1 Questions about networks? What is /what kinds? Purposes? Requirements? Functions? How ? Implementation and usage?

2 Networks and Services Communication Networks Information Services Network Evolution

3 What is a communication network ? Infrastructure consisting of equipment & facilities to transfer information between/among users in different locations (telephony, television, cellular, Internet) Like railroads, highways, & airline infrastructures which transport people & goods across space Communications networks transport information across space, but –at the speed of light and at very high rates The exchange of information enables interaction at a distance nearly instantaneously

4 Communication Networks VS. Transportation Networks Flow of people & goods Flow of information Flexible interconnectivity&various services (both) Roads & highways transmission lines Intersections & interchanges switches/routers Deciding the route (which exit in an interchange) routing Moving along the route forwarding Traffic lights & stop signs flow control Road maintenance, detour, …

5 Networks enable services Transportation systems enable one basic service – the transfer of objects, then (built on it) – postal / passenger / freight service Communications networks typically provide a basic information service – telephony: two-way exchange of voice signals – voice messaging, fax, modem – Internet: transfer of information packets – , web browsing, e-commerce...

6 Typical Network Interactions Gathering/ Concentration – this is the traditional interaction model such as – bill payments / registration – polling, voting, auctions Distribution – electronic newspaper delivery – television & radio broadcasting – mass e- mail.

7 Typical Network Interactions (cont.) Request/ Reply (typical client/server model) – catalog ordering – Web browsing Two & Multi-way Interactive – telephone (& conferencing) – video-on-demand

8 Typical services and Requirements Radio & television broadcasting, telephone service, cellular telephone service, , web-browsing, video-on-demand, audio- video-conferencing Delay, reliability of service, accuracy of transmission, volume of information, security and privacy, other specific requirements.

9 Two Examples of Networks & Services Telephone Networks were developed solely to provide two-way voice communications Internet was developed to provide a basic service (transfer of packets of information) which would provide a platform for developing many services –e- mail – web- browsing – video streaming – many variants of e- commerce

10 The caller picks up the phone triggering the flow of current in wires that connect to the telephone office. The current is detected and a dial tone is transmitted by the telephone office to indicate that it is ready to receive the destination number. The caller sends this number by pushing the keys on the telephone set. Each key generates a pair of tones that specify a number. (In the older phone sets the user dials a number which in turn generates a corresponding number of pulses.) The equipment in the telephone office then uses the telephone network to attempt a connection. If the destination telephone busy, then a busy tone is returned to the caller. If the destination telephone is idle, then ringing signals are sent to both the originating and destination telephones. The ringing signals are discontinued when the destination phone is picked up and communication can then proceed. Either of the users terminate the call by putting down a receiver. Telephone Office 1. Telephone Office 2. Telephone Office Telephone Office Telephone Office Telephone Office Figure 1.1 Telephone set up and communication

11 Requirements for telephone service Two-way communication Real-time, not much delay Reliable connection-oriented Voice quality reduces a little (not exact),OK Others: connection all the time, security&privacy sometime, 800 calls, caller ID, voice mail, call return, etc.

12 service and requirements Real time? Connection-oriented? NO Accuracy? YES Delivery confirmation and security and privacy (sometime) Others: reply, forward, list, etc Given receivers address, input subject and message, then Click send.

13 The user clicks on a link to indicate which document is to be retrieved. The browser must determine the address that contains the document. It does this by sending a query to its local name server. Once the address is known the browser establishes a connection to the specified machine, usually a TCP connection. In order for the connection to be successful, the specified machine must be ready to accept TCP connections. The browser runs a client version of HTTP, which issues a request specifying both the name of the document and the possible document formats it can handle. The machine that contains the requested document runs a server version of HTTP. It reacts to the HTTP request by sending an HTTP response which contains the desired document in the appropriate format. The TCP connection is then closed and the user may view the document Figure 1.4 Retrieving a web page

14 WWW service & requirements Not real time, but can not delay too long Not only text, but also audio, images Dynamic content, links to anywhere Concepts: HTTP protocol (HyperText Transfer Protocol) Client/server model (http client, http server), URL (Uniform Resource Locator): (protocol://host:port/file path and file name)

15 Realplayer example Figure 1.5 Copyright © , RealNetworks, Inc. All rights reserved. RealPlayer is a trademark of RealNetworks, Inc. Streamed audiovisual service: A RealPlayer segment

16 Streamed audiovisual services & requirements On-demand (user asks for a channel as CNN) Interactive is less tense (CNN server sends a stream of information to the user) Image quality is less required Others: stop, pause etc.

