데이터 링크 제어 DLC(Data Link Control)

Framing

• When a message is carried in one very large frame, even a single-bit error would require the retransmission of the whole frame

fixed-size framming

• there is no defining the boundaries of frame
• the size itself can be used a delimiter
• example : ATM, WAN

Variable size framming

Character-Oriented framming

• carried 8-bit character (such as ASCII)
• To seperate one frame from next one, an 8-bit flag is added at beginning and the end of the frame
• The Header carries the source and destination and other control infromation (8-bit)
• The Trailer carries error detection redandunt bits (8-bit)

Bit-Oriented framming

• delimetiter to seperate one frame from the other, Protocol uses a special 8-bit pattern flag (01111110)
• In dataword, If we encounter 011111, to avoid misunderstaning with flag, add single 0 bit; 01111110 -> 011111010 (It called bit stuffing)

Bit-Oriented frame

Bit Stuffing

Flow and Error control

Flow control

• If the items are produced faster than they can be consumed, the consumer can be overwhelmed and may need to discard some ltems. (Vice versa)

Buffer

• AIthough flow control can be implemented in several ways, one of the solutions is normally to use two buffers
• A buffer is a set of memory locatlons that can hold packets at the sender and receiver
• When the buffer of the receiving data-link layer is full, it informs the sending data-link layer to stop pushing frames.

Error control

• Error control at the data-link layer is normally very simple and implemented using one of the following two methods.
• In both methods, a CRC is added to the frame header by the sender and checked by the receiver.
  • In the first method, if the frame is corrupted, it is silently discarded; if it is not corrupted,  the packet is delivered to the network layer. This method is used mostly in wired LANs such as Ethernet.
  • ln the second method, if the frame is corrupted, it is silently discarded; if it is not corrupted, an acknowledgment is sent(for the purpose of both flow and error control) to the sender.

 

Data Link Control Protocols

• Simple
• Stop and Wait
• Go Back N (disapeared)
• Selective Repeat (disapeared)

Piggyback

• In sender site, not immediatly send ACK
• Only if we have data to transfer.
• Add(Piggybacking) an ACK field within an existing data frame.