• सुखार्थी त्यजते विद्यं विध्यार्थी त्यजते सुखम्सु sखर्थीन: कुतॊ विद्या कुतॊ विध्यार्थीन: सुखम् ||
  • “Luxury people leave knowledge, but a student leaves Luxury No knowledge for one who seeks Luxury, No luxury for student”
  • न चोर हार्यं न च राज हार्यं न भात्रू भाज्यं न च भारकारि |व्ययं कृते वर्धत एव नित्यं विद्याधनं सर्वधनप्रधानम ||
  • Cannot be snatched away by thief, cannot be snatched away by king, Cannot be divided among brothers, Not heavy either If spent daily, it always keeps growing. The wealth of knowledge is the precious of wealth of all”

Bridge




Bridge

A bridge operates in both the physical and the data link layer.

 

As a physical layer device, it regenerates the signal it receives.

 

As a data link layer device, the bridge can check the physical (MAC) addresses (source and destination) contained in the frame.



Filtering

 

A bridge has filtering capability which was not in repeater. It can check the destination address of a frame and decide if the frame should be forwarded or dropped.

If the frame is to be forwarded, the decision must specify the port. A bridge has a table that maps addresses to ports.


Example

As in Figure below two LANs are connected by a bridge. If a frame destined for station 612B13456182 arrives at port 1, the bridge consults its table to find the departing port.


According to its table, frames for 612B13456182 belong to LAN1 therefore, there is no need for forwarding, and the frame is dropped.

 

And if a frame for 752B13456112 arrives at port1, the departing port is port 2 the frame is forwarded.

 

In the first case, LAN 2 remains free of traffic; in the second case, both LANs have traffic.



Theoretically a bridge should be able to connect LANs using different protocols at the data link layer, such as an Ethernet LAN to a wireless LAN.



Factors to be considered while connecting two LANs working on different protocol

A bridge should be able to connect LANs using different protocols at the data link layer, such as an Ethernet LAN to a wireless LAN. However, there are many issues to be considered:

 

 1.      Frame format. Each LAN type has its own frame format

 

2.      Maximum data size. If an incoming frame's size is too large for the destination LAN, the data must be fragmented into several frames. The data then need to be reassembled at the destination. However, no protocol at the data link layer allows the fragmentation and reassembly of frames. Fragmentation is allowed at network layer level. The bridge must therefore discard any frames too large for its system.

 

 3.      Data rate. Each LAN type has its own data rate. (10-Mbps data rate of an Ethernet / 1-Mbps data rate of a wireless LAN.) The bridge must be able to buffer the frame to compensate for this difference.

 

4.      Bit order. Each LAN type has its own strategy in the sending of bits. Some send the most significant bit in a byte first; others send the least significant bit first.

 

 5.      Security. Some LANs, such as wireless LANs, implement security measures in the data link layer. Other LANs, such as Ethernet, do not. Security often involves encryption  When a bridge receives a frame from a wireless LAN, it needs to decrypt the message before forwarding it to an Ethernet LAN.

 

6.      Multimedia support. Some LANs support multimedia and the quality of services needed for this type of communication; others do not.