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 y. Make efficient use of shared transmission facilities y. Multiplexing y. Congestion control z Interfacing y. A device must interface with the transmission system z Signal Generation y. Signals must be x. Capable of being propagated through transmission system x. Interpretable as data at the receiver

Key Communications Tasks z Synchronization y. Receiver must know x. When a signal begins to arrive and when it ends x. Duration of a signal z Exchange Management y. Both devices may transmit simultaneously or take turns y. Amount of data sent at one time y. Data format y. What to do if an error occurs

Key Communications Tasks z Error detection and correction y. Required when errors cannot be tolerated z Flow Control y. Assure that source does not overwhelm destination by sending data faster than can be processed z Recovery y. Required when an information exchange is interrupted due to a fault in the system y. Resume activity at point of interruption or y. Restore system state to condition prior to beginning of exchange

Key Communications Tasks z Addressing and routing y A source system must indicate the identity of intended destination y Transmission system maybe a network through which various paths (routes) must be chosen z Message formatting y Both parties must agree on form of data to be exchanged z Security y Sender assured only intended receiver receives data y Receiver assured data not altered in transit and actually came from purported sender z Network Management y Configure system, monitor its status, react to failures and overloads.

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 z Communication networks classified into y. Wide area networks (WANs) y. Local area networks (LANs)

Simplified Network Model

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

Simple Switched Network

Circuit Switching z Dedicated communications path established for the duration of the conversation y e. g. telephone network z The path is a connected sequence of physical links between nodes z Inefficient y Channel capacity dedicated for duration of connection y If no data, capacity wasted z Set up (connection) takes time z Once connected, transfer is transparent z Developed for voice traffic (phone)

Packet Switching z Station breaks long message into packets sent one at a time to the network z Packets pass from node to node between source and destination z Data sent out of sequence z Used for computer to computer communications

Packet Switching z Advantages y. Line efficiency x. Single node to node link can be shared by many packets over time x. Packets queued and transmitted as fast as possible y. Data rate conversion x. Each station connects to the local node at its own speed x. Nodes buffer data if required to equalize rates y. Packets are accepted even when network is busy x. Delivery may slow down z Priorities can be used

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 z Original packet switching networks designed with a data rate of 64 Kbps z Frame relay designed with a data rate of 2 Mbps

Asynchronous Transfer Mode z ATM is evolution of frame relay z Frame relay uses variable length packets called frames z ATM uses fixed length packets called cells z Little overhead for error control z Data rate from 10 Mbps to Gbps z Constant data rate using packet switching technique

Integrated Services Digital Network z ISDN designed to replace public telecom system z Defined by standardization of user interfaces z Implemented as a set of digital switches and paths z Entirely digital domain z Supports voice and non-voice applications z Support for switched and non-switched applications z Reliance on 64 Kbps connections y Basic service: 192 Kbps y Primary service: 1. 544 Mbps and 2. 048 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

LAN Topologies

Protocols & Protocol Architecture z In addition to data path, we need to account for other factors in communication network: y. Source must identify the destination to the network y. Source must make sure that destination is prepared to accept data y. Security must be accounted for; data should go to the intended user on the receiver y. Incompatible file formats may need to be translated z Computer Communication y. Exchange of information between computers for cooperative action

Protocols z Communication must follow some mutually acceptable conventions , referred to as protocol z Set of rules governing the transfer of data between entities z Used for communications between entities in different systems z Communicating entities must speak the same language z Entities: anything capable of sending or receiving information y User applications, e-mail facilities, terminals z Systems: physically distinct object that contains one or more entities y Computer, terminal, remote sensor

Key Elements of a Protocol z Syntax y Data format and size y Signal levels z Semantics y Control information y Error handling y Actions to take in response to reception of different messages z Timing y Speed matching y Sequencing y When to discard a message, retransmit, give up

Protocol Architecture z Protocols can quickly become very complicated (and thus incorrect) z Implement functionality with several protocols z Layering is a popular way of structuring such a family of network protocols z Each layer represents a new level of abstraction with well defined function z Layer N defined in terms of layer N-1 only, providing total interface to layer N+1

