Communication concepts

Week 2 Lecture 1

Introduction to key concepts

Physical media Analogue & Digital transmission Multiplexing Circuit & Packet Switching Bandwidth & Latency

It will be at a broad principle level. We need to get these definitions clear before we can proceed. Some might find it simplistic.

Wide Area Network

Local Area Network

Home ISP

ISP & Telco (IBP)

The last kilometre

Business

WiFiNetwork

Media (electromagnetic spectrum)

PowerVoice

RadioInfrared

lightUV

lightX, Gamma

rays

Visible light

0 3khz 300ghz

Copper cable

Wireless 802.11 & 802.162-60ghz range

Optic fibre

Twisted pair

Colour coded pairs of insulated copper wires twisted around each other

One carries the signal, the other is grounded and absorbs interference

Electrical noise is a big issue – the tighter the twist the more resistant the cable is to noise

Lots of variations STP shielded twisted pair UTP unshielded twisted pair Quality of copper, number of twists, length of segment, devices

on the line

Limit of 90 metre lengths

UTP Categories

CAT 3 used for Ethernet segments to 10mbps,

CAT 5 – higher grade copper, more twists, used for fast Ethernet up to 100mbps – 4 wire pairs

CAT 5E and CAT 6 – current standards, up to 1000mbps

Fibre optic cable

Consists of One or more glass fibre at the core Clad by a glass layer that acts as a mirror Layer of plastic Braiding of Kevlar Plastic jacket

Data is transmitted by a pulsating light generated by a laser or LED

Fibre characteristics Currently used as a LAN backbone or in the WAN for

high capacity links It is marginally more expensive – starting to replace

copper to the desktop Basically unlimited bandwidth – up to 1gbps at

present Cannot be easily tapped Does not need repeaters or amplifiers Does not transmit in both directions –2 strands NIC and Hubs more expensive

Wireless

Big range of complex products being introduced here

We will look at Mobile telephones Packet Switched Data Networks Point to point links Wireless LANs - 802.11 - WiFi Last kilometer – 802.16 - WiMax Satellites

Mobile Network Generations 1G – Analogue – phased out in Oz 2G – Digital

GSM in Europe & Asia – other technologies in the US Data to 9.6kbps, SMS messages to 160 characters

2.5G - Digital Higher data rates at lower intro cost than 3G, GPRS (General Packet Radio Service) can go up to

115kbps, Oz intro first GPRS network at 24kbps to go to 48kbps

Mobile Network Generations (cont.) 3G –

Expected to offer data rates up to 2mbps Frequencies sold recently by auction in Europe &

Oz Three competing protocols (CDMA) each

backwards compatible with the 3 main 2G technologies

4G Some small implementations have been made Expected to go up to 10mbps Maybe data only

Other wireless products Packet Switched Data Networks

Used in the US and some other areas at low data rates

One new US carrier expects to offer up to 128kbps

Point to point links Usually line of sight from rooftops Micro wave links have been in use for many

years Laser links now offer up to 1gbps for 2km

Other wireless products (cont.)

Last kilometer – give access to local exchanges – may be very significant to countries without wired infrastructure 802.16 WiMax

Wireless LANs – allow a LAN to operate within a building without wires up to 11mbps WiFi

Wireless Personal Area Networks – Bluetooth – allow devices to be connected within 10 metres without wires

Satellites GEO – Geo-synchronous Earth Orbit MEO – Medium Earth Orbit LEO – Low Earth Orbit In use downstream in OZ now Main problem is latency

250 mille-seconds for GEO 10 to 100 mille-seconds for LEO

Analogue Transmission Until recently telephone networks were analogue Sound is carried along the wire in sine wave

form Put simply, there are three attributes, the height

and length of the wave, known as amplitude, frequency and phase.

Loudness varies the amplitude and pitch varies the frequency

Data can be carried by varying – Amplitude, Frequency or Phase

AmplitudeShiftKeying

ASK

FrequencyShiftKeying

FSK

PhaseShiftKeying

PSK

ASK – susceptible to noise degradationFSK – has band width limitationsPSK – can transmit multiple bits per cycle

Digital Transmission Digital signals do not use the sign wave It turns the electrical signal on and off.

“On” representing 1 and “Off” Zero Usually represented as a square wave

form Not as clear as On or Off because of

noise and voltage variation

- Problem of synchronisation if a long row of zeros or ones sent- This is one of a number of solutions – Return to Zero

Movement to Digital Telephone converting to digital ISDN – Integrated Services Digital Network

available for some years at 64kbps ADSL – Asymmetric Digital Subscriber Line Asymmetric means that downstream is

faster than upstream, mirroring the usual Internet pattern

ADSL Downstream up to 1.5mbps Upstream up to 256kbps Telephone on the same line “Always on” Internet connection Availability & bandwidth dependent on

Quality of copper line Length of segment Existence of devices on the line to the

exchange

Multiplexing Transmission capacity of the media is often

much greater than the needs of any one user

This capacity can be shared by allowing simultaneous transmission of multiple signals on a single data link

This technique is known as multiplexing There is one device to combine the signals

– a mux and one to separate them again – a demux – at each end of the link

Mux Demux

Approaches There are a range of approaches

FDM – Frequency Division Multiplexing divides the frequency into sub frequencies – used in ADSL to divide channel into Up and Down stream and Telephone channels

TDM – Divides the time into fixed sized frames and then into slots. A connection is then allocated a slot within the frame. Gives a fixed bandwidth whether used or not

Multiplexing (Cont.) Statistical Multiplexing allocates capacity to

those tasks that have data to transmit WDM – Wave Length Modulation is used in

fibre-optic cable, which allows multiple signals with different wavelengths to be transmitted simultaneously

CDMA – Code Division Multiple Access is used in mobile phone networks. Allocates a code to each bit and sends them at a higher rate – called a chip rate. See Kurose page 436 for details.

Serial & Parallel In Serial transmission the bits follow

each other down the one path In parallel multiple paths of wires are

used to transmit all the bits in a byte at the same time or in parallel

Parallel is now largely limited to printer cables and they are now being replaced by the USB (Universal Serial Bus)