CS101 Lecture 17: Networking Circuit Switching Packet Switching - PDF document

3/4/13 CS101 Lecture 17: Networking Circuit Switching Packet Switching Aaron Stevens (azs@bu.edu) 4 March 2013 Computer Science What You’ll Learn Today Computer Science  What is a communications network?  What are the implications of the network topology and transmission strategy?  How do computers connect to each other?  What are the building blocks of the Internet?  How is data transferred across a network?  How does my computer connect to the Internet? 1

3/4/13 Communications Networks Computer Science  What is a network?  What are its defining characteristics? The Telephone Network Computer Science POTS (the plain old telephone system), a.k.a PSTN To connect a phone call, the caller ’ s phone must be physically connected to connect to the receiver ’ s phone. Connecting these circuits (called switching) takes place at dedicated facilities called central offices. 2

3/4/13 POTS Circuit Switching Computer Science Telephone wires leave your house, and connect to the central office. Image from www.exegesis.uklinux. net. At the central office, connections are made to other telephone lines… POTS Circuit Switching Computer Science Telephone operators used to actually switch wires to connect the calls. In the PSTN, this connection (called switching) is done electronically. 3

3/4/13 Modem Computer Science Early computer networking used the phone network: dial up. A modem converts computer signals into sounds. Circuit Switched Network Computer Science Computers can be connected over a circuit switched network (e.g. phone lines), creating a circuit between the source and destination. A Switched Circuit connects devices A and B. Image from www.tcpipguide.com. 4

3/4/13 Circuit Switching: Details Computer Science  How many circuits do you need to connect 1 pair of computers? 2 pairs? N pairs?  What happens when a backhoe cuts the line? Bursty Data Transmissions Computer Science Computers send data in irregular bursts. Example: email. Using a switched network connection is wasteful – the circuits must be connected even if no data is being sent! 5

3/4/13 Packet Switching Computer Science Packet A unit of data sent across a network. Packet switching Messages are divided into fixed-sized, numbered packets. Packets are individually routed to their destination. Reassembled into messages. Router A network device that directs a packet between networks toward its final destination. Packet Switching Network Computer Science Nodes send packets of data along routes to a destination, without a dedicated circuit. Packets (even in the same transmission) Image from www.tcpipguide.com. can take different routes. Image from http://www.teach-ict.com/technology_explained/packet_switching/packet.switching.gif 6

3/4/13 Packet Switching: Details Computer Science  What happens to packets when they arrive?  Are they ready for consumption?  What happens if some packet(s) get lost? Packet Switching Computer Science What’s so great about packet switching?  Efficient use of wires/circuits  Multiple paths between source and destination  Do not need as much infrastructure as number of users increases 7

3/4/13 What’s Not To Love? Computer Science  Time required to reassembly messages, resend missing packets.  Why does this matter?  Speed of delivery: latency is not guaranteed.  Inefficient for small packets Circuit or Packet Switching? Computer Science  For which kind of information is circuit switching preferred?  For which kinds of information is packet switching preferred?  Are all packets treated the same by the network? Should some be prioritized? 8

3/4/13 Network Interface Controller Computer Science Each node has a network interface controller (NIC) connected to its circuit board. The CPU treats the NIC as an input/output device. It communicates by reading or writing bytes of data to the NIC. Each NIC has a unique Media Access Control (MAC) address, which distinguishes it from all other NICs. Types of Networks Computer Science Local-area Network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area. Wide-area network (WAN) A network that connects local-area networks over a potentially large geographic distance. Gateway/Router A particular computer on a LAN which directs all communication going between that LAN and other networks. 9

3/4/13 LAN/WAN Example Computer Science LANs separated by a great distance are connected by High speed communication links to create a WAN. Image source: http://www.air-stream.org.au/files/wide_area_network.gif What You Learned Today Computer Science  Circuit Switching  Packet Switching  Network Topology  How Your Computer Connects to the Internet 10

3/4/13 Announcements and To-Do List Computer Science  Readings this week:  Reed ch 3, pp 44-50 (today)  Watch: history of Internet video  linked to class schedule page  Start at time 19:45, to end (about 40 minutes)  Reed ch 3, pp. 51-53 (Wednesday))  HW07 due WED 3/6  QUIZ 3 will be on Friday 3/8  HTML forms, CSS, frames, images, audio 11