DATA COMMUNICATION AND NETWORKING

Chapter 12

Communication

sending or receiving information

• Beating of drums

• Mirrors reflecting sunlight

• Homing pigeons

• Telegraph, telephone

• Computers …

Data Communication

Data Communication

DATA COMMUNICATION

• Exchange of data between two devices

• wired or wireless transmission medium

Communication system (collection of hardware, software …)

Transfer of data Method of transfer

Preservation of the data during

the transfer process

Delivery

• The system must deliver data to the correct or the intended destination.

Accuracy

• The system must deliver data accurately (error free).

Timeliness

• The system must deliver data in a timely manner without enough time lags.

Five components of a data Communication System

1. Message

• It is the information to be communicated. Popular forms of information include text, pictures, audio, video etc.

2. Sender

• It is the device which sends the data messages. It can be a computer, workstation, telephone handset etc.

3. Receiver

• It is the device which receives the data messages. It can be a computer, workstation, telephone handset etc.

4. Transmission

Medium• It is the

physical path by which a message travels from sender to receiver. Some examples include twisted-pair wire, coaxial cable, radio-waves etc.

5. Protocol

• It is a set of rules that governs the data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating.

Data Transmission Modes

• Data in a simplex channel is always one way. Simplex channels are not often used because it is not possible to send back error or control signals to the transmit end. An example of a simplex channel in a computer system is the interface between the keyboard and the computer, in that key codes need only be sent one way from the keyboard to the computer system.

Simplex

• A half duplex channel can send and receive, but not at the same time. Its like a one-lane bridge where two way traffic must give way in order to cross. Only one end transmits at a time, the other end receives.

Half Duplex

• Data can travel in both directions simultaneously. There is no need to switch from transmit to receive mode like in half duplex. Its like a two lane bridge on a two-lane highway.

Full Duplex

Analog & Digital Transmission

• information changes continuously and can take on many different values. An analog clock’s hands move constantly, displaying time on a continuous scale.

Analog

• information is characterized by discrete states. A light bulb, for example, is on or off. A digital clock represents the time in one-minute intervals and doesn’t change its numbers again until the next minute. A digital clock can represent exact minutes but not the seconds that pass in between.

Digital

Analog & Digital Transmission …Analog to analog

Voice (Analog Data) Telephone Analog Signal

Digital to analogPC ( Digital Data) Modem Analog Signal

Analog to digitalVoice ( Analog Data) CODEC Digital Signal

Digital to DigitalPC ( Digital Data) Digital Transmitter Digital Signal

AnalogSignals- Sine waves

• Amplitude• Frequency• Wavelength

Data Communication Measurement

• 64 Kbps or less channels

Narrowband Services

• 1.5 Mbps to 45 Mbps (North American)• 2 Mbps to 34 Mbps (International)

Wideband Services

• 45 Mbps or greater (North American)• 34 Mbps or greater (International)

Broadband Services

Transmission Media

Radio Frequency Propogation

Modulation

Multiplexing

Multiplexing …

Synchronization- Controlling timeAsynchronous Transmission

Each character of data is treated independently

Synchronous TransmissionFor sending large blocks of dataControl schemes

Character-orientedBit-oriented

6.17

Switching

4 Employee Office• Direct Lines to each of them!• Point to point communication

Impractical & WastefulRoute calls by making temporary connection!

Use Switches or Exchanges

Some switches are directly connected to a comminication device while others are dedicated to route/forward information

Circuit switching

Major Attributes

• Dedicated fixed BW channel

• Data (sent/received) path is determined by the circuit

• Data path does not change within the lifetime of connection

• Simple method. Used in Public Service Telephone Network (PSTN)

Disadvantage?

• Waste when there is no data flowing

• Cannot change the circuit after it is established even if there are other (cheaper) routes possible

Packet Switching

Major Attributes

• Focuses on data communication (vs. voice)

• Break data into packets• Package Assembler and Disassembler (PAD)

• Packets from a single message will not necessarily follow the same route

• Packet Header• Destination

• Priority

• …

• Packet Switching Exchange (PSE)

• Efficient?

“Message Switching” is obsolete

Computer Network

Standalone versus Networked Environment

• How can we connect (physically)?

• Network Operating Systems

• Manage multiple computers on a network

• Client/Server (NOS)

• Client request and server serves

• Access to resources

• Peer to peer network (NOS?)

• Equal peer nodes• Both server & client

• Music/File sharing!

Client/Server

How it worksClient sends request for service to serverServer fulfills request and send results to

clientClient and server may share processing

BenefitsReduces volume of data trafficAllows faster response for each clientNodes can be less expensive computers

Attribute P2P Client/Server

Centralization of resource and data

No Yes

Maintenance cost Low HighNeeds staffs, support

Installation Process Simple Requires experts

Cost Low- no dedicated server

Requires a dedicated server

Security Low High

Reliability High- all independent nodes

Server down- network down

Standalone Vs. Networked Environment

Topology – Physical layout of components

Computer Network

LANMANWANCAN

LAN- Local Area Network

Connections over short distances through communications media

Connecting LANsBridge – connects networks with similar protocolsRouter – directs traffic via best pathIP switches

Replacing routersLess expensiveFaster

GatewayConnects LANs with dissimilar protocolsPerforms protocol conversion

Components

• PCs• Network

cable• NIC

WAN- Wide Area Network

Link computers in geographically distant locations

Network Topology

Bus topologyRing topologyStar topologyTree topologyMesh topology

Check the textbook

OSI Model

Textbook P 445

OSI Model …

Layer 1 - Physical

• Defines the cable or physical medium itself• e.g., thinnet, thicknet, unshielded twisted pairs (UTP).

• All media are functionally equivalent.

Layer 2 - Data Link

• Defines the format of data on the network• A network data frame, aka packet, includes

• checksum,

• source and destination address, and

• data.

• The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). net interface to handle connections to the outside world, and a loopback interface to send packets to itself.

OSI Model …

Layer 3 - Network

• NFS uses Internetwork Protocol (IP) which is responsible for routing, directing datagrams from one network to another.

• The layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram.

Layer 4 - Transport

• Subdivides user-buffer into network-buffer sized datagrams

• enforces desired transmission control.

• Two transport protocols sits at this layer• Transmission Control Protocol (TCP)

• User Datagram Protocol (UDP)

Reliability and speed are the primary difference between these two protocols.

OSI Model …

Layer 5 - Session

• Defines the format of the data sent over the connections

• The NFS uses the Remote Procedure Call (RPC) for its session protocol• RPC may be built on either TCP or UDP

Layer 6 - Presentation

• External Data Representation (XDR) sits at this level.

• Converts local representation of data to its canonical form and vice versa. • The canonical uses a standard byte ordering and

structure packing convention, independent of the host.

Layer 7 - Application

• Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS … network applications.

Network Devices

NICRepeaterHubBridgeSwitchRouterGateway

P449