Networks and Communications

Networks and Communications.

Lecturer - Eric Goodyer

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Lecture notes prepared by Dr Amelia Platt

with minor revisions and additions by Eric Goodyer

Last Revision January 2000

Recommended Reading:-

Computer Networks

by A Tanenbaum, 3rd edition, Prentice Hall

Data Communications, Computer Networks and Open Systems

by Fred Halsall, 4th edition, Addison Wesley

Computer Networks: A First Course

by Jean Walrand, Aksen Associates

Data and Computer Communications

by W Stallings, 3rd edition, Macmillan

Packet Switching and X25 Networks

by Simon Poulton, Pitman

1 Introduction

1.1 Different types of Networks

· Wide Area Networks (WANs)

· Local Area Networks (LANs)

· Metropolitan Area Networks (MANs)

1.2 Differences between WANs, LANs & MANs

Coverage Area

· LANs cover a small area, typically a room or building

· MANs cover a larger area, typically a city or county

· WANs have no limit on size or area covered

Ownership

· LANs are private - not owned by PTTs

· MANs can be private or public

· WANS can be private or public

Transmission Rates (Speed)

· LANs typically have high data rates compared to WANs

· MANs have higher rates than LANs

· WANs have low data rates

Topologies

· LANs and MANs typically have ring and bus topologies

· WANs have mesh topologies

Type of Transmission

· LAN and MAN broadcast by nature of topologies

· WANs private (user to user)

Signalling

· All represent 'bits' differently on transmission lines

However the above picture is changing rapidly and the distinction is becoming increasingly blurred

This course will consider these types of networks in terms of

· Network access techniques

· Protocols

2 Communication Architectures

2.1 Problems associated with communication

The task of a communication is very complex. To give an understanding of the complexity, below is a small sample of the type of problems which must be solved by the communications software:-

·  How to connect two users together - communications channel.

·  How to represent signals on a communications channel.

·  How to detect and correct errors on channel to ensure error free transmission.

·  How to allow users to gain access to the communications channel.

·  How to route data to the correct user across a network.

·  How to ensure the receiver interprets the data correctly - the receiving machine may differ from the sending machine.

·  How to allow the user to run applications over the channel.

Now consider a number of communication situations :-

Two users at either end of

· a piece of wire

· a network

· a set of interconnected networks

Clearly, connecting two users across a set of interconnected networks, is much more complex when compared to connecting the same users over a single piece of wire.

2.2 Communication Architectures

The solution is to break the overall problem down into a set of small, well defined tasks. The result is typically referred to as a communications architecture (or structure). There are a number of such architectures, and two of the most widely used are :-

· ISO Open Systems Interconnection (OSI) 7 layer reference model.

· TCP/IP reference model

2.3 OSI 7 Layer Reference Model

Each layer is intended to perform a specific task in the overall problem of communication. Each layer is independent of all the others. Communication with the layers immediately above and below is via a well defined interface. Layer N is said to request service from layer N-1 (below) and provide a service to the layer N+1 (above). Layer N in one protocol stack communications with the same layer in a remote protocol stack via the layers below. This is known as virtual or peer-to-peer communication.

In particular, the OSI 7 layer model defines 7 layers. At the end points, the 7 layer model can be viewed as the :-

· Upper layers (5-7) (Application layers - (7)Application, (6)Presentation, (5)Session)

· Transport layer (4) (Interface between subnet and application layers)

· Lower layers (1-3) (Subnet - (3)Network, (2)Link, (1)Physical)

2.4 TCP/IP (Transmission Control Protocol/ Internet Protocol)

Another reference model which is very widely used is the TCP/IP Reference Model - it is used for example in the Internet.. TCP/IP defines only 4 layers -

Host-to-network layer

Allows host to connect to the network so that IP Datagrams can be sent

Internet layer

Allows host to inject packets onto the network and to route packets

Transport layer

Provides a peer-peer link between source & destination

Application layer

Higher level protocols, such as TELNET, FTP

It is discussed in more detail in section 11.5 below

3 Overview of Switching methods

There are 2 types of switching methods

· Circuit switching

· Packet switching

3.1 Circuit switching

Set up a dedicated end-to-end connection. Switching implies that the connection is switched through a number of intermediate exchanges. How could this be modelled?

