CHAPTER 1

Data Communications Systems Overview

INTRO

1.1 A COMMUNICATIONS MODEL

Source generates data to be transmitted

Transmitter (eg. Modem: digitalanalog signal) Converts data into transmittable signals

Transmission System (eg. Telephone line) Carries data

Receiver Converts received signal into data

Destination Takes incoming data

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SIMPLIFIED COMMUNICATIONS MODEL - DIAGRAM

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SIMPLIFIED DATA COMMUNICATIONS MODEL

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key-in message

e-mail

char. string

sequence of bits

modem LAN,Tel. line

1.2 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

Wide Area Network (WAN) Local Area Network (LAN)

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WIDE AREA NETWORKS Large geographical area Crossing public rights of way Rely in part on common carrier circuits Alternative technologies

Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)

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CIRCUIT SWITCHING

Dedicated communications path established for the duration of the conversation Data transmitted as rapid as possible Route to outgoing channel without delay

e.g. telephone network

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A

B

C

D

F

E

G

I II III

PACKET SWITCHING Data senLonger t out of sequence

transmission – broken up into multiple packet Stored briefly then routed according to info. in header

Small chunks (packets) of data at a time Variable length block Contains data+ header with control info. (priority

codes, source & destination add.) Packets passed from node to node between

source and destination Used for terminal to computer and computer to

computer communications8

FRAME RELAY Packet switching systems have large overheads

to compensate for errors Variable length block

Stored briefly then routed according to info. in header Modern systems are more reliable Errors can be caught in end system Most overhead for error control is stripped out Advantage : high data rates and low error rates

Packet Switching network : 64 kbps Frame Relay network : up to 2 Mbps

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ASYNCHRONOUS TRANSFER MODE (ATM) ATM @ cell relay Evolution of frame relay Little overhead for error control Fixed packet (called cell) length Anything from 10Mbps to Gbps Constant data rate using packet switching

technique Stored briefly then routed according to info. in

header Frame relay vs ATM

Frame relay : variable-length packets called frames

ATM : fixed-length packets called cells 10

ATM LAN

LOCAL AREA NETWORKS (LAN) Smaller scope

Building or small campus Usually owned by same organization as

attached devices Data rates much higher Usually broadcast systems

Transmission from one station is broadcast to and received by all other stations

Only one station at a time can transmit a packet

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LAN CONFIGURATIONS Switched

Switched EthernetMay be single or multiple switchesConsist a switch with a no. of attached device(s) or no. of interconnected switches

ATM LAN use an ATM network in LAN

Fibre Channel Wireless

Mobility Ease of installation

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METROPOLITAN AREA NETWORKS MAN- designed to extend over an entire city Middle ground between LAN and WAN

Connecting a number of LANs Private or public network High speed Large area

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1.3 NEED FOR PROTOCOL ARCHITECTURE

Protocol Protocol Architecture E.g. File transfer

1. Source must activate communications path or inform network the identity of desired destination

2. Source must check destination is prepared to receive data or not

3. File transfer application on source must check destination file management system will accept and store file for his user

4. May need file format translation Task broken into subtasks Implemented separately in layers in vertical stack Same set of layered functions needed in both

systems Peer layers communicate 14

Figure

KEY ELEMENTS OF A PROTOCOL

Syntax Data formats Signal levels

Semantics Control information Error handling

Timing Speed matching Sequencing

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Structure/format of data

Meaning/ interpretation of a set of bits

When data should be sent+

The speed of transmission

Presentation Layer

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

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SIMPLIFIED FILE TRANSFER ARCHITECTURE

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(3) and (4)

(2)

(1)

File transfer

OSI Open Systems Interconnection ( OSI ) Developed by the International Organization

for Standardization (ISO) Seven layers

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1.4 OSI - THE MODEL A layer model Each layer performs a subset of the required

communication functions Each layer relies on the next lower layer to

perform more primitive functions Each layer provides services to the next

higher layer Changes in one layer should not require

changes in other layers

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OSI LAYERS

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•Application•Presentation•Session•Transport•Network•Data Link•Physical

Can U remember layers of OSI model?

THE OSI ENVIRONMENT

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Message : Balqis Please Help Me

Identify sender and intended reciever , is there an e-mail app

available?

Send as electrical signal over category 5 copper wiring at X volt

and X Mbps

Keep track of how many hops; open the shortest path first ; go to Ip

address 192.168.1.2

Make sure all data has arrived intact

Inititate and terminate the session according to X protocol

Decode data with X decoding key ; use ASCII characters

Is the initial connection set up? Put data into frames according to X

standard

Balqis Please Help Me

Identified sender and intended receiver , found available e-mail

app.

