? • Information collection / distribution • access to shared computing resources • provide high reliability • …...

3. Computer Networks from the perspectives of network users/programmers and network designers: • Perspective from a user: • What do you care? • What services? Email, FTP, reliable communication, unreliable. • Service quality: How fast? • Service cost • Service interface: send_packet, receive_packet

4. Perspective from a designer: • What do you care? • How to make users happy? • How to achieve that at the lowest price? (how to provide the services effectively.) • Design issues • Design alternatives • Considerations in deciding which alternative is better? • Example: How to interconnect the machines in a network? • Pairwise interconnection (complete graph) • point-to-point networks. • No direct connections between all pairs of nodes. Messages must pass a number of intermediate nodes to reach the destination. E.g. Tree, Mesh.

5. point-to-point networks. • No direct connections between all pairs of nodes. Messages must pass a number of intermediate nodes to reach their destinations. E.g. Tree, Mesh. • How to do this? • Packet switching and circuit switching • Packet switching: • Messages are cut into packets • When an intermediate node receives a packet, it determines the next node and forward the packet (store-and-forward networks). • Circuit switching networks: • set up a circuit before transmission • all data from one session (connection) follow the same circuit.

6. Broadcast networks. • Each node can send messages to all nodes in the system. E.g. bus connection. • Contention problem: two nodes may want to send messages simultaneously. • Resolve the contention: multiplexing. • Time division multiplexing: divide time domain into time slots, different nodes use different time slots to transfer. • Design choice # 1: static assignment • A uses slot 1, B uses slot 2, … • Design choice # 2: dynamic assignment • dynamically allocate slots to communication channels. • Need a mechanism to allocate channels, both ends must agree a slot for communication • How to evaluate these design choices?

7. Evaluation criteria • Application requirements: • functionality: Can I do it? • performance: How fast I can send data? • Cost constraints: the cost of the network has to be within certain budget. • Application performance requirements: • delay: time from the data are transmitted to the time the data are received. • Propagation delay + queueing delay + transmission delay • propagation delay = distance / signal propagation speed speed of light (in vacuum): 3.0*10^8 m/s = 300km/ms propagation delay across continental USA (NY to LA): 3000 miles = 4800km, delay = ??? Geosynchronous Satellite: 36000km, delay = ?

8. Application performance requirements: • delay: time from the data are transmitted to the time the data are received. • Propagation delay + queueing delay + transmission delay • propagation delay = distance / signal propagation speed • queueing delay: depend on the network load • transmission delay: how fast can you put bits on the wire? • E.g. Ethernet 10Mb/s (bandwidth) • 500KB message, transmission delay = ? • Throughput: amount of data transmitted / time • High throughput = low delay? • E.g: transfer 10Kb data using 10Mb/s link, delay = ? • Using 1Gb/s link, delay = ? • Conclusion?

9. Applying the criteria, some case studies: • Case 1: topology choices for local area network(LAN) and wide area network (WAN). • LAN: computer networks that cover a small area (a single building up to a few kilometers) with a small number of computers. • Topology choices: all directly connected, point to point and broadcast? • WAN: computer networks that cover a large area (continent) with a lot of computers. • Topology choices: all directly connected, point to point and broadcast?

10. Case 2: the switching technique for telephone networks and computer networks application traffic throughput delay tele. Interactive continuous 64kbps <200ms voice smooth computer rlogin bursty varied varied web ftp telephone networks: circuit switching or packet switching? Circuit switching + TDM Computer networks: circuit switching + TDM? Integrated Service Digital Network (ISDN): supporting both telephone traffic and computer traffic. ATM.

11. Case 3: Network for Military use • military concerns: survivability • I still need to hear your voice even when most of the network components were gone. • Idea: each packet can find its way through the network, end system recover from the lost packets (due to the damaged links or congestion). • How ARPANET works: • End system chops messages into packets, each packet has a header with destination address. • Each packet finds its way to the destination independently. • End system put together the message. • Why not circuit switching + TDM?