E.g. Present telephone networks, mobile cellular networks

3.2 Packet switching

Information is broken into segments. These segments are called packets at layer 3 and frames at layer 2. More generally they are called Protocol Data Units (PDUs).

Packets are sent individually through the network.

What problems could this cause for voice traffic?

E.g. Internet, Superhighway, most data networks

N.B. There are a number of variants of packet switching, but the same principle applies. See Figure at the end.

Advantages and disadvantages of the two switching methods

Circuit switching?

Private, secure, not subject to congestion

But inefficient use of bandwidth, pay for time call is connected regardless of amount of data

Packet switching?

Shared use of high cost components, efficient use of bandwidth, only pay for data in transit

But not secure or private, subject to congestion

4 Delays associated with networks

4.1 Propagation Delay

· Time taken for a signal to travel from the transmitter to the receiver

· Speed of light is the fastest a signal will propagate

3 X 108 m/sec through space

2 X 108 m/sec through copper

4.2 Transmission Delay (Time)

· Time taken to put the bits on the transmission media

Transmission speed of 2Mbps means

2 X 106 bits can be transmitted in 1 second

4.3 Processing Delay

· Time taken to execute protocols

check for errors

send Acks etc.

4.4 Queuing Delay

· Only in packet switched networks

· Time spent waiting in buffer for transmission

· Increases as load on network increases

4.5 Round Trip Delay

Round trip delay is defined as the time between the first bit of the message being put onto the

transmission medium, and the last bit the acknowledgement being received back by the

transmitter. It is the sum of the all the delays detailed above. The round trip delay is a critical factor in the performance of packet switched protocols and networks. Indeed, it has been stated that a good algorithm for estimating the round trip delay is at the heart of a good packet switch protocol.

5 Properties of Signals

5.1 Bandwidth

· Bandwidth is a measurement of the width of a range of frequencies and is measured in hertz (Hz).

· In data networks bandwidth is normally specified as bits per second (BPS)

· Shannon-Hartley Theorem states that

Dmax = Blog2(1 + S/N)

where Dmax is the maximum bit rate

B is the bandwidth in Hz

and S/N is the signal to noise ratio

All transmission mediums are degraded by ‘noise’. If the average power of the signal is given by S, and the average power of the noise is given by N, then the signal to noise ratio is given by S/N. The greater the value of S/N then the greater is the theoretical transmission rate of that medium.

5.2 Square wave properties

Square wave is composed of sine waves with frequencies:-

· Fundamental frequency F0+

· Odd harmonics 3F0+, 5F0+, 7F0+, 9F0+... (the 3rd, 5th, 7th, and 9th harmonics)

· Note, the fundamental frequency is equal to the basic repetition frequency of the wave form.

The amplitude of the harmonics are increasingly proportional to the harmonic number.

· Amplitude of the 3rd harmonic is 1/3 of the amplitude of the fundamental frequency.

· Amplitude of the 5th harmonic is 1/5 of the amplitude of the fundamental frequency.

Sine waves up to and including the 9th harmonic represent over 95% of the signal power.

- Implications.

· Don't need to receive all of the harmonics to receive the signal.

· Must receive at most (at least) up to the 9th harmonic.

Note : The more harmonics received, the flatter the peak or trough.

The graph below shows a square wave that consists of sine waves up to the 9th harmonic only.

5.3 Signal distortion

Attenuation

· Decrease in the amplitude of the transmitted signal.

· Attenuation increases with distance, repeaters must be used to restore signal to transmitted level.

· Attenuation increases with frequency, repeaters must also take this into consideration.

Propagation delay

· Propagation delay varies with frequency.

· So various frequencies of a signal propagate at different rates.

· Clearly they will incur different amounts of delay.

· As bit rate increases, so does the probability of frequencies from one signal interfering with the next.

· The longer the transmission media then the more is the ‘spread’ of the component frequencies of the original transmitted square wave.