Send as electrical signal over category 5 copper wiring at X volt

and X Mbps

Keep track of how many hops; open the shortest path first ; go to Ip

address 192.168.1.2

Make sure all data has arrived intact

Inititated and terminated the session according to X protocol

Decode data with X decoding key ; used ASCII characters

Initial connection is set up. Decoding data in frames according

to X standard

Level 1

Level 2

Level 3

Level 4

Level 5

Level 6

Level 7

1.5 TCP/IP PROTOCOL ARCHITECTURE Developed by the US Defense Advanced Research

Project Agency (DARPA) for its packet switched network (ARPANET)

Used by the global Internet No official model but a working one.

Application layer Host to host or transport layer Internet layer Network access layer Physical layer

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next

Corresponds to the 4 layers of OSI model

Represent application, presentation, session layer in OSI model

OSI V TCP/IP

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TCP/IP PROTOCOL ARCHITECTURE MODEL

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SUMMARY Data com. is the transfer of data from one device

(eg. computer) to another using some form of transmission medium (eg. wire cable)

5 basic components of data com. sys. – message/data, sender, receiver, medium and protocol.

Networks allow shared access to info. devices

Protocol is a set of rules that govern data com.

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SUMMARY CONT…. ISO created a model called OSI which allow multiple

sys. to communicate.

7-layer OSI model provides guidelines for the development of universally compatible architecture, hardware, and software.

Physical, data link, and network layers – network support layers

Session, presentation, and application layers – user support layers

TCP/IP ,a 5-layer hierarchical protocol suite developed before OSI model, is the protocol suite (set) 28

END!!!

KEY POINT PROTOCOL

In computing, a protocol is a convention or standard that controls or enables the connection, communication, and data transfer between two computing endpoints.

In its simplest form, a protocol can be defined as the rules governing the communication.

Most protocols specify one or more of the following properties: How to start and end a message How to format a message What to do with corrupted or improperly formatted messages (error

correction) How to detect unexpected loss of the connection, and what to do

next Termination of the session or connection. Common Protocols: HTTP (Hyper Text Transfer Protocol) FTP (File Transfer Protocol). IP (Internet Protocol).

Sample:29

KEY POINT TOPIC 1.3

PROTOCOL ARCHITECTURE

A layered structure of hardware and software that supports the exchange of data between sys. and supports distributed applications, such as electronic mail and file transfer.

At each layer of protocol architecture, one or more common protocols are implemented in communicating sys. Each protocol provides a set of rules for the exchange of data

between sys.

Most widely used protocol architecture : TCP/IP protocol suite –consist physical, network access, internet, transport, and application layer.

Another important protocol architecture is the seven- layer OSI model. 30

KEY POINT TOPIC 1.4

Open system Model that allows any two different sys. to

communicate regardless of their underlying architecture.

Purpose of OSI model To open communication between different sys. without

requiring changes to the logic of the underlying hardware and software.

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KEY POINT TOPIC 1.4 -2

Please Do Not Touch Steve’s Pet Alligator !

(Physical, Data Link, Network, Transport,Session, Presentation, Application) !!

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At the receiving node, the layered process that sent the message

on its way is reversed. The physical layer recalculates the checksum, confirms

arrival, and logs in packets.

The network layer recounts incoming packets for security.

The transport layer recalculates the checksum and reassembles the message segments.

The session layer holds the parts of the message until it is complete and sends it to the next layer.

The presentation layer decrypts, expands, and translates the message.

The application layer identifies the recipient, converts the bits into readable characters, and directs the data to the correct application.

INTERMEDIATE NODES

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An intermediate node calculates and verifies the checksum for each packet. A router might also reroute the message to avoid congestion on the network.

PHYSICAL LAYER

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The physical layer encodes the packets into the medium that will carry them – such as an analog signal, if the message is going across a telephone line – and sends the packets along that medium.

PHYSICAL LAYER Provide means for bits to be transmitted

across physical com. path. Defines the electrical and mechanical

requirement of com. sys. Eg. Type of cable to be used, type of connectors

to be employed, the bit rate(s) and signal levels to be used.

Defines topology of network Eg. Bus or ring

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DATA-LINK LAYER

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The data-link layer supervises the transmission. It confirms the checksum, and then addresses and duplicates the packets. This layer keeps a copy of each packet until it receives confirmation from the next point along the route that the packet has arrived undamaged.

DATA-LINK LAYER Divides the stream of bits received from network layer

into frames.

If frames are to be distributed to diff. sys. on network, it adds a header to frame the physical add.(on the same network) of sender and receiver.

To control the flow of data rate, not to overwhelm receiver.

Adding mechanisms to detect and retransmit damage or lost frames.

If >2 devices are connected, it determines which device has control over the link(network(s)) at any given time.

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NETWORK LAYER

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The network layer selects a route for the message. It forms segments into packets, counts them, and adds a header containing the sequence of packets and the receiving computer.

NETWORK LAYER Responsible for the source-to-destination delivery of a

packet possible across multiple network(links)

Not necessary for 2 sys. in the same network/link.

Adding logical add. (on different network/link) of sender and receiver.

