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ISO/OSI Network Model

 

The standard model for networking protocols and distributed applications is the International Standard Organization’s Open System Interconnect (ISO/OSI) model. It defines seven network layers.
 


Layer 1 – Physical - 
Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP).


Layer 2 – Data Link - 
Data Link layer defines the format of data on the network. The data link layer handles the physical and logical connections to the packet’s destination, using a network interface. The data link layer’s protocol-specific header specifies the MAC address of the packet’s source and destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used.


Layer 3 – Network- 
NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129.79.16.40. The leading 1-3 bytes of the IP identify the network and the remaining bytes identify the host on that network.


Layer 4 - Transport - 
Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. TCP keeps track of the packet delivery order and the packets that must be resent. UDP on the other hand provides a low overhead transmission service, but with less error checking.


Layer 5 - Session - 
The session protocol defines the format of the data sent over the connections. RPC may be built on either TCP or UDP.


Layer 6 - Presentation - 
External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.


Layer 7 - Application - 
Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications.


TCP/IP Network Model

 

TCP/IP is designed around a simple four-layer scheme. The four network layers defined by TCP/IP model are as follows.

  • Layer 1 – Link - This layer defines the network hardware and device drivers.
  • Layer 2 – Network- This layer is used for basic communication, addressing and routing. TCP/IP uses IP and ICMP protocols at the network layer.
  • Layer 3 – Transport - Handles communication among programs on a network. TCP and UDP falls within this layer.
  • Layer 4 – Application - End-user applications reside at this layer. Commonly used applications include NFS, DNS, arp, rlogin, talk, ftp, ntp and traceroute.

Difference between OSI & TCP/IP Model

 

 


Ethernet Physical Layer

A Comparison of Various Ethernet and IEEE 802.3 Physical-Layer Specifications
 

 


IEEE 802.3 Frame Structure

 

  • Preamble :Each frame starts with a preamble of 7 bytes, each byte containing the bit pattern 10101010. Manchester encoding is employed here and this enables the receiver’s clock to synchronize with the sender’s and initialise itself.
  • Start of Frame Delimiter :This field containing a byte sequence 10101011 denotes the start of the frame itself.
  • Dest. Address :The standard allows 2-byte and 6-byte addresses. Note that the 2-byte addresses are always local addresses while the 6-byte ones can be local or global.

 

  • Multicast : Sending to group of stations. This is ensured by setting the first bit in either 2-byte/6-byte addresses to 1

Broadcast : Sending to all stations. This can be done by setting all bits in the address field to 1.All Ethernet cards(Nodes) are a member of this group.

  • Source Address :Refer to Dest. Address.
  • Length : The Length field tells how many bytes are present in the data field, from a minimum of 0 to a maximum of 1500. The Data and padding together can be from 46bytes to 1500 bytes as the valid frames must be at least 64 bytes long, thus if data is less than 46 bytes the amount of padding can be found out by length field.
  • Data :Actually this field can be split up into two parts - Data(0-1500 bytes) and Padding(0-46 bytes).
  • Frame Checksum : It is a 32-bit hash code of the data. If some bits are erroneously received by the destination (due to noise on the cable), the checksum computed by the destination wouldn’t match with the checksum sent and therefore the error will be detected.

Ethernet Frame Structure

 

 


IEEE 802.5: Token Ring Network

  • Token Ring is formed by the nodes connected in ring format as shown in the diagram below. The principle used in the token ring network is that a token is circulating in the ring and whichever node grabs that token will have right to transmit the data.

750.png 


Token Format

The token is the shortest frame transmitted (24 bit) MSB (Most Significant Bit) is always transmitted first - as opposed to Ethernet

755.png 

SD = Starting Delimiter (1 Octet)

AC = Access Control (1 Octet)

ED = Ending Delimiter (1 Octet)


Starting Delimiter Format:

760.png 

J = Code Violation

K = Code Violation


Access Control Format:

765.png 


T=Token

T = 0 for Token

T = 1 for Frame

When a station with a Frame to transmit detects a token which has a priority equal to or less than the Frame to be transmitted, it may change the token to a start-of-frame sequence and transmit the Frame


P = Priority

Priority Bits indicate tokens priority, and therefore, which stations are allowed to use it. Station can transmit if its priority as at least as high as that of the token.


M = Monitor

The monitor bit is used to prevent a token whose priority is greater than 0 or any frame from continuously circulating on the ring. If an active monitor detects a frame or a high priority token with the monitor bit equal to 1, the frame or token is aborted. This bit shall be transmitted as 0 in all frame and tokens. The active monitor inspects and modifies this bit. All other stations shall repeat this bit as received.


R = Reserved bits

The reserved bits allow station with high priority Frames to request that the next token be issued at the requested priority.


Ending Delimiter Format:

770.png 

J = Code Violation

K = Code Violation

I = Intermediate Frame Bit

E = Error Detected Bit


Frame Format:

MSB (Most Significant Bit) is always transmitted first - as opposed to Ethernet
 

775.png 

  • SD=Starting Delimiter(1 octet)
  • AC=Access Control(1 octet)
  • FC = Frame Control (1 Octet)
  • DA = Destination Address (2 or 6 Octets)
  • SA = Source Address (2 or 6 Octets)
  • DATA = Information 0 or more octets up to 4027
  • CRC = Checksum(4 Octets)
  • ED = Ending Delimiter (1 Octet)
  • FS=Frame Status

Starting Delimiter Format:

780.png 

J = Code Violation

K = Code Violation


Access Control Format:

786.png 

T=Token

T = “0” for Token,

T = “1” for Frame.

When a station with a Frame to transmit detects a token which has a priority equal to or less than the Frame to be transmitted, it may change the token to a start-of-frame sequence and transmit the Frame.


P = Priority

Bits Priority Bits indicate tokens priority, and therefore, which stations are allowed to use it. Station can transmit if its priority is atleast as high as that of token.


M = Monitor

The monitor bit is used to prevent a token whose priority is greater than 0 or any frame from continuously circulating on the ring. if an active monitor detects a frame or a high priority token with the monitor bit equal to 1, the frame or token is aborted. This bit shall be transmitted as 0 in all frame and tokens. The active monitor inspects and modifies this bit. All other stations shall repeat this bit as received.


R = Reserved bits
 the reserved bits allow station with high priority Frames to request that the next token be issued at the requested priority


Frame Control Format:

791.png 

FF= Type of Packet-Regular data packet or MAC layer packet

Control Bits= Used if the packet is for MAC layer protocol itself


Data Format:

No upper limit on amount of data as such, but it is limited by the token holding time.


Checksum:

The source computes and sets this value. Destination too calculates this value. If the two are different, it indicates an error, otherwise the data may be correct.


Frame Status:

It contains the A and C bits.

A bit set to 1: destination recognized the packet.

C bit set to 1: destination accepted the packet.

This arrangement provides an automatic acknowledgement for each frame. The A and C bits are present twice in the Frame Status to increase reliability in as much as they are not covered by the checksum.


Ending Delimiter Format:

796.png 

J = Code Violation

K = Code Violation

I = Intermediate Frame Bit

If this bit is set to 1, it indicates that this packet is an intermediate part of a bigger packet, the last packet would have this bit set to 0.


E = Error Detected Bit

This bit is set if any interface detects an error.





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