Seminar Report On FIBER CHANNEL ARBITRATED LOOP
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14-06-2009, 01:53 AM
Seminar Report On Submitted by:VIMAL V.B.
DEPARTMENT OF COMPUTER SCIENCE
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY
COCHIN â€œ 682022
Fiber Channel is an ANSI interface that acts as like a general transport vehicle to simultaneously deliver the command sets of several existing interface protocols. Fiber Channel Arbitrated Loop is one topology used to connect two or more devices within the guidelines set by the ANSI standards. It supports both large and small data transfers, and can be used in systems ranging from supercomputers to individual workstations. Fiber Channel Arbitrated Loop is designed for high-bandwidth high-end systems and transfers data in the serial mode. FC-AL uses fiber optic cabling to produce a maximum data transfer rate of more than 200 MB/second. FC-AL is capable of supporting up to 127 devices as far as 10 kilometers apart, thus opening new perspective for remote data storage. FC-AL allows hot swapping of the connected drives without interfering with the work of the entire system. Also, FC- AL devices can be designed with two ports to double the data transfer rates. This interface is more expensive than the SCSI interface. Fiber Channel isnâ„¢t a channel or a network, but allows for an active intelligent interconnection scheme, called a Fabric, to connect heterogeneous computer systems and peripherals with all the benefits of the existing methods. In fact, fiber channel supports its own protocol plus some other higher-level protocols such as the SCSI, FDDI, HIPPI (High Performance Parallel Interface) and IPI. Peripherals can include storage devices such as disk or tape arrays. Nowadays, Fiber channel option is mandatory for high-end mass storage products, such as disk or tape arrays.
There are basically two different ways to transfer data between computers or between computers and peripheral devices. They are channels and Networks
An approach based on channels provides a direct or switched point â€œto- point connection between communicating devices. Usually this method is implemented as parallel transmission and is designed to be hardware intensive. Any needed error detection and correction is handled in hardware. It provides high speed block based data transfer with low overhead. This method stress on high performance and the hardware is optimized to meet the need. The protocol used is very simple and it minimizes the software involvement in data transfer operations. This method also provides in- sequence data delivery and highly reliable data transmission. But by using this method we can only transfer data to a very small distance from several centimeters to few meters. A SCSI interface is a good example of a fast and effective channel. It is ideally suited for high-speed point-to-point transfer of large data blocks data in parallel way over relatively short distances.
This method is based on connecting distributed elements like workstations, file servers and peripherals using serial protocol. Networks are software intensive and it is slower than a channel, and has high overhead. In this approach the burden of ensuring reliable data delivery is transferred from hardware to software. Transmission of data is not reliable as on channels. Networks are usually used for long distance communication. Because of they are unstructured; they can support large number of devices of different types. They provides file based data transfer using a wide variety of protocols. Networks are used for transferring data with error-free delivery and voice and video where delivery on time is the primary requirement with error- free delivery being a secondary consideration. Ethernet based IP network is an example of network. This method is typically used for transferring small data files over relatively longer distances.
1.3 FIBER CHANNEL
Both channel and network topologies fall short when it comes to reliable, high-speed transfer of vast quantities of data over longer distances. Fiber channel combines the best features form these two protocols. It has high data transfer rate, low error rate, multiple connection topologies, longer distance, scalability and more. In recent years several technical developments have converged to a bigger than ever need for extremely fast data links. High performance computers have become the focus of much attention in the data communications industry. Performance improvements have spawned increasingly data-intensive and high-speed networking applications, such as multimedia and scientific visualization. However, the existing network interconnects between computers and I/O devices are unable to run at the speeds needed.
The intention of the Fiber Channel is to develop practical, inexpensive, yet expendable means of quickly transferring data between workstations, mainframes, supercomputers, desktop computers, storage devices, displays and other peripherals. Fiber Channel is the general name of an integrated set of standards being developed by the American National Standards Institute (ANSI). Fiber channel is not a channel or a network. It has an active intelligent interconnection scheme called. We can use optical fiber as well as copper wire as a communication media and its architecture does represents neither a channel nor a network. Fiber channel is first implemented using optical fiber as a transmission media and is used for transferring data between high-speed storage devices, so it gets this name. Fiber channel is a new superior I/O interface with the following features.