Noise

· There are different sorts of noise, which effect different media :-

· Thermal noise. All electronic components generate ‘noise’ internally; the level of this noise is related to the temperature of the electronic components, thus the term ‘thermal noise’

· Atmospheric noise. This is electrical interference induced into the electronics as a result of external electromagnetic radiation. This includes interference from nearby electrical equipment, such as computers, mains switches and CRTs, and also interference from radio waves. These sources of interference are also known as RFI, or Radio Frequency Interference, and EMC, or Electro-Magnetic Coupling.

RFI/EMC can be reduced by ‘good wiring practice’. This means ensuring that there are quality connections between cables, that good earth connections are made , and that protective shields are wrapped around transmission cables.

· Ringing. If cables are incorrectly terminated, then some of the energy in the transmitted signal is reflected back down the cable from which it came. This results in an effect that is similar to an optical interference pattern, with a characteristic ‘ringing’ distortion of the received signal. It is a major cause of distortion in high speed networks that are not correctly terminated, or have used the wrong (typically cheaper) cables.

6 Physical Layer

These describe the electrical and mechanical interface necessary to establish a communications path.

Layer 1 protocols are concerned with the physical and electrical interfaces. It defines for example:-

· Connection types and allocation of signals to pins

· Electrical characteristics of signals which includes bit synchronisation and identifying a signal element as a 0 or 1

Put simply, layer 1 is responsible for transmitting and receiving the signals.

6.1 RS232/V.24

Signal voltage levels

· -3V to -25V binary 1 for data, OFF for a control signal

· +3V to +25V binary 0 for data, On for a control signal

25 Volts is the maximum rating for a line without a load. In practice RS232/V24 signals are set to typically be +-12V

Use of RS232/V.24 as DTE/DCE interface standard

Ground Signals

· Pin 1 (SHG) Protective Ground / Shield Ground to reduce external interference

· Pin 7 (SIG) Signal Ground - provides a reference for other signals

Transmit and Receive

· Pin 2 (TxD) Transmit Data

· Pin 3 (RxD) Receive Data

Maintaining a Connection / ‘Hardware Handshaking’

· Pin 6 (DSR) Data Set Ready, Modem indicates to DTE that it is ready, i.e. connected to a telephone wire

· Pin 20 (DTR) Data Terminal Ready, DTE uses this to prepare the modem to be connected to the telephone line. If it is placed in an OFF condition it causes the modem to drop any connection in progress. Thus the DTE ultimately controls the connection.

‘Hardware’ Flow Control

· Pin 4 (RTS) Request to Send, Sent by DTE to modem to prepare it for transmission.

· Pin 5 (CTS) Clear to Send, Modem indicates to DTE that it is ready to transmit.

· Pin 8 (CD) Carrier Detect, Sent by modem to DTE, to inform it that a signal has been received from the other end of link.

Other

· Pin 22 (RI) Ring Indicator, sent by modem to DTE to inform it that a ringing signal has been received from the other end of the link. Used by auto-answer modems to wake-up the attached terminal.

6.2 X.21 interface

X.21

· Full duplex

· Synchronous Interface

· 15 pin connector, but only 8 defined - these are explained below

Ground Signals

·  G - Ground Signal

·  Ga - DTE common return

Clocks

·  S - Signal element Timing (bits)

·  B - Byte Timing

DTE to DCE

· T - transport

· C - control

T carries bit stream

C indicates how it should be interpreted

Three inactive states are defined as follows:-

T = 1 C=OFF interpreted as DTE ready

T = 1 C=OFF interpreted as DTE not ready due to abnormal condition

T = 0101.. C=OFF interpreted as DTE operational but not ready. Used for flow control.

DCE to DTE

· R - Receive

· I - Indication

R carries bit stream

I indicates how it should be interpreted

Three inactive states are defined in a similar way as for DTE to DCE

7 DIGITAL AND ANALOGUE SIGNALS

There are two types of data :-

Digital

Analogue

There are two types of transmission :-

Digital

Analogue

This gives rise to 4 potential situations :-

·  Digital data - Analogue transmission

·  Digital data - Digital transmission

·  Analogue data - Analogue transmission

·  Analogue data - Digital transmission

What are examples of digital data?

What are examples of analogue data?

When the types of data and the transmission are not the same, data must be changed to suit the transmission media.

Analogue Signals