Overview/Questions What is a communications network? For example, - PDF document

CS101 Lecture 5: Networking: Topology, Packet Switching John Magee 8 July 2013 1 Overview/Questions – What is a communications network?  For example, the phone network – Why connect computers together? – How do computers connect to each other? – What are the typical models for creating computer networks? – How is data transferred across a network? 2 1

The Telephone Network 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. POTS Circuit Switching 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… 4 2

POTS Circuit Switching Telephone operators used to actually switch wires to connect the calls. In the PSTN, this connection (called switching) is done electronically. 5 POTS Today 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) 6 3

Computer Networking 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? 7 Clients and Servers Computer networks operate in a client/server model. Examples: – Browser / web server – File transfer client / FTP server – What about Peer-to-Peer? 8 4

Networking Terms 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. 9 OSI Model Open Systems Interconnection Layered network protocol design OSI Layers Examples 10 5

Network Nodes 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) 11 Modem Early computer networking used the phone network: dial up. A modem converts computer signals into sounds. 12 6

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. 13 Network Topologies Local-area Network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area. Network Topology Describes the physical wiring plan for connections between nodes on a network. Also describes how messages are sent between nodes. 14 7

Ring Network Topology 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 Star Network Topology 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

Bus Network Topology On a bus network nodes are connected to a single communication line that carries messages in both directions. How many hops? Complexity? Ethernet The industry standard bus technology for local-area networks. Image source: http://www.edrawsoft.com/Network-Topologies.php Ethernet 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) 18 9

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. 19 WAN Example LANs separated by a great distance are connected by High speed communication links to create a WAN. 20 Image source: http://www.air-stream.org.au/files/wide_area_network.gif 10

Circuit Switched Network Computers can be connected over a circuit switched network (e.g. phone lines), creating a circuit between the source and destination. Image from A Switched Circuit connects devices A and B. www.tcpipguide.com. Network Core: Circuit Switching network resources (e.g.,  dividing link bandwidth bandwidth) divided into “pieces” 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

Circuit Switching: FDM and TDM Example: FDM 4 users frequency time TDM frequency time Introduction 1-23 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? 24 12

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! 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, then reassembled into messages. Router A network device that directs a packet between networks toward its final destination. 13

Packet Switching Network Nodes send packets of data along routes to a destination, without a dedicated circuit. Image from www.tcpipguide.com. Packets (even in the same transmission) can take different routes. 27 Image from http://www.teach-ict.com/technology_explained/packet_switching/packet.switching.gif Network Core: Packet Switching resource contention: each end-end data stream divided into packets  aggregate resource 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 1-28 14

Packet Switching: Statistical Multiplexing 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 1-29 Packet-switching: store-and-forward L R R R  takes L/R seconds to Example: transmit (push out)  L = 7.5 Mbits packet of L bits on to  R = 1.5 Mbps link at R bps  transmission delay = 15  store and forward: entire sec packet must arrive at router before it can be transmitted on next link  delay = 3L/R (assuming zero propagation delay) more on delay shortly … Introduction 1-30 15

Packet switching versus circuit switching 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 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 Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Introduction 1-32 16

Packet Switching: Details – What happens to packets when they arrive? Are they ready for consumption? – What happens if some packet(s) get lost? 33 Advantages of Packet Switching – Efficient use of wires/circuits – Multiple paths between source and destination – Slow growth of network infrastructure as number of customers increases 34 17

Disadvantages of 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. 35 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? – What current events topic relates to this? 36 18