17 Network design and evolution Network functions and network topology Telegraph networkmessage switching Telephone networkcircuit switching Computer networks and the Internet--- packet switching

18 Network Design Applications impose requirements on the services provided by a network – delay; reliability; accuracy – volume and rate – cost & convenience, even security. The network design – meets these requirements cost- effectively – must address a common set of functions (what?) – may take fundamentally different approaches

19 Essential Network Functions Basic user service --Voice connections or packet transfer. Terminal – the end system that connects to the network, telephone or computer. Transmission system – the means for transmitting information across a physical medium: copper telephone wires, coaxial TV cable, optical fiber. Information representation – the format of the information handled by the network, voice or bits. Addressing – the means for identifying points of connection to the network: telephone number or IP address

20 Essential Network Functions (cont.) Routing – the means for determining the path across the network Switching/forward approach – the means of transferring information flows between communication lines. Multiplexing – the means for connecting multiple information flows into shared connection lines. Flow & congest control.

21 t0t0 t1t1 Network Figure 1.6 A Network transfers information among users A single block of information or a stream of information.

22 Network Topology: How a Network Grows A network involves the interconnection of transmission lines using switches to convey information among users The growth of a network from a few users in close proximity to a very large community over a wide geographic region leads to a hierarchical network structure

23 Network (a) A switch provides the network to a cluster of users (b) A multiplexer connects two access networks Access network Figure 1.7 n*(n-1)/2 lines n lines

24 Metropolitan network A consists of access subnetworks a, b, c, d. National network consists of regional subnetworks. Metropolitan network A is part of regional subnetwork. A A Metropolitan 1* a c b d (a) (b) Figure 1.8 Hierarchical network topology

25 Approaches to Network Design FunctionTelegraph Network Telephone Network Internet Basic User Service Transmission of telegrams Bi-directional real- time transfer of voice signals Datagram&reliable stream service between computers Switching Approach Message switching Circuit switchingConnectionless packet-switching TerminalTelegraph, Teletype Telephone, modemComputer Information representation Morse, Baudot, ASCII Analog voice or PCM digital voice Any binary information

26 Approaches to Network Design (cont.) FunctionTelegraph Network Telephone Network Internet Transmission System Digital over various media Analog and digital over various media Digital over various media AddressingGeographical addresses Hierarchical numbering plan Hierarchical address space RoutingManual routingRoute selected during call setup Each packet routed independently MultiplexingCharacter and Message multiplexing Circuit multiplexingPacket multiplexing

27 Telegraphy Networks & message switching Samuel B. Morse invented telegraph in 1837 – binary communications using dots & dashes – Morse code maps alphabet to sequences of dots/ dashes – text messages transmitted hop- by- hop across a network of relay stations – Manual routing, routing decision is done by an operator – Operator stores a message, finds next station according to the destination and forwards it, so message switching – Multiplexing, the symbols from several operators into the same communication line. (20bits/sec 120bits/sec)

28 Morse code (See Table 1.1 at page 3 or 15) LetterCodeProb.NumberCodeProb. A B-… C …-- D ….- E ….. ……. T-0.104

29 Telephone networks & circuit switching 1875, Alexander G. Bell invented telephone Voice (analog) signal, telephone terminal is very simple, anybody can use it Switches were introduced because of the vast cost of dedicated lines (recall n*(n-1) lines to n lines) Human operator performs switching, manual routing Connection-oriented, dedicated end-to-end connection (dedicated lines and switches), therefore, circuit switching Routing decision at setting up of the connection

30 (a) A switch in the form of an operator with a patch cord panel (not shown) (b) Cords interconnect user sockets providing end-to-end connection Figure 1.10

31 Advances of telephone networks Digital (0,1) transmission, PCM technique Multiplexing: 20 voice calls, 1.5Mbps Digital switches Hierarchical telephone networks: local/ tandem/toll switches Hierarchical numbering (addressing) system: area code + exchange number + phone number Automatic connection by computer A separate singling network for management Enhanced telephone services:credit-card call, long distance call, 800, caller ID, voice mail, cellular phone (mobility).

32 Tandem CO Toll CO Tandem Figure 1.11 Hierarchical telephone network structure CO: Center Office switch Tandem switch Toll switch

33 Topics discussed (a brief summary) What is a communication network? What is the purpose of it? Provides services An infrastructure to transfer information over space What are the main requirements? Delay, reliability, accuracy What are four typical interactions? Gathering, distribution, request/reply, two& multiple way interaction What does connection-oriented mean? A connection between two parties must be set up ahead of information transmission

34 Topics discussed ( a brief summarycont. ) What are the main network functions? Switching, information representing, addressing, routing, multiplexing What is the generic network topology? Hierarchical network structure What are approaches implementing the functions? In telegraph networks: In telephone networks: Message switching, Morse code, geographical address, manual routing, char&message multiplexing Circuit switching, analog or digital, hierarchical numbering, call setup manually/automatically, circuit multiplexing