CS408/533 Computer Networks Text: William Stallings Data and Computer Communications, 6th edition Chapter 1 - Introduction

A Communications Model Source generates data to be transmitted Transmitter Converts data into transmittable EM signals Transmission System Carries data Receiver Converts received signal into data Destination Takes incoming data

Simplified Communications Model - Diagram

Key Communications Tasks (also an overview of the course) Transmission System Utilization - efficient use of resources (e.g. Multiplexing, Congestion Control) Interfacing and Signal Generation - hook up and generate transmittable and interpretable signals Synchronization - to know beginning and end of data Error detection and correction Flow control - do not overwhelm the destination Addressing and routing - destination identification and path selection Security - only the intended recipient must read Network Management – Complex System, need to be managed (planning, configuration, status monitoring)

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

Simplified Network Models

Wide Area Networks Large geographical area Resemble “highways” interconnected switching nodes Data are switched from one node to another towards the destination These nodes are not interested in the data Alternative technologies Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)

Circuit Switching Dedicated communications path established for the duration of the connection Channels are pre-allocated Minimal (if not none) delay at each node e.g. telephone network

Packet Switching No dedicated channels Data sent out in a sequence Small chunks (packets) of data at a time Packets passed from node to node between source and destination Packets are received stored briefly (necessary for packet processing) forwarded to the next node

Frame Relay Packet switching systems have large overheads to compensate for errors Modern transmission systems are more reliable The ideas behind frame relay Errors can be caught at the end points Let’s take the advantage of high data rates and low error rates Most overhead for error control is stripped out Data Rate 2 Mbps as compared to 64Kbps of X.25 (packet switching)

Asynchronous Transfer Mode ATM Evolution from frame relay Little overhead (even less than frame relay) for error control higher layers of end users are responsible for error check Fixed packet (called cell) length 48 bytes of data + 5 bytes of header Anything from 10Mbps to Gbps range

ISDN Integrated Services Digital Network Voice, Data and other traffic has a single interface network devices will understand the required service and process accordingly Designed to replace Public Telephone Network could not do it Narrowband ISDN (or just ISDN) Broadband ISDN (B-ISDN)

Local Area Networks Smaller scope Building or small campus Usually owned by same organization as attached devices requires set up and maintenance Data rates much higher than WANs Usually broadcast systems Now some switched systems and ATM are being introduced

Protocols Cooperative action is necessary computer communications is not only to exchange bytes huge system with several utilities and functions. For examples error detection encryption For proper communication, entities in different systems must speak the same language Mutually acceptable conventions what is communicated? when is communicated? how is communicated? Those conventions and associated rules are referred as “PROTOCOLS”

Protocol Architecture Task of communication broken up into modules For example file transfer could use three modules File transfer application Communication service module Network access module

Simplified File Transfer Architecture File Transfer Application Layer: Application specific commands, passwords and the file(s) transmission – high level data Communications Service Module: reliable transfer of those data – error detection, ordered delivery of data packets, etc. Network Module: actual transfer of data and dealing with the network – if the network changes, only this module is affected, not the whole system

A General Three Layer Model Generalize the previous example for a generic application we can have different applications (e-mail, file transfer, …) Network Access Layer Transport Layer Application Layer

Network Access Layer Exchange of data between the computer and the network Sending computer provides address of destination so that network can route Software is dependent on type of network used (LAN, packet switched etc.) Independent of upper layers

Transport Layer Reliable data exchange To make sure that all the data packets arrived in the same order in which they are sent out Independent of network being used Independent of application

Application Layer Support for different user applications e.g. e-mail, file transfer

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

Protocol Architectures and Networks X Z Y

Protocol Data Units (PDU) User data is passed from layer to layer Control information is added/removed to/from user data at each layer header each layer has a different header Data + header = PDU (Protocol Data Unit) each layer has a different PDU

Transport PDU Transport layer may fragment user data Each fragment has a transport header added Destination SAP Sequence number Error detection code This is transport protocol data unit

Network PDU Adds network header network address for destination computer optional facilities from network (e.g. priority level)

Operation of a Protocol Architecture