SAR Segmentation and Reassembly

SAR (Segmentation and Reassembly) is a technique used in network communications to transmit data over networks that have a smaller maximum transmission unit (MTU) than the size of the data being transmitted. It is commonly used in protocols such as ATM (Asynchronous Transfer Mode) and Frame Relay.

The purpose of SAR is to break down large data packets into smaller segments that can be transmitted over the network and then reassembled at the receiving end. This segmentation process ensures that the data can be transmitted in smaller units that fit within the network's MTU size.

Here is a detailed explanation of the SAR process:

Segmentation:

• The SAR process starts with the segmentation phase. The data packet, which is larger than the network's MTU, is divided into smaller segments or cells.
• Each segment typically consists of a header and a payload. The header contains control information, such as sequence numbers, to facilitate reassembly at the receiving end.
• The payload contains a portion of the original data packet, which is broken down into smaller pieces.
• The segmentation process continues until the entire data packet is divided into segments.

Reassembly:

• At the receiving end, the SAR process involves reassembling the segments back into the original data packet.
• The segments are received out of order, so the reassembly process requires organizing them correctly based on the sequence numbers in the headers.
• The receiving system collects the segments and uses the sequence numbers to determine the correct order of the segments.
• Once all the segments are received and arranged in the correct order, they are combined to reconstruct the original data packet.

Error Handling:

• SAR also includes error handling mechanisms to ensure data integrity during transmission.
• The segmentation process often includes adding error detection codes, such as checksums or cyclic redundancy checks (CRC), to each segment.
• At the receiving end, these error detection codes are used to verify the integrity of the segments.
• If any segments are found to be corrupted or lost during transmission, the receiving system can request retransmission of those specific segments.

Overall, SAR is a critical technique for transmitting data over networks with limited MTU sizes. It allows large data packets to be broken down into smaller segments, transmitted separately, and then reassembled at the receiving end. By dividing the data into smaller units, SAR enables efficient and reliable transmission, ensuring that the network's MTU limitations are not a barrier to effective communication.