SERIAL DATA TRANSFER
Serial data transfer provides greater distance and simplicity. By using shielded twisted pair we can get up to 10M between each device. If we are using coaxial cable we will get up to 30M.Upto 2Km can be achievable using multi mode optical fiber, and can get up to 10 KM if we are using single mode optical fiber. This 10KM is not the total span of the network, its only the maximum possible distance between two devices.
COMPATABILITY WITH OTHER PROTOCOLS
Fiber channel supports its own protocol plus some other higher-level protocols such as SCSI, IPI, FDDI, HIPPI, IP, SBCCS and more.
1.3.3 RELIABLE DATA TRANSFER
This is the most reliable form of communication, transmission protocol ensures that packets are not lost or damaged during data transmission. It has low error rates and error handling is implemented in hardware, and uses separate cables for transmitting and receiving data. 1.3.4 PACKETIZING DATA During a fiber channel data transfer hardware in the sending device segments each data block into a sequence of data structures called frames. Each frame can contain up to 2112 bytes of data plus addressing and error correction information. If the information unit fits within a single frame, a single frame sequence is used to transport the information to the recipient. When the information unit is very large, the fiber channel hardware combines multiple frames to a sequence. Up to 2 frames can be concatenated into a single sequence. Use of sequences reduces the amount of processor overhead during the transfer of any large file by minimizing the time required to process individual frames. The sequenced data is transmitted over a serial cable to the recipient. The recipient breaks each sequence into frames and reassembles the information units into the frames to reconstruct the original data. Which is then delivered to the recipient CPU for higher level processing. Hardware in the sending and receiving devices handles the segmentation and reassembly of frames into data blocks.
1.3.5 SUPPORT FOR LARGE NUMBER OF DEVICES
You can connect a large number of devices that is over 16 million. There is a unique identifier for each device; devices can be storage devices such as disk drives, tape units, high bandwidth graphics terminals, and laser printers.
2.FIBER CHANNEL TOPOLOGIES
Just like a network topology, there are accepted layouts that are used to define how Fiber Channel devices are connected together. Fiber Channel defines three topologies, namely Point-to-Point, switched fabric and Arbitrated Loop.
A point-to-point topology is the simplest of all the three topologies. It consists of two and only two Fiber Channel devices connected directly together. The transmit fiber of one device goes to the receive fiber of the other device, and vice versa. There is no sharing of the media, which allows the devices to enjoy the total bandwidth of the link. A simple link initialization is required of the two devices before communication can begin. The simplicity of the point-to-point configuration makes it the cheapest way to implement Fiber Channel.
An example of such a configuration would be a server connected directly to a storage array.
2.2 SWITCHED FABRIC
This topology is similar to the star topology used on networks that use twisted pair cabling. In this case, a Fiber Channel switch provides a central connectivity point for all of the devices in the fabric. This allows each node to access all other nodes that are connected to the fabric. The configuration of a fabric topology can be either cascaded or non- cascaded. In a cascaded configuration, all switches are interconnected to form a larger mesh. This allows a device connected to one switch to access any other node connected to a switch within the fabric. In a non-cascaded configuration, the switches are not interconnected, making this design easier to implement than the cascaded; however, the performance of the fabric will significantly degrade should a switch fail. Also, the mesh design of the cascaded fabric allows for greater fault tolerance.
