William Stallings Data and Computer Communications Chapter 1

William Stallings Data and Computer Communications Chapter 1 Introduction

A Communications Model z Source ygenerates data to be transmitted z Transmitter y. Converts data into transmittable signals z Transmission System y. Carries data z Receiver y. Converts received signal into data z Destination y. Takes incoming data

Simplified Communications Model - Diagram

Key Communications Tasks z Transmission System Utilization z Interfacing x. Data rates, duration, and spacing z Signal Generation z Synchronization x. Synchronous or Asynchronous transfers z Exchange Management x. Cooperation between sender and receiver z Error detection and correction x. Due to signal distortion

Key Communications Tasks z Addressing and routing x. How to deliver data to another entity outside my network z Recovery x. In case of any failure? How do communicating entities recover z Message formatting x. What is the format of the message to be understandable by the receiver z Security z Network Management x. Status, overloads, failures, future growth

Simplified Data Communications Model

Networking z Point to point communication not usually practical y. Devices are too far apart y. Large set of devices would need impractical number of connections z Solution is a communications network

Simplified Network Model

Internet model router server workstation mobile local ISP regional ISP company network

Wide Area Networks z Large geographical area z Crossing public rights of way z Rely in part on common carrier circuits z Alternative technologies y. Circuit switching y. Packet switching y. Frame relay y. Asynchronous Transfer Mode (ATM)

Circuit Switching z Dedicated communications path established for the duration of the conversation z e. g. telephone network xnetwork resources (e. g. , bandwidth) divided into “pieces” xpieces allocated to calls xresource piece idle if not used by owning call (no sharing)

Packet Switching z Data sent out of sequence z Small chunks (packets) of data at a time z Packets passed from node to node between source and destination z Used for terminal to computer and computer to computer communications each end-end data stream divided into packets z Two packets share network resources z each packet uses full link bandwidth z resources used as needed

Packet switching: Internet case A statistical multiplexing B C 1. 5 Mbs queue of packets waiting for output link D E Sequence of A & B packets does not have fixed pattern statistical multiplexing.

Frame Relay z Packet switching systems have large overheads to compensate for errors z Modern systems are more reliable z Errors can be caught in end system z Most overhead for error control is stripped out y. Packet switching 64 kbps y. Frame Relay 2 Mbps

Asynchronous Transfer Mode z ATM z Evolution of frame relay z Little overhead for error control z Fixed packet (called cell) length z Anything from 10 Mbps to Gbps z Constant data rate using packet switching technique

Integrated Services Digital Network z ISDN z Designed to replace public telecom system y. Digital switches z Wide variety of services y Supports a broad range of traffic type z Entirely digital domain y. Narrowband ISDN 64 kbps y. Broadband ISDN 100 s of Mbps

Local Area Networks z Smaller scope y. Building or small campus z Usually owned by same organization as attached devices z Data rates much higher z Usually broadcast systems z Now some switched systems and ATM are being introduced

Computer communication z Information exchanged between two computers for the purpose of cooperative actions is referred to as computer communications. z Two computers are interconnected y. Computer Networks

Protocols z Used for communications between entities in a system z Must speak the same language z Entities y. User applications ye-mail facilities yterminals z Systems y. Computer y. Terminal y. Remote sensor

Protocols Hi Hi Got the time? 2: 00 time

Key Elements of a Protocol z Syntax y. Data formats y. Signal levels z Semantics y. Control information y. Error handling z Timing y. Speed matching y. Sequencing

Protocol Architecture z Task of communication broken up into modules z For example file transfer could use three modules y. File transfer application x. File password, file command, file records. y. Communication service module x. Make sure that the file data is sent in reliable fashion y. Network access module x. Responsible for sending data

Simplified File Transfer Architecture

A Three Layer Model z Network Access Layer z Transport Layer z Application Layer

Network Access Layer z Exchange of data between the computer and the network z Sending computer provides address of destination z May invoke levels of service z Dependent on type of network used (LAN, packet switched etc. )

Transport Layer z Reliable data exchange z Independent of network being used y. Passes the data to the network, regardless of its type z Independent of application y. Data can be a web page, an email, the transport layer does not know the difference

Application Layer z Support for different user applications z e. g. e-mail, file transfer, web page, etc. .

Addressing Requirements z Two levels of addressing required z Each computer needs unique network address z Each application on a (multi-tasking) computer needs a unique address within the computer y. The service access point or SAP

Protocol Architectures and Networks

Protocols in Simplified Architecture

Protocol Data Units (PDU) z At each layer, protocols are used to communicate z Control information is added to user data at each layer z Transport layer may fragment user data z Each fragment has a transport header added y. Destination SAP y. Sequence number y. Error detection code z This gives a transport protocol data unit

Network PDU z Adds network header ynetwork address for destination computer y. Facilities requests

Operation of a Protocol Architecture

TCP/IP Protocol Architecture z Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) z Used by the global Internet z No official model but a working one. y. Application layer y. Host to host or transport layer y. Internet layer y. Network access layer y. Physical layer

Physical Layer z Physical interface between data transmission device (e. g. computer) and transmission medium or network z Characteristics of transmission medium z Signal levels z Data rates z etc.

Network Access Layer z Exchange of data between end system and network z Destination address provision z Invoking services like priority

Internet Layer (IP) z Systems may be attached to different networks z Routing functions across multiple networks z Implemented in end systems and routers

Transport Layer (TCP) z Reliable delivery of data z Ordering of delivery

Application Layer z Support for user applications z e. g. http, SMPT

TCP/IP Protocol Architecture Model

OSI Model z Open Systems Interconnection z Developed by the International Organization for Standardization (ISO) z Seven layers z A theoretical system delivered too late! z TCP/IP is the de facto standard

OSI Layers z Application x. Access to user + distributed services z Presentation x. Independence from application processes, data same format z Session x. Responsible of establishing, managing, terminating sessions between two communicating applications. z Transport x. Reliable transport, end-to-end error recovery, flow control z Network x. Provides switching capabilities to connect between system x. Responsible for establishing, maintaining, terminating connections z Data Link x Responsible of sending packets, synchronizing, error control, flow control z Physical x Communicating unstructured bit stream, mechanical, electrical, physical access

OSI v TCP/IP

Standards z Required to allow for interoperability between equipment z Advantages y. Ensures a large market for equipment and software y. Allows products from different vendors to communicate z Disadvantages y. Freeze technology y. May be multiple standards for the same thing

Standards Organizations z Internet Society y. Internet, x. IETF (Internet Engineering Task Force) x. IAB (Internet Active Board) x. IESG (Internet Engineering steering Group) x. RFC z ISO (international Organization for Standardization) z ITU-T International communication Union (formally CCITT) z ATM forum