Lecture 4 Data Transmission Modes and Switchingeng.staff.alexu.edu.eg/.../Lecture_4_Data_Transmission_Switching.pdf · Data Transmission Modes How are data sent via computer networks

Lecture 4

Data Transmission Modes and Switching

Data Transmission & Switching 1-1

Agenda

Introduction to Data Transmission Modes

Parallel and Serial Transmission

Transmission Media

Guided and unguided media

Data Switching Techniques

Circuit Switching

Packet Switching


The Heterogeneity in the Internet


billions of connected computing devices: hosts = end systems

running network apps Home network

Institutional network

Mobile network

Global ISP

Regional ISP

router

PC

server

wireless laptop

cellular handheld

wired links

access points

communication links

fiber, copper, radio, satellite

transmission rate = bandwidth

routers: forward packets (chunks of data)

Data Transmission Modes

How are data sent via computer networks and

related connecting wires/cables ?

Do we send 1 bit at a time ?

Do we group bits into larger groups ?

The transmission of binary data across a link

can be accomplished in either parallel or serial

mode


Data Transmission Modes


Parallel Data Transmission Binary data may be organized into groups of n bits

each (i.e. n parallel channels) Multiple bits (usually 8 bits or a byte/character) are

sent simultaneously on different wires/frequency channels with each clock tick

Major advantage: speed

Major disadvantage: cost


Serial Data Transmission

One communication channel rather than n to transmit data between two communicating devices one bit is sent with each clock tick

Major advantage: reduced cost

Major disadvantage: speed

Note: communication within devices is parallel, so there is a need to a converter (some overhead time is needed)



Serial transmission occurs in one of three ways

Asynchronous

• groups of bits are sent as independent units with start/stop

flags and no data link synchronization




Synchronous

• sending bits one after another without start or stop bits or

gaps. It is the responsibility of the receiver to group the bits.




Isochronous

• For real-time data transmission (e.g. audio and video

applications)

• Guaranteeing delays between an entire sets of frames to be

accepted (synchronous fails in guaranteeing fixed delay for

data arrivals)

• Allowing a part of the access time of a channel to send

isochronous real time data



Transmission Media

Transmission media are actually located below the physical layer and are directly controlled by the physical layer

A transmission medium can be broadly defined as anything that can carry information from a source to a destination free space, metallic cable, or fiber-optic cable


Transmission Media

Some Challenges in Transmission Media Unreliability due to the poor quality of the wires

• Noise

• Contention

Long distances • Power attenuation

Various applications and operating communication frequency bands

• Wide frequency spectrum

Electromagnetic energy and its impacts • Reactive power


Transmission Media Two broad categories

Guided media • A signal traveling along any of these media is directed and

contained by the physical limits of the medium

Unguided media • Signals are normally broadcast through free space and thus are

available to anyone who has a device capable of receiving them


Transmission Media


Guided Transmission Media Twisted-Pair Cable (shielded and unshielded)

Telephone lines

Coaxial Cable Cable TV networks

Fiber Optic Cable Backbone networks


Unguided Transmission Media

Unguided signals can travel from a source to a destination in several ways Ground propagation

• radio waves travel through the lowest portion of the atmosphere

Sky propagation • higher-frequency radio waves radiate upward into the

ionosphere where they are reflected back to earth

Line-of-sight propagation • very high-frequency signals are transmitted in straight lines

directly from antenna to antenna


Data Switching

A network is a set of connected devices

how to connect them to make one-to-one communication possible ? point-to-point connection between each pair of

devices

between a central device and every other device

impractical and wasteful when applied to very large networks !!!

A practical solution is network switching


Switched Network

switched network consists of a series of interlinked nodes, called switches

Switches are devices capable of creating temporary connections between two or more devices linked to the switch


Taxonomy of Switched Networks



Message Switched Networks

Data routed in its entirety from the source node to

the destination node, one hope at a time



Message Switched Networks

In message switching, each switch stores the

whole message and forwards it to the next

switch

• No packetization

• No resource allocation

It is still used in some applications like

electronic mail (e-mail)



Circuit Switched Networks

made of a set of switches connected by physical

links, in which each link is divided into n channels




A connection between two stations is a dedicated

path made of one or more links

Each connection uses only one dedicated channel on

each link

Each link is normally divided into n channels by using

multiplexing techniques, such as FDM or TDM

(making shared medium)




FDM

frequency

time

TDM

frequency

time

4 users

Example:


Circuit Switched Networks Circuit switching takes place at the physical layer

Before starting communication, the stations must make a reservation for the resources to be used during the communication (resource reservation)

The data are a continuous flow sent by the source station and received by the destination station (continuous data flow)

There is no addressing involved during data transfer (no addressing)


Example (circuit switched networks) Design and sketch a circuit-switched network that connects computers in two remote offices of a private company. The offices are connected using a T-1 line leased from a communication service provider.

