Untitled Lesson

Communication

Specification

Key Concepts

Symbol

A symbol is a particular pattern of bits represented by a signal.

For example, a symbol of four bits might be 1101.

We would say that this is a signal size of 4 bits.

Key Concepts

Baud rate

A communication system’s Baud rate is the number of signal changes in the
medium per second.

1 Baud (or 1Bd) is equal to 1 signal change per second.

Key Concepts

Bit Rate

A communication medium’s bit rate refers to the number of bits that are
transmitted over the medium per second.

This is often measured in bits per second (bps).

Therefore

A communication system’s bit rate is equal to its Baud rate multiplied by the

number of bits per signal in the communication medium.

A communication medium’s bit rate will be higher than its Baud rate if there is more than
one bit sent per signal.

Bandwidth

Bandwidth, expressed in Hertz, relates to the range of frequencies that a
communication medium is capable of transmitting.

There is a direct proportional relationship between bandwidth and bit rate.

Higher bandwidth results in a higher bit rate.

Latency

Latency in a communication medium, often measured in milliseconds, is the
difference in

time between an action being initiated and its effect being noticed.

For example, if you press the “R” key on your keyboard and the letter R appears
on screen 26ms later, the latency in the link between your keyboard and the
application you are using is 26ms.

Latency usually increases with distance.

Protocol

A protocol is a set of rules relating to communication between devices.

International organisations decide upon and publish protocols which allow devices
made by different manufacturers in opposite ends of the world to communicate
seamlessly.

Task

Complete Slides 2-7 on the activities.

Serial and parallel data

transmission

Serial Data Transmission

In serial data transmission, data is sent one bit at a time over one communication
line (this is usually a metal wire, but could also be an optical fibre or a wireless
channel).

Serial data transmission is frequently used for transmitting data over medium to
long distances (computationally speaking), such as from wired peripherals like
mice and keyboards to your computer.

Parallel Data Transmission

Parallel data transmission uses numerous parallel communication lines in order to send
multiple bits between components in a computer simultaneously.

The more lines that a parallel communication medium uses, the more data can be transferred

simultaneously.

Parallel Data Transmission

The more lines that a parallel communication medium uses, the more data can be
transferred simultaneously.

Each of the communication lines that forms part of a
parallel communication medium will have slightly
different electrical properties.

meaning that the time taken for one bit to be
transferred will differ slightly from line to line.

This means that bits sent together may not be
received together, a problem referred to as skew.

Skew

Skew is worst over long distances and, in extreme cases, can lead to bits from different

pulses overlapping, causing corruption of data.

Furthermore, parallel communication mediums are more expensive than their serial
counterparts because of their use of multiple lines.

For these reasons, parallel data transmission is most often used over short distances,
such as between parts of the processor and within RAM.

Crosstalk

Another issue, referred to as crosstalk, can occur with parallel data transmission.

When communication lines are tightly packed, signals from one line can “leak”
into another, another cause of data corruption.

Serial Vs Parallel

Serial data transmission doesn’t suffer from skew or crosstalk making it a
more reliable communication method, especially over long distances.

Serial communication mediums, which use just one line, are cheaper to
install than parallel mediums which use more than one line.

Task

Complete Slides 8-13.

Synchronous and Asynchronous

data transmission

Synchronous Transmission

When data is transmitted using synchronous transmission, a clock signal (which is
shared by both the sender and the receiver) is used to time when signals are sent.

Synchronous data transmission is used within the busses of a computer’s processor
in the fetch-execute cycle.

Synchronous Transmission

The signals, which are sent at regular intervals, will be received in the same order
that they were sent.

This makes synchronous data transmission suitable for transmitting information in
real-time systems.

Asynchronous transmission

In asynchronous data transmission, the requirement for a shared clock
signal is done away with by using start and stop bits to indicate the
duration of a transmission.

The start bit can be either a 0 or a 1 and the stop bit is always the
opposite of the start bit.

The sender and receiver must use the same Baud rate and need
only synchronise their clocks for the duration of data transmission.

Task

Complete Slides 14-18.