There are several benefits of this fabric topology, including support for a large number of nodes (over 16 million (2 24 ) in fact), increased fault tolerance when implementing a cascaded configuration, and improved scalability. The major benefit of this topology is that many devices can communicate at the same time; the media is not shared. Of course, it also requires the purchase of a switch. When the N_Ports log into the Fabric, the Fabric will assign Native Address Identifiers (S_ID). Other functions of the Fabric include a multicast server, broadcast server, alias server, quality of service facilitator, and directory server. Some Fabrics have FL_Ports, allowing Arbitrated Loops to be connected to the Fabric. 2.3 ARBITRATED LOOP
The Fiber Channel Arbitrated Loop, or FC-AL as it is known, is a Fiber Channel topology that combines the advantages of the fabric topology (support for multiple devices) with the cost savings of the point-to-point topology (where there's no need for a central switch). In a FC-AL implementation, devices are connected to a central hub, which is typically cheaper than a switch. As with Ethernet networks, the hub offers no additional functionality to the connected devices beyond serving as a centralized connection point. Arbitrated Loop has become the most dominant Fiber Channel topology, but it is also the most complex. Unlike the other two topologies, the media is shared among the devices, limiting each device's access. Not all devices are required to operate on an Arbitrated Loop; the added functionality is optional. Thus, for a Loop to operate, all devices must be Loop devices. There are two types of configurations.
2.3.1 PUBLIC LOOP
An Arbitrated loop with a fabric port is considered as a public port. In this configuration devices are connected to a central hub, and to one optional switch port.
2.3.2 PRIVATE LOOP
In a private loop, nodes are connected in a daisy-chained fashion. This will raise problems if one of the devices in the chain failed to operate. This is not to be a problem in the case of public loop. Private loop uses multiple ports for redundancy, if one port fails it switches its operation to the next port.
Arbitrated Loop is not a token-passing scheme. When a device is ready to transmit data, it first must arbitrate and gain control of the Loop. It does this by transmitting the Arbitrate (ARBx) Primitive Signal, where x = the Arbitrated Loop Physical Address (AL-PA) of the device. Once a device receives its own ARBx Primitive Signal, it has gained control of the Loop and can now communicate with other devices by transmitting an Open (OPN) Primitive Signal to a destination device. Once this happens, there essentially exists point-to-point communication between the two devices. All other devices in between simply repeat the data. If more than one device on the Loop is arbitrating at the same time, the x values of the ARB Primitive Signals are compared. When an arbitrating device receives another device's ARBx, the ARBx with the numerically lower AL_PA is forwarded, while the ARBx with the numerically higher AL_PA is blocked. Thus, the device with the lower AL_PA will gain control of the Loop first. Once that device relinquishes control of the Loop, the other device can have a chance. Unlike token-passing schemes, there is no limit on how long a device may retain control of the Loop. This demonstrates the "channel" aspect of Fiber Channel. There is, however, an Access Fairness Algorithm, which prohibits a device from arbitrating again until all other devices have had a chance to arbitrate. The catch is that the Access Fairness Algorithm is optional. There can be up to 126 nodes connected to a loop, but when the number of nodes increases, the performance will decrease. If only 2 devices are left in the loop, the topology becomes point-to-point
3. FIBER CHANNEL PORTS
Fibre Channel port is the entity on a node or switch within a storage area network that performs data communication and transfer over Fibre Channel connections. Ports are the link to the outside world for a Fiber Channel node. Each node has at least one port to provide access to other nodes; usually each port has two ports. Each port uses a pair of fibers; one to carry information into the port and one to carry information out of the port. This pair of fibers (actually copper wire) is called a link and is part of each topology. The Fiber Channel ANSI specification also supports fibers made of optical strands as a medium for data transfer. The ports are categorized into different types. NL_port : NL_port is the node connection pertaining to hosts or storage devices in an arbitrated loop topology. Also known as Node Loop port. FL_port : FL_port is the fabric connection in a public loop for an arbitrated loop topology. It is connected to a switch. Also known as Fabric Loop port. An F_Port uses the address identifier 0xFFFFFE L_port : L_port is the loose term used for NL_port and FL_port in an arbitrated loop. Also known as Loop port. N_port : N_port is the node connection pertaining to hosts or storage devices in a point-to-point or switched fabric topology. Also known as Node port.
