Data Comm.

 

 

Layers of TCP/IP

• Application
  
• Transport
  
• Network
  
• Data link
  
• Physical
  

  
   

 

Layer 7: The application layer - The application layer is responsible for providing services to the user.
This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.)

Layer 6: The presentation layer-This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer.

Layer 5: The session layer-This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination.

Layer 4: The transport layer-
The transport layer is responsible for delivery of a message from one process to another.
This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer.

Layer 3: The network layer-
The network layer is responsible for the delivery of packets from the original source to the
final destination. This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding.

Layer 2: The data-link layer-
The data link layer is responsible for transmitting frames from
one node to the next. This layer provides synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management.

Layer 1: The physical layer- The physical layer is responsible for transmitting individual bits from one node to the next. This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.

 

 

Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values.

 

 

 

 

Time and frequency domains

 

The bandwidth is a property of a medium: It is the difference between the highest and the lowest frequencies that the medium can satisfactorily pass.

 

 

Bit rate and bit interval

 

Error Detection and Correction

 

 

Detection

• A simple method of error detection is required to maintain data integrity
  
• The challenge in error detection is recognizing the different types of errors that occur in a simple and objective manner    
  

  
   

Redundancy

Redundancy means adding extra bits for detecting errors at the destination.

 

There are 4 types of redundancy checks:

• vertical redundancy check (VRC)
  
  • VRC can detect all single-bit errors.
    
  • VRC can detect multiple-bit and burst errors only if the total number of errors is odd.
    

  
   

1. Longitudinal redundancy check (LRC).
   

• Can detect multiple-bit and burst-errors.     
  
• Cannot detect errors where an even number of bits are inverted.
  

  
   

1. Cyclic redundancy check (CRC).
   

• Requires the addition of a frame check sequence (FCS) or error-detecting code to each synchronous frame.
  
• Performs a complex mathematical operation on the data (based on binary division).     
  
• Message is treated as one long binary number.     
  
• Divides entire numeric binary value of the block of data by a constant called a generator polynomial(n+1 bits).     
  
• Quotient is discarded, and the remainder (the CRC) is appended to the block and transmitted along with the data.     
  
• The receiver divides the data unit by the same number. If there is no remainder, message is intact.
  

  
   

1. checksum
   

• The error detection method used by the higher layer protocols of the OSI model is called checksum .     
  
• Computing a sum based upon the ASCII character
  

  
   

  
   

  
   

Error Correction

VRC, LRC, checksum, and CRC are effective in detecting errors, but cannot automatically correct the data.     

• Error correcting codes are more sophisticated than error-detection codes and require more redundancy bits.     
  
• The number of bits required correcting a multiple-bit or burst error is so high that it becomes too inefficient to correct.
  

      
  

  Single-Bit Error Correction
  

• The object of error correction is to locate the invalid bit or bits.     
  
• A redundancy code should adequately reflect all possible error locations.
  

Hamming Code

• The Hamming code is a method of error detection and error correction.     
  
• These redundancy bits are a function of the length of the data bits.
  
• For a data unit of m bits, the formula 2 r > m + r + 1 to determine r , the number of redundant bits needed.     
  
• Each r bit is the VRC bit for one combination of data bits; r 2 is the VRC bit for another combination of data bits, etc. (e.g. a 7-bit ASCII string would require 4 redundancy bits)     
  
• The receiver recalculates the VRC using the same sets of bits used by the sender plus the relevant parity ( r ) bit for each set.     
  
• It then assembles the new parity values into a binary number in order of r position.     
  
• Once a bit error is detected, the receiver can reverse its value and correct the error.
  

 

 

Single-bit error

 

Error

Detection

 

Multiple-Bit Error Correction

• Redundancy bits calculated on overlapping sets of data bits can also be used to correct multiple-bit errors.     
  
• Requires more sophisticated schemes.
  

 

5 component of data communication:

1. Sender
   
2. Receiver
   
3. Medium
   
4. Massage
   
5. Protocol
   

 

Type of Data Flow:

• Simplex : mainframe to monitor only
  
• Half duplex: between 2 workstation one direction way at one time.
  
• Full duplex: both directions at one time.
  

 

2 type of connection:

• Point to point
  
• Multipoint
  

 

4 type of topology:

• Mesh    
  
• Star
  

  
  

• Bus
  

  
  

• Ring    
  

 

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Chaper 2