Classes of Service

Fibre Channel defines several communication strategies called Classes of service. The Class used greatly depends on the type of data to be transmitted. The major difference between the Classes is the types of flow control used. If two N_Ports are to communicate or if an N_Port is to successfully log into a Fabric, there must be at least 1 common Class of service supported between them, since Sequences and Exchanges must take place using a single Class of service. This information is exchanged during Fabric Login and N_Port Login.

  

Class 1

In Class 1, a dedicated connection is established between two N_Ports. Once established, the two N_Ports may communicate using the full bandwidth of the connection; no other network traffic affects this communication. Due to this, frames are guaranteed to arrive in the order in which they were transmitted. In addition, the media speeds must be the same for all links which make up the dedicated connection. Because of the nature of the dedicated connection, there is no need for buffer-to-buffer flow control; the Fabric does not need to buffer the frames as they are routed. Thus, only end-to-end flow control is used in Class 1. Class 1 would be used when the data needs to be continuous and time critical, such as voice or video.
  

Intermix  

Intermix is an option of Class 1 whereby Class 2 and Class 3 frames may be transmitted at times when Class 1 frames are not being transmitted. The Class 2 and Class 3 frames may or may not be destined to the same N_Port as the Class 1 frames. Both N_Ports as well as the Fabric must support Intermix for it to be used.

Class 2  

Class 2 is referred to as multiplex due to the fact that it is a connectionless Class of service with notification of delivery and non-delivery of frames. Since no dedicated connection needs to be established, a port can transmit frames to and receive frames from more than one N_Port. As a result, the N_Ports share the bandwidth of the links with other network traffic. Frames are not guaranteed to arrive in the order in which they were transmitted, except in the point-to-point or Loop topologies. Also, the media speeds may vary for different links which make up the path. Both buffer-to-buffer and end-to-end flow control are used in Class 2. Class 2 is more like typical LAN traffic, such as IP or FTP, where the order and timeliness of delivery is not so important.

Class 3  

Class 3 is very similar to Class 2. The only exception is that it only uses buffer-to-buffer flow control. It is referred to a datagram service. Class 3 would be used when order and timeliness is not so important, and when the ULP itself handles lost frames efficiently. Class 3 is the choice for SCSI.

Class 4

Class 4 provides fractional bandwidth allocation of the resources of a path through a Fabric that connects two N_Ports. Class 4 can be used only with the pure Fabric topology. One N_Port will set up a Virtual Circuit (VC) by sending a request to the Fabric indicating the remote N_Port as well as quality of service parameters. The resulting Class 4 circuit will consist of two unidirectional VCs between the two N_Ports. The VCs need not be the same speed.Like a Class 1 dedicated connection, Class 4 circuits will guarantee that frames arrive in the order they were transmitted and will provide acknowledgement of delivered frames (Class 4 end-to-end credit). The main difference is that an N_Port may have more than one Class 4 circuit, possibly with more than one other N_Port at the same time. In a Class 1 connection, all resources are dedicated to the two N_Ports. In Class 4, the resources are divided up into potentially many circuits. The Fabric regulates traffic and manages buffer-to-buffer flow control for each VC separately using the FC_RDY Primitive Signal. Intermixing of Class 2 and 3 frames is mandatory for devices supporting Class 4.

Class 5

The idea for Class 5 involved isochronous, just-in-time service. However, it is still undefined, and possibly scrapped altogether. It is not mentioned in any of the FC-PH documents.

Class 6

Class 6 provides support for multicast service through a Fabric. Basically, a device wishing to transmit frames to more than one N_Port at a time sets up a Class 1 dedicated connection with the multicast server within the Fabric at the well-known address of hex'FFFFF5'. The multicast server sets up individual dedicated connections between the original N_Port and all the destination N_Ports. The multicast server is responsible for replicating and forwarding the frame to all other N_Ports in the multicast group. N_Ports become members of a multicast group by registering with the Alias Server at the well-know address of hex'FFFFF8'. The Class 6 is very similar to Class 1; Class 6 SOF delimiters are the same as used in Class 1. Also, end-to end flow control is used between the N_Ports and the multicast server.

The following table summarizes the classes of service in Fibre Channel.

Class of service	Fibre Channel description
Class 1	Dedicated connection In-order delivery, acknowledge first frame only No flow control after first frame of connection
Class 2	"Connectionless Frame switched Out-of-order delivery possible Acknowledge each frame Buffer-to-buffer and end-to-end flow control for all frames"
Class 3	Frame switched Out-of-order delivery possible No acknowledgments Buffer-to-buffer frame control for all frames
Class 4	Connection oriented Virtual circuit In-order delivery
Class 5	Reserved
Class 6	Connection oriented Multicast service