What Youll Learn Today What is a communications network? What are - - PDF document

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Aaron Stevens

31 January 2011

CS101 Lecture 5: Networking and Computer Networks

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What You’ll Learn Today

– 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?

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Communications Networks

– What is a network? What are its defining characteristics?

The Telephone Network

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.

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POTS Circuit Switching

Telephone wires leave your house, and connect to the central office. At the central office, connections are made to other telephone lines…

Image from www.exegesis.uklinux. net.

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POTS Circuit Switching

Telephone operators used to actually switch wires to connect the calls. In the PSTN, this connection (called switching) is done electronically.

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Modem

Early computer networking used the phone network: dial up. A modem converts computer signals into sounds.

Circuit Switched Network

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.

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Circuit Switching: Details

– 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

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!

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Packet Switching

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.

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Nodes send packets of data along routes to a destination, without a dedicated circuit. Packets (even in the same transmission) can take different routes.

Image from http://www.teach-ict.com/technology_explained/packet_switching/packet.switching.gif

Packet Switching Network

Image from www.tcpipguide.com.

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Packet Switching: Details

– What happens to packets when they arrive? – Are they ready for consumption? – What happens if some packet(s) get lost?

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What’s So Great about packet switching?

–Efficient use of wires/circuits –Multiple paths between source and destination –Slow growth of network infrastructure as number of users increases

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What’s Not To Love about Packet Switching?

–Time required to reassembly messages, resend missing packets.

• Why does this matter?

–Speed of delivery: latency is not guaranteed. –Inefficient for small packets

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Circuit or Packet Switching?

– 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?

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Network Interface Controller

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.

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Types of Networks

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.

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LAN/WAN Example

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

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What You Learned Today

– Circuit Switching – Packet Switching – Network Topology – How Your Computer Connects to the Internet

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Announcements and To-Do List

– Readings this week:

• Reed ch 3, pp 44-50 (today)
• Watch: history of Internet video

– Start at time 19:45, to end

• Reed ch 3, pp. 51-53 (Wednesday)

– HW02 (HTML) due Wednesday 2/2 – Quiz 1 on Friday 2/4

• Covers lectures 1-5