ASYNCHRONOUS

Sends A letter at a time. The devices are not synchronised. The letter is framed by start and stop bits. The start bits are to ‘wake up’ the other device and stop bits to say that’s it finished.

10 11010011 110

FRAME with start and stop bits

Used in low bandwidth situations

SYNCHRONOUS

Here the devices are synchronised, they are both reading the line at the same speed.

A start frame is sent to set the synchronisation. A stream of data follows, then a stop frame.

Start frame… DATA …Stop frame

Used in high bandwidth situations

PARITY CHECKS

As ASCII is a 7 bit code, we have a spare bit in each byte.

If we use this to ensure the number of ones and zeros is even, then this gives an error check.

Letter: 1000011

Even parity: 11000011

If an even number of ones is not received, then an error has occurred.

PARITY CHECKS

This same can be done with odd parity.

Parity checks are very simple to implement BUT:

What if two bits are wrong?

Uses up 12.5% of the message, so high redundancy.

CYCLIC REDUNDANCY CHECK

All the bytes in the packet of data are subject to a calculation according to some formula.

The result is divided by a set number and the remainder is sent.

At the other end the same calculation is performed and if they do not get the same remainder, then they request the packet to be resent.

CSMACDCarrier Sense Multiple Access Collision Detection

•A node senses the bus to see if it is clear.

•It puts its message on the bus.

•If another node does this at the same time, a collision occurs.

•The node that first detects the collision sends out a signal to clear the bus.

•The two nodes wait a random time interval before retransmitting.

WPANWireless Personal Area Network

Very local, one room, about 10 metres radius

Connect mobile, pda, laptop, mp3 player, psp

All devices must be using the same standard and have the necessary transmitter / receiver.

Bluetooth is the most common standard.

WLAN

Wireless Local Area Network

Often used in homes to share

broadband.

Each machine must have a wireless network interface card (or a USB receiver).

The standard is 802.11, the latest version 802.11g has a speed of 108 Mbps.

WWAN

Wireless Wide Area Network

This is possible using equivalent of mobile phone transmitters or satellite receivers.

INTERNET CONNECTIONS

•Dial up

•Cable modem

•Leased line

•ISDN

•ADSL

Dial-up

A device called a ‘Modem’ was developed to allow Digital data to be carried over the analogue telephone system.

MODEM

Digital signal

‘1’

‘0’

One Tone representsBinary ‘1’

A different TonerepresentsBinary ‘0’

Telephone line

This is why dial modems can be heard to ‘whistle’ as they communicate. You are hearing the tones that represent digital information

•A dial-up connection makes use of a 56 Kilobits per second Modem.

•Your PC literally dials (hence the name) a phone number (provided by your ISP) and connects to the server and therefore the internet.

•You are circuit switched to your ISP and are paying a phone call.

Dial Up

Same as making a phone call, so ties up your phone.

No special equipment apart from in-built modem.

Need to sign up with an ISP.

Good for casual users, but speed very frustrating.

(the World Wide Wait)

Cable Modem

An always on connection.

Huge bandwidth possible (50Mbps), but you share your line with others.

Effective speed is only 2 - 8 Mbps.

Cable Modem

Good for home users, small businesses,

You can get a cheap deal if you have cable TV as well

but because you share your line, not very secure

(subject to ‘packet sniffing’).

Bandwidth can drop at peak times.

Leased Line

Here you rent your own dedicated line from BT.

Very expensive.

Often used by businesses to connect branches to head Office.

Very secure, reasonable bandwidth.

ISDN

Integrated Services Digital Network.

Needs a separate digital line to be connected.

Like dial up you pay per call. Carries 64Kbps but uses two lines to get 128Kbps or more.

Don’t need a modem.

Standard ISDN is 64Kb but can be multiplied for higher speed, however you pay multiple charges.

•The ISDN service allows you to use the telephone at the same time – it does not tie up the line.

•It is fast to connect.

Gives you a dedicated (circuit switched) line while you are connected.

Good for business use, cheaper than leased but still very secure.

Excellent for video conferencing. Used by TV companies for outside broadcasts.

Expensive to install and run. Limited speed nowadays but there is a 1.5Mbps version.

Providing bandwidth is expensive – it needs special equipment at the exchanges and your telephone line has to be of good quality .

Telephone companies realised that most domestic customers would be downloading far more than they would be uploading. And so it made sense to develop a DSL technology that provided fast download speeds but slower upload speeds.

This service is called Asymmetric Digital Subscriber Line (ADSL) otherwise known as ‘Broadband’

ADSLThe connections work by splitting your phone line into two separate channels, one for data (internet) and one for voice (phone calls) which means you can talk on the phone and be connected to the internet at the same time.

Excellent for home users and businesses

Speeds of 1Mbps – 8Mbps (download)

Upload about 1/8 or less of download.

Advantages of ADSL

• Faster downloads compared to dial-up or ISDN

• No need for a second phone line

• ADSL connections are Always on, so no dialling up, but you do re-connect.

Disadvantages of ADSL• ADSL connections are not available to

everyone, you need to be within 3 miles of an ADSL enabled exchange.

• You will need a special ADSL modem and ADSL filters.

• Because ADSL connections are Always on you will need a firewall to protect your PC

Network Interface Card

Must be present in any device connected to a LAN.

Like all interfaces they have to carry out data conversion and buffering.

They also organise data in frames with start and stop bits, add error checks and add headers with addresses

MAC Address

Every NIC has its own 48 bit unique Media Access Control address.

Although LANs often use private IP addresses, they have to be translated into MAC addresses so that the NIC recognises it.