Provide routing When independent networks or links are connected

together to create an internetwork (a network of networks) /a large network, the connecting devices(called routers/gateways) route the packets to their final destination. 40

TRANSPORT LAYER

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The transport layer protects the data being sent. It subdivides the data into segments and creates checksum tests- mathematical sums based on the contents of data- that can be used later to determine whether the data was scrambled. It also makes backup copies of the data. The transport header identifies each segment’s checksum and its position in the message

TRANSPORT LAYER Responsible for source-to-destination (end-to-end)

delivery of the entire message. Network layer oversees end-to-end delivery of

individual packet

Ensure that the whole message arrives intact in order

Adding port add./service-point add. (process (running prog.) add.) of sender and receiver.

Ensures frames arrive at the destination in their correct order

Provide end-to-end correction and data flow.42

SESSION LAYER

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The session layer opens communications. It sets boundaries (called brackets) for the beginning and end of the message and establishes whether the message will be sent half duplex, with each computer taking turns sending and receiving, or full duplex, with both computers sending and receiving at the same time. The details of these decisions are placed into a session header.

SESSION LAYER Handles logon and logoff procedures and then

it established, and later clear down, the connection between two terminals.

Controls the transfer of messages over network and overall error detection.

Controls how a message starts and finishes.

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PRESENTATION LAYER

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The presentation layer translates the message into a language that the receiving computer can understand (often ASCII, a way of encoding text as bits). This layer also compresses and perhaps encrypts the data. It adds another header specifying the language as well as the compression and encryption schemes.

PRESENTATION LAYER Concerned with the syntax and semantics of

the info. exchange between two sys.

To translate, encrypt, and compress data

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APPLICATION LAYER

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For a message, file, or any other data to travel through a network, it must pass through several layers, all designed to make sure the data gets through intact and accurate. The first layer, the application layer, is the only part of the process a user sees, and even then the user doesn’t see most of the work the application does to prepare a message for sending over a network. The layer converts a message’s data into bits and attaches a header identifying the sending and receiving computers.

APPLICATION LAYER Enables the user,whether human or software, to access the

network

Provides user interfaces and support for services Eg. E-mail, remote file access and transfer, shared database

management etc.

Allows user to log on to a remote host

Allows user to access files in remote computer (change/read data), retrieve files from a remote comp., manage/control files in a remote comp.

Provide the basis for e-mail forwarding and storage

Provides distributed database sources and access for global info. about various abject and services.

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ROUTERS

A processor that connects 2 networks

Primary function is to relay data from 1 network to the other on its route from the source to the destination end sys.

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SMTP( SIMPLE MAIL TRANSFER PROTOCOL )

The TCP/IP protocol that specifies the process of management in the Internet

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An application service for retrieving a web document

http (Hypertext Transfer Protocol )

INTRO Data com. : transfer of data from one device to

another via some form of transmission medium (perantaraan)

How people share data info? 1970s- data wrote in tapereceiver’s comp. by

armored car/airplane -1x/week Data floppy diskreload onto comp.

Time consuming, errors, size lim., virus Connecting PC comp. network (comp. can talk to

each other)

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Isn't it amazing???

INTRO (2) Why did U choose to learn about computer? Isn’t it amazing that data/instructions in

computers ONLY using 0s and 1s??? Isn’t it amazing that INTERNET works without

actually ANYONE manages it? Isn’t it amazing that the message (e-mail with

moving char!) you sent your mother could reach her just like what you sent from your computer in just a few minutes?

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Isn't it amazing???

THE LITTLE NET THAT GREW.. 1957-Soviet Union launched 1st satellite,

Sputnik Advanced Research Projects Agency

(ARPA) to get the U.S into space. Role was replaced by NASA,and ARPA

sponsor of advanced research projects at univ. and contractor.

1960-Paul Baran wrote paper to Pentagon of impact in case of Soviet nuclear attack. Military command ,messages signals should be

carried over a distributed network. Break message into blocks, send separately

over network.53

THE LITTLE NET THAT GREW… 1967-ARPA looking for a way its member to

distribute messages, data for research. 1969-ARPAnet (distributed network), build

interface message processor (IMP), which connected comp. at univ. research centers.

Incorporated a tech. called TCP/IP (transmission control protocol/Internet protocol) developed at National Science Foundation (NCF).

1969- computers at univ. all over US were linked to ARPAnet.

First letter –”L”

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THE LITTLE NET THAT GREW! 1972- ARPAnet connected 23 host sites. By 1975, one new installation/month. CSNET(Computer Science Network) was

designed, cheaper than ARPAnet. 1974- Bob Kahn and Vint Cerf came up with

idea “network of networks”-let dissimilar networks communicate with one another.

By 1982, diff. networks adopting TCP/IP as comm. standard – “Internet”

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THE LITTLE NET THAT GREW….. For years, Internet was the territory of colleges and

defense contractors. 1991- NSF lifted restrictions on the commercial use

of the Net. People sent out advertising over Internet-spam World Wide Web – txt-only origins world of

graphics, sound and video.

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