Frame Type Identification
The problems with the broadcast of the signal. For every signal broadcasted frame on the network each computer uses computational resources and then places the contents into memory, which interrupt the Processor. It makes the system software to make the decision whether to take or reject the frames. Another cause of problem is that if a pair of computer use broadcasting in place of sending them directly to all other computers waste Processors time while rejecting the frames.
Multicasting:
The solution to above problem is multicasting. It is the restricted form of broadcasting. It works like broadcasting but it does not forward frames automatically to the Processor. The hardware interface is programmed in advance to accept certain frames that have multicast address as the destination address. If an application program wants to receive certain frames then it program the hardware interface to accept an additional set of addresses.
The interface hardware frame then begins accepting three types of frames:
The interface hardware frame then begins accepting three types of frames:
- Multicast frames
- Broadcast frames
- The frames that are distend to the station itself.
Multicast Addressing:
If a computer is running an audio application. They can receive audio frames if the interface are programmed to received them and the other computers that are not running that audio application will not waste resources.
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Identifying Packet Contents:
The destination must get some clue about how to interpret frame data. For this purpose it can use two types which are as follows.
Explicit Frame Type:
The identifying value is included with frame that describes the types of the included data.
Implicit Frame Type:The identifying value is included with frame that describes the types of the included data.
In implicit frame the receiver must infer from frame data.
Headers and Frame Format:
LAN technology standards define frame format for each technology. All contemporary standards use the following general format.
- Frame header
- Payload
Frame header has address and other identifying information. Information typically in fields has fixed size and location. The data area may vary in size.
The different friends of ether frame format and their purposes is explained below:Field Purpose
Preamble Receiver synchronization
Destination Address Identifies intended receiver
Source Address Hardware address of sender
Frame Type Type of data carried in frame
Data Frame payload
CRC 32-bit CRC code
Ethernet Fields:
The preamble and CRC is often not shown in frame. The destination address of all is the broadcast address. There is special value reserved for frame type field.
Frame Without Type Fields:
Some LAN technologies do not include a type field. Sender and receiver can agree on interpretation, which is as follows:
They agree on single data format and use only that format this limits to one type of data.In this type all computers on LAN must use one format. They also agree to encode the data format into first few bytes of the data field.
Encoding the Data Type:
To ensure interoperability format of encoding area should be universally agreed upon it typically set by standards only.
Unknown Types:
For either encoding format some computer may not be prepared to accept frames of some types, which are unknown e.g. protocol type is not installed and the newly defined type. The receiving computer examines the field and discards any frame with unknown type.
Network Analyzers:
A network analyzer also called network monitor or a network sniffer is used to examine the performance of or debug a network. It can report statistics such as capacity utilization, distribution of frame size, collision rate or token circulation time.
Operation of Network Analyzers:
The operation of network analyzer is a computer with a network interface that receives all frames, which is called promiscuous mode. Many desktop computers have interface that can be configured for promiscuous mode. When combined with software computer can examine any frame on LAN. In this way the communication across LAN is guaranteed to be private. This computer receives and displays (but does not respond to) frames on the LAN. Network analyzer can be configured to filter and process frames. It can count frames of specific type of size. It displays only frames from or to specific computers. In general it can be configured to match any value of any field and capture only these frames meeting the filter specifications.
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