Tuesday, January 26

Study Notes - Computer Network - Network Topology

A network may be represented as a graph with nodes representing computer or network devices like switches, routers etc and the links representing communication links.
Modes of communication may be broadcast or point to point (both may be shared or dedicated links)

LAN (Local Area Network) Topologies
1) Star
2) Ring
3) Bus

Shared broadcast links
  • point to point communication
  • Each pair of communicating nodes use the link for a short time
  • Other nodes ignore the communication
  • There has to be a distributed protocol to decide who gets to use the link.
Bus Topology
  • Single cable connects to all the computers
  • Each computer has a connector to a shared cable
  • Computers must synchronize and allow to transmit only one at a time

Advantages of Bus topology
  • Inexpensive to install
  • Easy to add stations
  • Use less cable than other topologies
  • Works well for small networks
Disadvantages of Bus topology
  • No longer recommended
  • Backbone breaks, whole network down
  • Limited number of devices can be attached
  • Difficult to isolate on problems
  • Sharing the same cable slows the response rate

Direct point to point communication (not shared)

  • Computers connected by communication channels that each connect exactly two computers
  • Forms mesh or point to point network
  • Allows flexibility in communication hardware, packet formats, etc
  • Provides security and privacy because communication channel is not shared

Star Topology

  • All computers connected to a hub
Extended Start Topology
  • A star network which has been expanded to include an additional hub or hubs.


Ring Topology
  • No beginning or end
  • All devices have equal access to media
  • Single ring - Data travels in one direction
  • Double ring - Allows fault tolerance
  • Each devices has to wait its turn to transmit
  • Most common type is Token ring (IEEE 802.5)
  • A token contains the data, reaches the destination, data extracted, acknowledgement of receipt sent back to transmitting device, removed, empty token passed on for another device to use.
Advantages of Ring Topology
  • Data packets travel at great speed, reason being it is a synchronous operation
  • No Collisions
  • Easier to find faults
  • Terminators not required
Disadvantages of Ring Topology
  • Requires more cable then bus
  • A break in the ring will bring it down
  • Not as common as the bus - less devices available
  • Ring is the most common topology in a wide area network
Token Ring
  • Many LAN technologies that use ring topology use token passing for synchronized access to the ring.
  • Ring itself is treated as a single, shared communication medium
  • Bits pass from transmitter, past other computers and are copied by destination
  • Hardware must be designed to to pass token even if attached computer is powered down.
Using the Token
  • When a computer wants to transmit, it waits for the Token.
  • After transmission, computer transmits token on ring
  • Next computer ready to transmit receives the token and then transmit

Mesh Topology
  • Not common in LAN
  • Most common used in WAN's to interconnect LAN's
  • Each node is connected to every other node
  • Allows communication to continue in the event of a break in any one connection. It is Fault Tolerant.
Advantages of Mesh topology
  • Improves fault tolerance
  • Can carry more data
Disadvantages of Mesh topology
  • Expensive
  • Difficult to install
  • Difficult to manage
  • Difficult to troubleshoot

Physical v/s Logical Topology
  • The actual layout of a network and its media is its physical topology
  • The way in which the data access the medium and transmits the packets is the Logical topology.

Factors included while deciding a topology

  • Cost
  • Scalability
  • Bandwidth capacity
  • Ease of installation
  • Ease of fault finding and maintenance


Bandwidth

  • Capacity of a media to carry information
  • Total capacity may be divided into channels
  • A channel is a portion of the total bandwidth used for a specific purpose
Analog Signals V/S Digital Signals

Digital
  • Less error prone
  • Distortion of the signal between source and destination can be eliminated
  • In Digital communications, it is often possible to reconstruct the original signal even after it has been affected by noise
Analog
  • Old technology
  • Little control over distortion
  • when noise affects an analog signal, it is hard to deduce the original signal
Nyquist's Limit
  • Suppose we know the bandwidth (H) of a channel and the number of signal levels (V) being used. What is the maximum number of bits that we can transmit?
  • Nyquist's limit Max_bps = 2*H*log(base 2)(V) bits/sec
  • Example, If the bandwidth is 3100Hz and we are using 16 level modulation then the maximum bits per second that can be transferred is
    max_bps = 2*3100*log(base 2)(16) = 24800 bps
  • So higher the bandwidth, more bits can be communicated.

Benefits of Digital Transmission
  • Reliability
    - Can regenerate slightly damaged signals
    - There are only two states. Change to the closest. Eg. If the two states are voltages +10V (1) and -10V (0) and the signal is +8V, the signal is concluded to be 1
  • Error detection and correction
  • Encryption
    - Makes it possible to carry sensitive information over the network.
  • Compression
    - Compress before transmission and decompress at the destination end. So the overall load is lighter. 
Digital signalling current state encoding
  • Data is encoding by the presence or absence of a signal
  • A positive voltage may be either represented as 1 or a 0
  • The current state indicates the value of the data
Transmission System
  • Analog Transmission
    - Not used with digital signal
    - Transmits analog signals without regard whether it represents digital or analog data
    - Uses amplifiers - also boosts noise
    - ok for voice
    - Can distort digital data
  • Digital Transmission
    - Concerned with the content of the signal
    - Uses repeaters which recover the pattern of 0's and 1's and re-transmits
    - Can be used with analog signals if it carries digital data. It recovers the digital data from the analog signal and generates new clean analog signal
    - It is becoming standard.

Data Encoding techniques
Analog Encoding of Digital Information
  • Amplitude shift keying
    - Two binary numbers (0,1) represented by two different amplitude of the carrier wave
    - Rather inefficient
    - Uses up to 1200 bps on voice grade lines
    - Used to transmit digital data over optical fiber
  • Frequency shift keying
    - Two binary numbers (0, 1) represented by two different frequencies of the carrier wave
    - Less susceptible to error than ASK
    - Used up to 1200 bps on voice grade lines
    - Commonly used for high frequency radio (4 to 30Mhz)
  • Phase shift keying
    - Two binary numbers represented by phase shift of the carrier wave.
    - More efficient and noise resistant than FSK
    - Used up to 9600 bps on voice grade lines

Baud rate - Maximum number of times Signal Changes per second
Bit rate - Bits per second

Digital Encoding of Analog Information
  • Commonly used in digitizing voice
  • Technique is called Pulse Code Modulation (PCM)
    - Sampling theory states if a signal is sampled at a rate twice the highest frequency then the samples contain all the information of the original signal.
    - Voice data in the telephone system is limited to a maximum of 45Khz
    - 8000 samples / sec should be sufficient
    - Each of these samples is assigned a binary code (8 bits)
    - Typically 8 bits are used which gives 256 levels

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