Data communication model

Key Concept

Terminology

Data transmission occurs between a transmitter & receiver via some medium
Data
- Entites that convey meaning
Signals & Signalling
- Electric or electromagnetic representations of data, physically propagates along medium
Transmission
- Communication of data by propagation and processing of signals

Communication Model

Source: Generates information/message to be transmitted
Transmitter: Converts information/messages into transmittable electrical signals
Transmission System (or Channel): Carries signals
Receiver: Converts received electrical signal into information/message
Destination: Takes incoming information/message

Data Transmission

Source Data:

Digital data - discrete, e.g. text and integers
Analog data - continuous, e.g. audio

Transmitted Signal:

Digital signal - a sequence of voltage pulses transmitted over a medium
Analog signal - a continuous varying electromagnetic wave

4 Types of Data Transmission

Analog Tranmission

Analog data, analog signal: Amplitude Modulation (AM), FM, PM.
Digital data, analog signal: Amplitude Shift Keying (ASK), FSK, PSK.

Digital Transmission

Digital data, digital signal: NRZ-L, NRZI, Bipolar-AMI, …
Analog data, digital signal: Pulse Code Modulation (PCM)

Communication Network

In practice, many devices want to communicate.

Point to point (Fully Connected) communication is not usually practical
- Devices are too far apart
- Large set of devices would need impractical number of connections $(\frac{N(N-1)}{2})$ $(\frac{N ( N - 1 )}{2})$
  - If N is very large, then we need a huge number of connection.

So we use a Communication Network. A Network is the interconnect of a set of devices capable of communication.

A device can be a host
A device can also be a connecting device

Hosting device can be any computer, workstationn, cellular phone, or security system.

Connecting Network Device

Hub - Now we seldom use it
Router - (Layer 3) to link up to different networks
Switch - (Layer 2) to form local network

Devices are connected using wired or wireless media

Open vs Closed Networking

Closed networks	Open networks
Each vendor designs / builds their own (Secure)	Mulitple groups collaborate to define a technology (Competitive approach)
Given technology owned by vendor	To insure interoperability, specifications written in Standards documents that are available to everyone (Known to public)
vendor may license technology to other groups	Companies build products according to standards

Organizations Issue Standards

IEEE - governs local network and wifi standard
IETF - governs internet protocol standard
ITU - governs telecommunication standard
ISO
W3C
and many others

Switching Techniques of Communication Network

Circuit Switching - Dedicated circuit per call: Telephone net
Packet Swtiching - Data sent through network in discrete “chunks”

Overview of Internet

In practice, all devices cannot be simply connected to one single network.

Near-by devices will be connected to a network, different networks will then be linked up through the network device (router). This interconnection of networks formed Internet (Internetwork).

Internet Structure

The Internet Structure can be divided into 3 parts.

Network edge

Applications and hosts

Network core

A group of Routers
Network of networks

Internet uses packet switching: Data sent through network in discrete “chunks”

Access network

Connect hosts to the network core

Internet Today

Peers means entities are in the same levels.

Internet Administration

Various groups that coordinate Internet issues have guided this growth and development.

Internet Society (ISOC)

Protocols and Services

A protocol is a set of rules and formats that govern the communication between communicating peers.
When communication is complex, we need several protocols at different levels/layers, or protocol layering.

Key Elements of a Protocol

When we set up a protocol, we need to specify the format.

Syntax

Data formats
Signal

Semantics

Control information
Error handling

Timing

Speed matching
Sequencing

Protocol Layering

A complex task of communication is broken up into modules.

Signal generation
Interfacing
Synchronization
Error detection and correction  Addressing and routing
Recovery
Message formatting
Security

Each module is performed at different level/layer.

Base on the idea, people developed two models:

OSI 7-Layer Model

OSI means Open Systems Interconnection.

Protocol details are publicly available
Changes are managed by an organization whose membership and transactions are open to the public

Each layer performs a subset of the required communication functions
Each layer relies on the next lower layer to perform more primitive functions
Each layer provides services to the next higher layer
Changes in one layer should not require changes in other layers

The 7-Layers

You need a

Physical connection to
Link to a
Network in order to
Transport data of a communication
Session for
Presentation (Presenting them in an
Application

Organization of the Layers

User Support Group
- Application Layer
- Presentation Layer
- Session Layer
Transport (Bridging)
- Transport Layer
Network Support Group
- Network Layer
- Data link Layer
- Physical Layer

TCP/IP Model

It is similar to the OSI model.

Encapsulation and Decapsulation

Addressing Level

Packet names and Addresses are called differently in different layers.

Packet names	Layers	Addresses
Message	Application layer	Names
Segment	Transport layer	Port numbers
Packet	Network layer	Logical/IP address
Frame	Data-link layer	Physical/MAC/ethernet addresses
Bits	Physical layer

OSI vs TCP/IP

Performance Measures

Performance measures of a computer network.

Two common ways in evaluating the performance of a computer network:

Analytical Methods
Computer Simulations

Throughput

Throughput means the actual output rate (bits/s) of the system.

$\text{Throughput} = \frac{\text{Transfer Size (bits)}}{\text{Transfer time (s)}} = \frac{\text{Data Volume (bits)}}{\text{Required time (s)}}$

Utilization

Utilization means how we utilize the bandwidth (resources).

End-to-End Delay

End-to-End Delay means how long it takes to travel from source to the destination.