Protocol Architecture

Protocol Architecture z Interfaces are primitive objects, operations, services provided by one layer to its higher layers z Task of communication broken up into modules z For example file transfer could use three modules y. File transfer application y. Communication service module y. Network access module

Protocol Architecture z File transfer application y. Transmitting passwords, file commands, file records y. Performat translation if necessary z Communication service module y. Assure that the two computers are active and ready for data transfer y. Keep track of data being exchanged to assure delivery z Network access module y. Interface and interact with the network

Simplified File Transfer Architecture

A Three Layer Model z Communications involve three agents: applications, computers, and networks z File transfer operation: y Application=>Computer=>Network=> Computer=>Application z Communication tasks organized into three layers y. Network access layer y. Transport layer y. 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 such as priority z Dependent on type of network used (LAN, packet switched, circuit switching, etc. ) z Communication software above network access layer need not know type of network

Transport Layer z Data must be exchanged reliably and in same order as sent z Contains mechanisms for reliable data transportation z Independent of network being used z Independent of application z Provides services useful to variety of applications z Sharing of communication resources

Application Layer z Contains logic needed to support various user applications ye. g. e-mail, file transfer z Separate module for each application

Addressing Requirements z Every entity in overall system must have a unique address 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

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, called a transport protocol data unit, has a transport header added y Destination SAP y Sequence number y Error detection code z Network PDU adds network header y network address for destination computer y Facilities requests like priority level

Protocol Data Units (PDU)

Operation of a Protocol Architecture

Protocol Architectures z Two protocol architectures as the basis for development of interoperable communications standards y. TCP/IP protocol suite y. OSI reference model z TCP (Transmission Control Protocol)/IP (Internet Protocol) is the most widely used interoperable architecture z OSI (Open Systems Interconnection) model is the standard model for classifying communications functions

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 Nature of signals z Data rates

Network Access Layer z Exchange of data between end system and network z Destination address provision z Invoking services like priority z Different standards are used for circuit switching, packet switching (X. 25), LANs (Ethernet) z Mainly concerned with access and routing data between two computers in same network

Internet Layer (IP) z Systems may be attached to different networks z Routing functions across multiple networks z Implemented in end systems and routers z Routers connect two networks and relay data from one network to the other

Transport Layer (TCP) z Reliable delivery of data z Ordering of delivery z Most common protocol is the transmission control protocol (TCP)

Application Layer z Contains logic to support various user applications z Separate module for each application ye. g. http, ftp, telnet

TCP/IP Protocol Architecture Model

OSI Model z Open Systems Interconnection z Developed by the International Organization for Standardization (ISO) z A model for computer communications architecture z Framework for developing protocol standards z Seven layers z TCP/IP is the de facto standard

OSI Layers z Application z Presentation z Session z Transport z Network z Data Link z Physical

OSI Layers

OSI Layers z Physical layer: y. Transmits unstructured bit stream over transmission medium y. Mechanical, electrical, functional, and procedural characteristics to access medium z Data link layer: y. Reliable transfer of information across physical layer y. Sends blocks/frames with synchronization, error control, and flow control

OSI Layers z Network layer: y. Separates data transmission and switching technologies from upper levels y. Establishes, maintains, and terminates connections z Transport layer: y. Reliable and transparent transfer of data between end points y. End-to-end error recovery and flow control

OSI Layers z Session layer: y. Control structure for communication between applications y. Establishes, maintains, and terminates sessions between cooperating applications z Presentation layer: y. Makes applications independent from differences in data presentation z Application layer: y. Access to the OSI environment y. Distributed information services

OSI vs. TCP/IP

Standards z Required to allow for interoperability between equipment z Govern physical, electrical, and procedural characteristics of communication 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 z ISO (International Organization for Standardization) z ITU-T (International Telecommunication Union) yformally CCITT (International Telegraph and Telephone Consultative Committee) z ATM forum