Bridges

Bridge:

  1. A bridge has two layers: Physical layer and the data link layer.
  2. It can perform all the task that device with the physical layer performs additional because of the data link layer it can perform some other tasks.
  3. It is used to connect two different types of LANs.
  4. It is a store and forward device. Inside a bridge, a collision domain is very less.

Because of the data link layer, it has some other functionality:-

  1. It can see the mac address contained in the frame.
  2. It has filtering capacity so Collison is very less or negligible.
  3. It can't change the mac address.

A bridge has two types of tables:  static table and dynamic table.

Bridge uses tables to map the incoming frames to respective ports.

 

Transparent Bridges:

As the name suggests a transparent, all the other stations are unaware of the existence of that bridge.

A system with a transparent bridge must meet the following criteria:-

Forwarding Learning and No looping.

1. Forwarding: means bridge should forward the frame to the correct destination.

 

2. Learning: means automatically updating the table with the help of flooding. To make a table dynamic, we need a bridge that gradually learns from the frame movements. To do this, the bridge inspects both the destination and the
source addresses.

 

3. Looping:

 

When two LANs are connected using two bridges looping problem happens.

Bridges use a spanning tree algorithm to solve the looping problem.

let's understand this looping problem with one diagram:

 

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Data link layer

Data Link Layer:
 It is responsible for node-to-node delivery of data.
 It receives the data from network layer and creates FRAMES, add physical address to these frames & pass them to physical layer.
 It consist of 2 layers:
 Logical Link Layer (LLC).
 Medium Access Control (MAC).
 Functions of Data Link Layer:
 It is used for synchronization and error control for the information.
 It divides the bits received from Network layer into frames.
 It provides Flow Control.
 To enable the error detection, it adds error detection bits to the data which is to be transmitted.

CRC in a trailer(reason):-

1. The Data Link Layer is the second layer of OSI Model.
2. It is responsible for node-to-node delivery of data.
3. It receives the data from network layer and creates FRAMES, add physical address to these frames & pass them to physical layer.
4. Data Link Layer delivers the frames using Hardware Address.
5. The CRC is computed while the packet is being transmitted and then incorporated in a trailer.
6. Similarly, the receiver computes the CRC and compares it with the transmitted one. 7. Thus it is more efficient to put CRC in a trailer. 8. Because only one pass needs to be made over the packet as it computes the CRC while scanning the packet, and then outputs it at the end (trailer). 9. If the CRC were in the header, then two passes would be necessary - one to compute the CRC, and one more to append it to the front of the packet.
10. Therefore using the trailer cuts the work in half.

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