Communication and Networks Assignment
Communications and Networks
version 1.0
Diploma in Information Technology
Copyright © 2020 by Singapore Institute of Management Pte Ltd. All rights reserved.
Lesson 12: Transmission Modes
1
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
2
Lesson 12 Outline
Transmission Modes
Serial Transmission Types
Channel Types
3
Serial vs Parallel Transmission
Source: https://www.youtube.com/watch?v=myU2x27FIIc
4
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
5
Transmission Modes Types
Source: Douglas, C (2016) Computer Networks and Internets
6
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
7
Parallel Transmission Illustration
Source: Douglas, C (2016) Computer Networks and Internets
Each wire carries one bit
8
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
9
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
10
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
11
Serial Transmission Illustration
Source: Douglas, C (2016) Computer Networks and Internets
12
Parallel & Serial Interface
Source: Douglas, C (2016) Computer Networks and Internets
MSB: Most Significant Bit
LSB: Least Significant Bit
Diagram shows the conversion
13
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
14
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
15
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
16
Byte Big-endian Bit Little-endian
Source: Douglas, C (2016) Computer Networks and Internets
What happens if bit big-endian and byte little-endian
17
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
18
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
19
Lesson 12 Outline
Transmission Modes
Serial Transmission Types
Channel Types
20
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
21
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
22
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
23
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
24
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
25
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
26
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
27
Synchronous Transmission Illustration
Async:
Sync:
Source: Douglas, C (2016) Computer Networks and Internets
28
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
29
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
30
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
31
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
32
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
33
Lesson 12 Outline
Transmission Modes
Serial Transmission Types
Channel Types
34
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
35
Channel Types Illustration
Source: Douglas, C (2016) Computer Networks and Internets
36
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
37
DTE and DCE
Source: https://www.youtube.com/watch?v=62tQoiqt2cc
38
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
39
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
40
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
41
DCE and DTE Illustration
Source: Douglas, C (2016) Computer Networks and Internets
42
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?
43
Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 9
44
End of Lesson
45