Multicasting
There are problems with broadcasting.
It wastes CPU time to discard the frames for the computers who didn't need the message.
In multicast, the interface hardware is programmed in a way to accept multicast address.
The frames are not directly forwarded to the CPU.
There are 3 types of frames the interface hardware now deals with:
1. Multicast frames.
2. Broadcast frames.
3. Frames that were directly destined to the stations (through physical address).
Multicast Addressing
Example: imagine 2 computers running an audio application.
Their interface is programmed to accept the audio frames.
Other computers will ignore the frames, who are not running that audio application.
Identifying Packet Contents
There needs to be a way for destination to know how to interpret the incoming frames.
Explicit Frame Type
In this type, the identifying value is included with the frame which describes the frame data.
Implicit Frame Type
In this type, the frame data has to be read and its type is deduced from that data.
Header Frame Format
This figures shows the Ethernet frame format
Those numbers represent bytes.
| Fields | Purpose |
|---|---|
| Preamble | Receiver Synchronization |
| Destination Address | Identifies intended receiver |
| Source Address | Hardware address of sender |
| Frame Type | Type of data carried in the frame |
| Data | Frame Payload |
| CRC | 32-bit CRC code |
Frame without Type Fields
Some LAN technologies do not use the frame type field.
This causes all the computers in the network to agree upon using one single data format.
Encoding Data Type
The data type is encoded within the frame data.
IEEE 802.2 LLC
This standard uses Logical Link Control Sub Network Attached Point header.
OUI stands for organizationally unique identifier.
Unknown Types
Some computers might not have the protocol type installed which makes them blind to the type of frame they are receiving.
Hence, they discard it.
Network Analyzers
A network analyzer is also called network monitor or network sniffer.
It is used to analyze the performance of a network or debug it.
It shows information like capacity utilization, frame distribution size, collision rates, token circulation time etc.
A computer can be configured as promiscuous mode which allows it to see all frames (for analysis).