Ethernet in Networking | Ethernet Frame Format

Local Area Network-

 

• A Local Area Network (LAN) is a network of computers.
• It is confined to a small area which may be a room, building or a group of buildings.
• A LAN may be wired, wireless or a combination of the two.

 

LAN Technologies-

 

Standard technologies used to build a wired LAN are-

 

 

1. Ethernet
2. Token Ring

 

In this article, we will discuss about Ethernet and its Frame Format.

 

Ethernet-

 

• Ethernet is one of the standard LAN technologies used for building wired LANs.
• It is defined under IEEE 802.3.

 

Characteristics-

 

Point-01:

 

• Ethernet uses bus topology.
• In bus topology, all the stations are connected to a single half duplex link.

 

 

Point-02:

 

• Ethernet uses CSMA / CD as access control method to deal with the collisions.

 

Point-03:

 

• Ethernet uses Manchester Encoding Technique for converting data bits into signals.

 

Point-04:

 

• For Normal Ethernet, operational bandwidth is 10 Mbps.
• For Fast Ethernet, operational bandwidth is 100 Mbps.
• For Gigabit Ethernet, operational bandwidth is 1 Gbps.

 

Ethernet Frame Format-

 

IEEE 802.3 defines the following Ethernet frame format-

 

 

1. Preamble-

 

• It is a 7 byte field that contains a pattern of alternating 0’s and 1’s.
• It alerts the stations that a frame is going to start.
• It also enables the sender and receiver to establish bit synchronization.

 

2. Start Frame Delimiter (SFD)-

 

• It is a 1 byte field which is always set to 10101011.
• The last two bits “11” indicate the end of Start Frame Delimiter and marks the beginning of the frame.

 

NOTES The above two fields are added by the physical layer and represents the physical layer header. Sometimes, Start Frame Delimiter (SFD) is considered to be a part of Preamble. That is why, at many places, Preamble field length is described as 8 bytes.

 

3. Destination Address-

 

• It is a 6 byte field that contains the MAC address of the destination for which the data is destined.

 

4. Source Address-

 

• It is a 6 byte field that contains the MAC address of the source which is sending the data.

 

5. Length-

 

• It is a 2 byte field which specifies the length (number of bytes) of the data field.
• This field is required because Ethernet uses variable sized frames.

 

NOTES The maximum value that can be accommodated in this field = 216 – 1 = 65535. But it does not mean maximum data that can be sent in one frame is 65535 bytes. The maximum amount of data that can be sent in a Ethernet frame is 1500 bytes. This is to avoid the monopoly of any single station.

 

The following three fields collectively represents the Ethernet Header–

• Destination Address (6 bytes)
• Source Address (6 bytes)
• Length (2 bytes)

Thus, Ethernet Header Size = 14 bytes.

 

6. Data-

 

• It is a variable length field which contains the actual data.
• It is also called as a payload field.
• The length of this field lies in the range [ 46 bytes , 1500 bytes ].
• Thus, in a Ethernet frame, minimum data has to be 46 bytes and maximum data can be 1500 bytes.

 

Minimum Length of Data Field   Ethernet uses CSMA / CD as access control method to deal with collisions. For detecting the collisions, CSMA / CD requires- Minimum length of data packet = 2 x Propagation delay x Bandwidth Substituting the standard values of Ethernet, it is found that minimum length of the Ethernet frame has to be 64 bytes starting from the destination address field to the CRC field and 72 bytes including the Preamble and SFD fields. Therefore, minimum length of the data field has to be = 64 bytes – (6+6+2+4) bytes = 46 bytes   Maximum Length of Data Field   The maximum amount of data that can be sent in a Ethernet frame is 1500 bytes. This is to avoid the monopoly of any single station. If Ethernet allows the frames of big sizes, then other stations may not get the fair chance to send their data.

 

7. Frame Check Sequence (CRC)-

 

• It is a 4 byte field that contains the CRC code for error detection.

 

Advantages of Using Ethernet-

 

• It is simple to understand and implement.
• Its maintenance is easy.
• It is cheap.

 

Limitations of Using Ethernet-

 

Point-01:

 

• It can not be used for real time applications.
• Real time applications require the delivery of data within some time limit.
• Ethernet is not reliable because of high probability of collisions.
• High number of collisions may cause a delay in delivering the data to its destination.

 

Point-02:

 

• It can not be used for interactive applications.
• Interactive applications like chatting requires the delivery of even very small amount of data.
• Ethernet requires that minimum length of the data must be 46 bytes.

 

Point-03:

 

• It can not be used for client server applications.
• Client server applications require that server must be given higher priority than clients.
• Ethernet has no facility to set priorities.

 

Token Ring overcomes these limitations of Ethernet.

 

Important Concept-

 

For data transmission-

• TCP segment sits inside the IP datagram payload field.
• IP datagram sits inside the Ethernet payload field.

 

 

Next Article- Practice Problems On Ethernet

 

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