There are two 4 X 8 (4 inputs and 8 outputs) switches in this network.

For each switch, four output ports are folded into the input ports to allow communication between computers in the same office.

Four other output ports allow communication between the two offices.


Example (circuit switched networks)


Solution

Structure of Circuit Switches (Example)


Time Division Switch (TDS) Time Slot Interchange (TSI)


Operation Phases

Setup Phase

• A dedicated circuit needs to be established

Data Transfer Phase

• After the establishment of the dedicated circuit, the two

parties can transfer data

Teardown Phase

• When one of the parties needs to disconnect, a signal is

sent to each switch to release the resources. Data Transmission & Switching 1-30

Delay in a Circuit-Switched Network


Setup the channel

The 𝑡𝑜𝑡𝑎𝑙 𝑑𝑒𝑙𝑎𝑦 = 𝑇𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛+ 𝑇𝑝𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 + 𝑇𝑆𝑒𝑡𝑢𝑝 + 𝑇𝑡𝑒𝑎𝑟𝑑𝑜𝑤𝑛


Packet Switched Networks: Datagram Networks

The data message is divided into packets of fixed or

variable size

The size of the packet is determined by the network

and the governing protocol

There is no resource pre-allocation for a packet

No reserved bandwidth on the links

No scheduled processing time for each packet


Taxonomy of Switched Networks Packet Switched Networks: Datagram Networks

Each packet (called datagram) is treated

independently of all others (stop and forward)

Packets belong to the same message may travel

different paths to reach the destination


Delay in a Datagram Network


For n links,

The 𝑡𝑜𝑡𝑎𝑙 𝑑𝑒𝑙𝑎𝑦 = 𝑛 × 𝑇𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛+ 𝑛 × 𝑇𝑝𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 + (𝑛 − 1) × 𝑇𝑤𝑎𝑖𝑡𝑖𝑛𝑔

Switching in the Internet Switching in the Internet is done by using the datagram approach to

packet switching at the network layer


Taxonomy of Switched Networks Packet Switched Networks: Virtual Circuit

Switched Networks A virtual-circuit network is a cross between a circuit-

switched network and a datagram network

There are setup and teardown phases

Resources can be allocated during the setup phase

Data are packetized and each packet carries an address in the header (common with datagram)

All packets follow the same path established during the connection (common with circuit switching)

Normally implemented in the data link layer (shared medium)

• Circuit-switched network is implemented in the physical layer

• Datagram network in the network layer 1-36

Delay in a Virtual Circuit Switched Network


For n links,

The 𝑡𝑜𝑡𝑎𝑙 𝑑𝑒𝑙𝑎𝑦 = 𝑛 × 𝑇𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛+ 𝑛 × 𝑇𝑝𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 + 𝑇𝑆𝑒𝑡𝑢𝑝 + 𝑇𝑡𝑒𝑎𝑟𝑑𝑜𝑤𝑛

Addressing in Virtual Circuit Switched Networks

Global Addressing

A source or a destination needs to have a global

address (i.e. an address that can be unique)

Local Addressing

Virtual-Circuit Identifier (VCI)

• A small number that has only switch scope

• It is used by a frame between two switches

• When a frame arrives at a switch, it has a VCI; when

it leaves, it has a different VCI Data Transmission & Switching 1-38

Comparison between circuit and packet switching


Exercise (a) Use circuit switching and packet switching networks techniques to allocate bandwidth for 4 network users

(b) Study the special case for a packet switching network when you have 20 users and there will be 4 active users

Each user generates 250kbps when “active”

Probability that user is active 25% of time

Hint: calculate the probability that there are 4 or less simultaneously active users


router 1 Mbps 4 users

Link

Lecture-related Questions

Why does a circuit-switched network need end-to-end

addressing during the setup and teardown phases? Why

are no addresses needed during the data transfer phase

for this type of network?

What is the role of the address field in a packet traveling

through a datagram network?



Lecture Summary Covered material

Introduction to Data Transmission Modes Parallel and Serial Transmission

Transmission Media Guided and unguided media

Data Switching Techniques Circuit Switching

Packet Switching

Material to be covered next lecture Introduction to the Data Link Layer

Error Detection and Correction Flow and Error Control

Lecture 4 Data Transmission Modes and Switchingeng.staff.alexu.edu.eg/.../Lecture_4_Data_Transmission_Switching.pdf · Data Transmission Modes How are data sent via computer networks

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