Fibre channel Protocol stack - Storage Basics-8

 There are five layers in FC protocol stages which are quite similar to the OSI layer.

1. FC0:- It is equivalent to the physical Layer in OSI model. It belongs to the physical layer and defines the cable and connector used for the FC traffic. It sends the data in the form of bits “0” and “1” sequentially.

FC hubs are working in FC0 layer.

2. FC1:- It is responsible for the data encoding. It ensures the data error correction will be done in case of error.

It also performs the link creation and maintenance. It is equivalent to data link layer of OSI model.

8b/10b encoding is used in 1Gig, 2Gig, 4Gig, 8Gig whereas 10Gig and 16Gig standard uses 64b/66b encoding.

3. FC2:- It is the most important layer in FC protocol stack which perform various critical functions. It is equivalent to network OSI layer and is defined in FC-PI-2 standard.

FC switches works on FC2 layer.

FC2 layer performs the below functions:-

  A. Data block size handling: -   It defines how big data payloads   can  be send over the network.  Below are the few key words:-

• Exchange:- Exchange is the session built between end machines to transmit and receive data. There can be multiple Exchanges between the hosts.
• Sequence: - Payload or data is made up of big data units called sequence. It ensures the correct order of delivery as well and eliminates the out of order issues.
• Frames: - Since each link can only sent a defined amount of data size. The big data blocks are broken down to smaller chucks known as Frames. Frames can consist of up to 2112 bytes. If one frame is lost then the entire sequence has to be retransmitted

FC2 not only ensures that the frame has been received successfully at the receiver end but it also makes sure that the frames are been sent or received in sequence.

B. Flow control: - It provides the flow control to avoid the situation at the receivers end and is known as credit limit. It makes sure that both transmitter and receiver are in sync and transmitter will not overload the receiver. It performs two types of flow control:-

• Buffer-to-buffer credit: - It is also known as link flow control in which both end of the link negotiate data speed.
• End-to-End flow control:- In End-to-End flow control, the speed is negotiated between transmitter and receiver

C. Addressing:-  Each device in the fabric has its own unique WWNN (world wide node name) and each port gets a 64-bit address called WWPN (world wide port name). As soon as port is up neighbor switch will assign a 24 bit value called FCID to it.

4. FC3:- It performs the below mentioned services as mentioned below. It is not used in FC protocols and can be used by using additional software but not implemented yet.

• Encryption
• Mirroring or RAID
• Compression

5. FC4:- It is used to map the protocol data to the below layer. It encapsulates the data units and sends it to FC2 to perform the lower layer functions.