Networking Circuit Switching Packet Switching Aaron Stevens - PDF document

9/27/2012 CS101 Lecture 07: Networking Circuit Switching Packet Switching Aaron Stevens (azs@bu.edu) 27 September 2012 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

9/27/2012 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 (Public Switched Telephone Network) 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

9/27/2012 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

9/27/2012 POTS/PSTN Today Computer Science Modern telephone systems are hybrid. The audio is digitized at the exchange, then converted back to analog at the receiving exchange. A Nortel switch, which servers tens of thousands of customers. (Image from Wikimedia Commons) 7 Modem Computer Science Early computer networking used the phone network: dial up. A modem converts computer signals into sounds. 4

9/27/2012 Computer Networking Computer Science Computer Network A collection of computing devices connected in order to communicate and share resources. Connections between computing devices can be physical using wires or cables, or wireless using radio waves or infrared signals. Why connect computers together? 9 Clients and Servers Computer Science Computer networks often operate in a client/server model. Examples:  Browser / web server  File transfer client / FTP server  What about Peer-to-Peer? 10 5

9/27/2012 Networking Terms Computer Science Client A computer which uses (consumes) resources from the network. Server A computer that stores and manages files or applications for multiple users on a network. Example: Web Server A computer dedicated to responding to requests (from the browser client) for web pages. 11 Network Nodes Computer Science A network node is any device on a network:  Computer (desktop, laptop, PDA, etc.)  Server (web server, mail server, etc. )  Router (device which directs traffic)  Firewall (access control device) 12 6

9/27/2012 Network Topologies Computer Science Local-area Network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area. LAN Network Topology Describes the physical wiring plan for connections between nodes on a network. Also describes how messages are sent between nodes. 13 Ring Network Topology Computer Science A ring network connects all nodes in a closed loop on which messages travel in one direction.  Each node has two neighbors How many hops? Complexity? Image source: http://www.edrawsoft.com/Network-Topologies.php 7

9/27/2012 Bus Network Topology Computer Science On a bus network nodes are connected to a single communication line that carries messages in both directions. How many hops? Complexity? Image source: http://www.edrawsoft.com/Network-Topologies.php Star Network Topology Computer Science A star network centers around one node to which all others are connected and through which all messages are sent. How many hops? Complexity? Image source: http://www.edrawsoft.com/Network-Topologies.php 8

9/27/2012 Ethernet Computer Science Where does the name come from? CSMA/CD – Carrier Sense Multiple Access/Collision Detection  How do you have a conversation in a large group? Originally used Bus topology (10Base-2, Coax)  Advantages/Disadvantages? Moved to Star topology (10Base-T, 100Base-TX; UTP) 17 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. 9

9/27/2012 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. 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 10

9/27/2012 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. Network Core: Circuit Switching Computer Science network resources (e.g.,  dividing link bandwidth into “pieces” bandwidth) divided into “pieces”  frequency division  time division  pieces allocated to calls  resource piece idle if not used by owning call (no sharing) Introduction 1-22 11

9/27/2012 Circuit Switching: FDM and TDM Computer Science Example: FDM 4 users frequency time TDM frequency time Introduction 1-23 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? 12

9/27/2012 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! 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. 13

9/27/2012 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 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? 14

9/27/2012 Network Core: Packet Switching Computer Science resource contention: each end-end data stream  aggregate resource divided into packets demand can exceed  user A, B packets share amount available network resources  congestion: packets  each packet uses full link queue, wait for link use bandwidth  store and forward:  resources used as needed packets move one hop at a time Bandwidth division into “pieces”  node receives complete Dedicated allocation packet before forwarding Resource reservation Introduction 1-29 Packet Switching: Statistical Multiplexing Computer Science 100 Mb/s C A Ethernet statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link D E  sequence of A & B packets has no fixed timing pattern  bandwidth shared on demand: statistical multiplexing .  TDM: each host gets same slot in revolving TDM frame. Introduction 1-30 15

9/27/2012 Packet switching versus circuit switching Computer Science Packet switching allows more users to use network! Example:  1 Mb/s link N  each user: users 1 Mbps link • 100 kb/s when “active” • active 10% of time  circuit-switching:  10 users Don’t mix up…  packet switching: Mb/s vs. MB/s  with 35 users, probability > Bits vs. Bytes 10 active at same time is less than .0004 Introduction 1-31 Packet switching versus circuit switching Computer Science Is packet switching a “slam dunk winner?”  great for bursty data  resource sharing  simpler, no call setup  excessive congestion: packet delay and loss  protocols needed for reliable data transfer, congestion control  Q: How to provide circuit-like behavior?  bandwidth guarantees needed for audio/video apps  still an unsolved problem (can’t do it!) Introduction 1-32 16

9/27/2012 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?  What current events topic relates to this? 33 Packet Switching Computer Science 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 17