Transmission Time

Also known as Transmission Delay.

the time to send out data bits into medium

$\text{Transmission time} = \frac{\text{Data size (bits)}}{\text{Data Rate(bps)}}$

Propagation Time

Also known as Propagation Delay.

the time for the data bits (electrical signal) to travel

$\text{Propagation Time} = \frac{\text{Distance}(m)}{\text{Speed of light}(3 \times 10^8m/s)}$

The transmission delay is the amount of time required for the router to push out the packet.

The propagation delay is the time it takes a bit to propagate from one router to the next.

Queueing Delay

If there is a router, there is a buffer in the router for processing the data bits when the packet come in.

Processing Delay

the router need time to process the packet and update the header of packet.

Loss Rate

the percentage that the data bits (packets) are lost

Bandwidth Delay Product

Also known as Delay bandwidth product.

the maximum number of bits that could be in transit through the physical medium at any given instant

$\text{Bandwidth Delay Product} = \text{Data rate} \times \text{Propagation Delay}$

Bandwidth describes the maximum data transfer rate of a network or Internet connection.

Key Concept Review Q&A

Review Questions

What are the major components of a data communication system?

What is a computer network?

How do we classify a computer network?

What is a protocol?

What are the advantages of using layering protocol architecture?

List the layers of the Internet model and the OSI model.

Which layers in the Internet model are the network support layers?

Which layer in the Internet model is the user support layer?

What is a peer-to-peer process?

How does information get passed from one layer to the next in the Internet model?

What are headers and trailers, and how do they get added and removed?

What is the difference between a port address, a logical address, and a physical address?

Name some services provided by the application layer in the Internet model.

How do the layers of the Internet model correlate to the layers of the OSI model?

Problems

Note:

ps: 10^-12 s

ns: 10^-9 s

$\mu$ s: 10^-6 s

ms: 10^-3 s

KB : 10^3 B

MB : 10^6 B

GB : 10^9 B

TB : 10^12 B

Q1.

(a)

Distance: $55 \times 10^9 m$

Since: $\text{Propagation Time} = \frac{\text{Distance}(m)}{\text{Speed of light}(3 \times 10^8m/s)}$

Propagation Time = $\frac{55 \times 10^9 m}{3 \times 10^8 m/s} = 183.33s = 184s$

Round Trip Time means the Time Travel to and from.

Round Trip Time = 2(Propagation Time) = $368s$

(b)

$\text{Bandwidth Delay Product} = \text{Data rate} \times \text{Propagation Delay}$

Data rate = 128kb/s

Propagation Time = 184s

Therefore Bandwidth Delay Product = $128kb/s \times 184s = 23552kb$

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Image Size: $5 \times 10^6Byte$

Total Time = Propagation Time + Transmission Time

Since $\text{Transmission time} = \frac{\text{Data size (bits)}}{\text{Data Rate(bps)}}$

Transmission Time = $\frac{5 \times 10^6 \times 8 bit}{128 \times 10^3 bit/s} = 312.5s$

Total Time = $184 + 312.5 = 496.5$ seconds

Q2.

How “wide” is a bit on a 10-Gbps link? It means how much times it occupy for 1 bit. In other words, How long does it take to send 1 bit?

$\frac{1 bit}{10 \times 10^9bps} = 10^{-10}s = 0.1ns$

The length in the wire of such a bit is:

$0.1ns \times 2.3 \times 108 m/s = 2.3 \times 10^{-2} = 23mm$

Q3.

For a. We don’t really care about the time, We only care we can see the email or not.

For b. We don’t really care about the time, We only care the file.

For c. Video is a delay-sensitive data. When the delay is very long, it will pause/payback the video.

Q4.

a. Route determination - Routing is belong to Network Layer

b. Flow control - Link Layer and Transport Layer

c. Interface to transmission media - Physical Layer

d. Provides access for the end user - Application Layer

Q5.

a. Reliable process-to-process message delivery - Transport Layer

b. Route selection - Network Layer

c. Define frames - Link Layer

d. Provides user services such as e-mail and file transfer - Application Layer

e. Transmission of bit stream across physical medium - Physical Layer

Q6.

Total Header: $5 \times 20$ = 100 bytes

Message size: 150 bytes

$\text{Transmission Efficiency} = \frac{\text{useful data}}{\text{total data sent}}$

Transmission Efficiency = $\frac{150}{100 + 150} \times 100\% = 60\%$

As we can see the efficiency is not that high, it is better to reduce size of header.

Q7.

TCP Layer: 1500 + 160 = 1660 bits

IP Layer: (1500 + 160) + 160 = 1820 bits

Then transmitted through 2 networks (destination), each of which uses a 24-bit frame header. The destination network has a maximum frame size of 800 bits.

Therefore at most the data field can store 800 - 24 = 776 bits.

Since 1820 > 776, It cannot We need to carry fragmentation to divide our IP packet in smaller piece.

Note if we divide our IP packet, each piece should be a completed IP packet including Header and Data.

In destination network, at most the data field can store 800 - 24 - 160 = 616 bits.

Therefore 1660 = 616 + 616 + 428
We divided into 3 IP packets, then add back the header.

Therefore the 3 packets are: 776, 776, 588

The 3 IP packets will be encapsulated into 3 frames, and 3 24-bit headers will be generated. Thus, the total number of bits delivered = $(776 + 776 + 588) + 3 \times 24 = 2212$ bits.

Note In practice, We cannot divide like that because a fragment should be a multiple of 64 bits.