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

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Computer Science

CS101 Lecture 17: Networking

Circuit Switching Packet Switching

Aaron Stevens (azs@bu.edu)

4 March 2013

Computer Science

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?

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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.

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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?

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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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Packet Switching

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

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What’s Not To Love?

• 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 Computer Science

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
• 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