chapter 2 different modes of data transmission 2.1bandwidth bandwidth 2.2serial and parallel...

Chapter 2 Different Modes Chapter 2 Different Modes of Data Transmissionof Data Transmission

2.12.1 Bandwidth

2.22.2 Serial and Parallel Transmissions

2.32.3 Simplex and Duplex

2.42.4 Multiplexing

2.52.5 Concept of Packets

2.62.6 Access Methods

Chapter 2 Data Transmission

2.1 Bandwidth2.1 Bandwidth

Bandwidth of a channel is the maximum data transmitted per second

indicate the capacity of the channel

has unit bps (bits per second). Kbps, Mbps, or Gbps

Note: bandwidth is not the physical speed of signal movement

it does not measure how long it takes for data to travel from one point to the other.


2.2 Serial and Parallel Transmissions

Two modes of transmission: parallel

several lines for several bits to move simultaneously

seriala single line onlybits move one after the other


2.2 Serial and Parallel Transmissions


2.2.1 Comparing Serial and Parallel

Under identical conditions, parallel transmission is faster, because

more channels are used (This explains why

multiple parallel lines are found on printed circuit boards) Disadvantages of parallel transmission:

very thick cables high cost difficult to install

Therefore, data transmission in networking media is mainly serial.

Note: modern technology has increased the rate of transmission over serial

cable.


2.2.2 Serial and Parallel Conversion

Why conversion is needed? Transmission inside the computer is parallel, but is serial

outside. How?

Done by Network Interface Card (NIC) which converts between

• parallel data streams from the CPU, and

• serial data along the cables


2.2.2 Serial and Parallel Conversion


2.3 Simplex and Duplex

Simplex one way data flow

e.g. TV broadcast. Duplex

bi-directional data transmission half-duplex

transmission in one direction at a timeWhen A is sending data, B has to

wait until the transmission is over. e.g. walkie-talkies, hubs

full-duplex two-way transmission at the same time

e.g. telephone, switches


2.4 Multiplexing (1/2)

Multiplexing means using a single line to carry signals from several devices

e.g. You may surf the Web, while speaking over the phone – both use the same phone line (DSL broadband).

by interleaving the signals


2.4 Multiplexing (2/2)

Multiplexing (cont’) allows better use of the bandwidth of a channel Typical examples

mobile phone systems, cable TV and DSL broadband

requires the bandwidth of the shared channel several times higher than the transmission rate of individual device

De-multiplexing interleaved signals are sent to the corresponding receivers


2.5 Concept of Packets

Almost all networks break messages into smaller pieces of data before transmission takes place.

Packets (or frames) are fundamental units of information carriage in a computer

network.


2.5.1 Reasons for Using Packets

1. Avoid monopolisation Packets from different nodes can be interleaved

2. Even load distribution between lines Packets may use different paths

3. Avoiding complete failure Packets may be routed round the problem

4. Easier error recovery Only packets with errors need re-transmission


2.5.2 Packet-switching

Packet-switching involves packets

may use different paths involves routers

which directs the packets is designed to make full use of bandwidth is adopted by WANs and the Internet

Circuit-switching a path is established between the sender and receiver until

communication is over is used in traditional phone connection

the available bandwidth is not fully utilised


2.5.3 Packet Structure (1/2)

Each packet must carry data content addresses

sender address• for acknowledge receipt of

data

• for re-transmissionreceiver address

• for delivery


2.5.3 Packet Structure (2/2)

Each packet must carry (cont’) control information

sequencing• for re-assembling

– because packets may arrive out of ordererror-control

• for detecting or correcting errorstime-to-live (TTL)

• for avoiding undelivered packets to move forever.


2.5.4 Packets Creation and Receive

The NIC creates outgoing packets

adds MAC addresses • of its own and the receiver

sends the packet onto the networking medium.

examines incoming packetsaccepting those targeting for it, and ignoring those not targeting for it.

The receiver computer reassembles the packets

to re-construct the original messageby making use of the sequencing information


2.5.4 Packets Creation and Receive


2.6 Access Methods

Data collision occurs in shared networking medium

when two computers send data at the same timeBoth data will be destroyed


2.6 Access Methods

Access methods ensure that data can be sent successfully over shared

medium including

CSMA/CDCSMA/CA Token passing


2.6.1 CSMA/CD

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Computer ready send data 1. listens to the channel 2. If the channel is free,

2.1 sends the data. 2.2 listens to see whether collision occurs or not. 2.3 If there is a collision,

• waits a random period of time before re-send A random period is necessary, otherwise, the cycle “wait-send-

collide” would occur repeatedly. the most popular access method

used by Ethernet networks


2.6.2 CSMA/CA

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)

The computer ready to send data 1. listens to the channel 2. If the channel is free,

2.1 sends an “intend-to-send” signal 2.2 listens to see if there is another “intend-to-send” signal2.3 If there is another “intend-to-send” signal

• wait a random period of time, and repeat the above process – until the channel is absolutely free.

3. sends the data. CSMA/CA is commonly used by wireless LAN

because collisions cannot be detected.


2.6.3 Token Passing (1/2)

Token is a small piece of code created when the network is turned on moving around continuously

reaching each computer in turn

Token passing can only work in ring networks e.g. Token Ring



Token Passing Computer ready to send data

1. grabs the token2. adds data to the token

• including addresses and data content to it3. sends the token out as a packet

If the packet is delivered successfully,4.1.1 The receiver returns a message to acknowledge arrival 4.1.2 The sender sends out a new token

If no receiver is found, 4.2.1 The packets returns to the sender computer4.2.2 The sender either

• re-try or sends out a new token

chapter 2 different modes of data transmission 2.1bandwidth bandwidth 2.2serial and parallel...

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