Data And Computer Communications - Basics
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
- 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.
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
Propagation Time
Also known as Propagation Delay.
the time for the data bits (electrical signal) to travel
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
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
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:
Since:
Propagation Time =
Round Trip Time means the Time Travel to and from.
Round Trip Time = 2(Propagation Time) =
(b)
Data rate = 128kb/s
Propagation Time = 184s
Therefore Bandwidth Delay Product =
©
Image Size:
Total Time = Propagation Time + Transmission Time
Since
Transmission Time =
Total Time = 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?
The length in the wire of such a bit is:
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: = 100 bytes
Message size: 150 bytes
Transmission Efficiency =
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 = bits.
Note In practice, We cannot divide like that because a fragment should be a multiple of 64 bits.
Q8.
[SOURCE ADDRESS][DESTINATION ADDRESS][SOURCE IP][DESTINATION IP][DATA]
Then