Communication and Networks Assignment

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Communications and Networks

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Diploma in Information Technology

Lesson 12: Transmission Modes

Lesson 12 Learning Outcomes

Distinguish between parallel and serial transmission

Distinguish between asynchronous and synchronous transmission

Define bits, bytes, blocks and frames

Distinguish between simplex, half-duplex and full-duplex transmission

Explain what is meant for an equipment to be DCE and DTE

Lesson 12 Outline

Transmission Modes

Serial Transmission Types

Channel Types

Serial vs Parallel Transmission

Source: https://www.youtube.com/watch?v=myU2x27FIIc

Transmission Modes

Transmission mode: manner in which data is sent over the underlying medium

Two fundamental categories:

Serial: one bit is sent at a time

Further categorised according to timing of transmissions

Parallel: multiple bits are sent at the same time

Transmission Modes Types

Source: Douglas, C (2016) Computer Networks and Internets

Parallel Transmission

Allows multiple data bits sent at the same time over separate media

Mostly used with a wired medium that uses multiple, independent wires

Signals on all wires are synchronised

a bit travels across each of the wires at precisely the same time

Parallel Transmission Illustration

Source: Douglas, C (2016) Computer Networks and Internets

Each wire carries one bit

Parallel Transmission Notes

In addition to parallel wires that each carry data, a parallel interface usually contains other wires

To allow sender and receiver to coordinate

To make installation and troubleshooting easy, wires for parallel transmission system are placed in a single physical cable

Parallel Transmission Benefits

High speed: can send N bits at the same time

parallel interface can operate N times faster than an equivalent serial interface

Match to underlying hardware: computer and communication hardware uses parallel circuitry

parallel interface matches internal hardware well

Serial Transmission

Parallel is superior but most communication systems use serial mode

Serial can be extended over long distances at much less cost

Using only one wire means there will never be a timing problem caused by one being slightly longer than another

However, devices must contain a hardware to converts data from parallel circuitry to serial form for the wire

Serial Transmission Illustration

Source: Douglas, C (2016) Computer Networks and Internets

Parallel & Serial Interface

Source: Douglas, C (2016) Computer Networks and Internets

MSB: Most Significant Bit

LSB: Least Significant Bit

Diagram shows the conversion

Serial Transmission Hardware

Hardware needed to convert data between parallel and serial can be straightforward or complex

Depend on type of serial communication

Universal Asynchronous Receiver and Transmitter (UART): chip that performs the conversion

Universal Synchronous-Asynchronous Receiver and Transmitter (USART): chip that handles conversion for synchronous networks

Serial Transmission Order

What should sender transmit first:

Most Significant Bit (MSB): big-endian to describe a system that sends the MSB first

Least Significant Bit (LSB): little-endian to describe a system that sends the LSB first

Either form can be used, but sender and receiver must agree

Ethernet Transmission Order

Order in which bits are transmitted does not settle the question of transmission order

Data in a computer is divided into bytes, and each byte is further divided into 8bits

Possible to choose a byte order and a bit order independently

Ethernet technology specifies data is sent byte big-endian and bit little-endian

Byte Big-endian Bit Little-endian

Source: Douglas, C (2016) Computer Networks and Internets

What happens if bit big-endian and byte little-endian

Serial Transmission Timing

Asynchronous: transmission can occur at any time

arbitrary delay between transmission of two data items

Synchronous: transmission occurs continuously

no gap between transmission of two data items

Isochronous transmission occurs at regular intervals

fixed gap between transmission of two data items

Practice 12.1

Suppose each character is represented by 8-bit, suggest the possible transmission modes for the following and explain why.

Achieve high speed

Achieve long distance at low cost

Lesson 12 Outline

Transmission Modes

Serial Transmission Types

Channel Types

Asynchronous Transmission

System is asynchronous if it allows physical media to be idle for some time between two transmissions

Asynchronous style of communication is well-suited to applications that generate data at random

User typing on a keyboard or clicks on a link

Asynchronous Transmission Issues

Issues arises from the lack of coordination between sender and receiver

Receiver don’t know how long medium will remain idle before more data arrives

Asynchronous usually arrange for sender to transmit a few extra bits before each data item

Preamble or Start bits: extra bits to inform data transfer is starting and allow receiver to synchronise with incoming signal

RS-232 Character Transmission

Consider transfer of characters across copper wires between a computer and a device keyboard

Each data item represents one character

Standardized by Electronic Industries Alliance (EIA)

Most widely used for character communication

Known as RS-232-C, and commonly abbreviated RS-232

EIA Specification for RS-232

EIA standard specifies the details like:

physical connection size: max cable length 50 feet long

electrical details: between -15v +15v

the line coding being used

Can be configured to control exact number of bits per second

Can be configured to send

RS-232 Illustration

Voltage varies at different stages

When a start bit, eight bits of a character, and a stop bit are sent

Diagram shows voltage for 8-bits character

Source: Douglas, C (2016) Computer Networks and Internets

Synchronous Transmission

Synchronous mechanism transmits bits continually

No idle time between bits

after transmitting final bit of one data byte, sender transmits bit of the next data byte

Sender and receiver constantly remain synchronized

Less synchronization overhead

Synchronous vs Asynchronous

Asynchronous RS-232 requires an extra start bit and stop bit

8-bit character requires minimum of 10-bit time, even if no idle time

On a synchronous system

each character is sent without start or stop bits

Synchronous Transmission Illustration

Async:

Sync:

Source: Douglas, C (2016) Computer Networks and Internets

Framing

Sometimes sender does not have data ready to send

Can make use of the idea of framing

Framing: interface added to a synchronous mechanism that accepts and delivers a block of bytes

Frame: blocks of bytes in framing

Framing Synchronisation

To ensure sender and receiver stay synchronized, a frame starts with a special sequence of bits

Idle sequence or idle byte: transmitted when sender has no data to send

Source: Douglas, C (2016) Computer Networks and Internets

Isochronous Transmission

Isochronous transmission: delivering data at a steady rate

provide steady bit flow for multimedia applications

Important to reduce jitter that disrupt reception

cause pops or clicks in audio/make video freeze for a short time

Jitter: variability of delay

Isochronous Network

Isochronous network is designed to accept and send data at a fixed rate, R

Data must be handed to the network for transmission at exactly R bits per second

Suppose an isochronous mechanism designed to transfer voice operates at a rate 64000bits/second

Sender generate digitized audio continuously

Receiver must be able to accept and play the stream

Practice 12.2

Propose and explain for each of the following type of scenarios, what will be a suitable serial transmission to use.

Minimum idle time in the physical medium

User typing on a keyboard

Real-time streaming video

Lesson 12 Outline

Transmission Modes

Serial Transmission Types

Channel Types

Channels Types

Depending on direction of transfer:

Simplex: transfer data in a single direction

analogous to broadcast radio or television

Full-Duplex: allows transmission in two directions simultaneously

analogous to a voice telephone conversation

participant can speak even if they are able to hear background music at the other end

Half-Duplex: transmission in two direction but one at a time

Channel Types Illustration

Source: Douglas, C (2016) Computer Networks and Internets

Half-duplex Transmission

Uses shared transmission medium

Shared medium can be used for communication in each direction

Analogous to using walkie-talkies where only one side can transmit at a time

Additional mechanism is needed at each end to coordinate transmission to ensure only one side transmits at a given time

DTE and DCE

Source: https://www.youtube.com/watch?v=62tQoiqt2cc

DTE and DCE Terms

Data Communications Equipment (DCE) and Data Terminal Equipment (DTE) were originally created by AT&T to distinguish between

DCE are communications equipment owned by phone company

DTE are terminal equipment owned by a subscriber

DTE and DCE Example

If a business leases a data circuit from a phone company

Phone company installs DCE equipment at business

Business purchases DTE equipment that attaches to the phone company’s equipment

Ownership Concept

Concept of DCE-DTE distinction is not ownership of the equipment

It lies in the ability to define an arbitrary interface for a user

If underlying network uses synchronous transmission

DCE equipment can provide either synchronous or isochronous interface to user equipment

DCE and DTE Illustration

Source: Douglas, C (2016) Computer Networks and Internets

Practice 12.3

Suppose a user sign up with an Internet Service Provider (ISP) for a fiber plan. The ISP will have to setup a modem in his premise. On this modem, the user plugs in his own network device onto the modem and connects his end-user devices to the network device.

What are some examples of end-user devices?

What kind of network device can the user attach to the modem. How can his end-user devices connect to it?

Which device(s) is a DCE and which is a DTE?

Reading

Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 9

End of Lesson