F_port : F_port is the fabric connection on the switch side of a switched fabric topology. Also known as Fabric port. E_port : E_port is the connection between 2 fibre channel switches. Also known as Expansion port. When 2 E_ports between 2 switches form a link, that link is referred to as ISL. TE_port : TE_port is the term used for multiple E_ports trunked together to create high bandwidth between switches. Also known as Trunking Expansion port. G_port : G_port is the connection that can be classified or emulated as F port or E port. It has no function until it has logged into a fabric. Also known as Generic port. GL_port: GL_port is a G_port with Loop capabilities. Also known as Generic Loop port.
4. FIBRE CHANNEL LAYERS
It is often easier to understand a communications protocol if it is first broken down into parts or layers. Some people are familiar with the International Standards Organization (ISO) - Open Systems Interconnect (OSI) model, which breaks the communications puzzle into seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Fiber Channel does not follow this model, but instead, the protocol has been broken into six layers: FC-0, FC-1, FC-AL, FC-2, FC-3, and FC-4. FC-0 to FC-2 contains the physical description of Fiber Channel defined in the FC-PH standard, while FC-3 and FC-4 describe the interface with other applications, as well as some protocol specific information. Each is described below along with the main functions it defines. FC-0 and FC-1 can be thought of as defining the Physical Layer of the OSI model. FC-AL is actually a sub layer of FC-2, we can call it as FC-1.5, and this layer is further added to the protocol stack. FC-2 is similar to what other protocols define as a Media Access Control (MAC) layer, which is typically the lower half of the Data Link layer. Fiber Channel, however, does not define the concept of a MAC. FC-3 is not really a layer at all. It is still a largely undefined set of services for devices having more than one port. An example is striping, where data is transmitted out of all ports at the same time in order to increase bandwidth. FC-4 defines how other well-known higher layer protocols are mapped onto and transmitted over Fiber Channel. Thus, one can roughly think of the Fiber Channel layers defining up through the Transport layer of the OSI model.
4.1 FC-0: PHYSICAL INTERFACE
FC-0 is called the Physical interface Layer. This Fiber Channel layer defines the physical media used to link two Fiber Channel ports, including transmitters, receivers, connecters, and cabling types used. It also describes the optical and electrical parameters for a variety of data rates, maximum transfer distances, and noise limits.
4.1.1 TRANSMISSION SPEED
Fiber Channel offers a very wide range of media speeds. One of the goals of Fiber Channel is to allow HIPPI to map to it. HIPPI is a 100MB/s technology, thus Fiber Channel's primary data rate allows for data to travel 100MB/s. After factoring in 8B/10B encoding, frame headers, and other overhead, the transmission speed is 1063 Mb/s. This speed is referred to as full speed. There also exists half speed, quarter speed, and eighth speed. In addition, double and quadruple speeds are defined. The following table illustrates. By far, the most common speed is full speed, with some quarter speed devices also in existence.
4.1.2 TRANSMISSION MEDIA
Despite the name, Fiber Channel can run over both copper and fiber media. The main tradeoffs are that although longer distances can be achieved with fiber, it is more expensive. Speeds up to 100MB/s can run on both copper and fiber; 200MB/s and 400MB/s require fiber media. For copper, the following cable types are used: video cable, miniature cable, and shielded twisted pair. The most common by far is shielded twisted pair, using a DB-9 connector. For fiber, the choices are: 62.5mm multi-mode, 50mm multi-mode, and single-mode. The SC connector is used. Also for fiber, both long wave and short wave lasers can be used. Short wave seems to be most popular now. The short wave transmitters have the option to implement the Open Fiber Control (OFC). The data rates on these media ranges from 100 MB/S to 400MB/S. Depending on the data rate and media used, the point-to-point distance for a link can range from 50 meters to 100 Kilometers.
4.1.3 OPEN FIBER CONTROL
We are using short wave laser for communication. The FC-0 specifies a safety system - the Open Fiber Control system (OFC) - for short wave laser data links, since the optical power levels exceed the limits defined by the laser safety standards. This is a scheme designed to prevent disconnected fibers from continuously transmitting light, as a safety precaution. There is a simple protocol where each transmitter on a link periodically transmits short pulses of light to the other receiver. When the receivers detect this, the transmitters can operate normally. If an open fiber condition occurs in the link, the receiver of the Port the fiber is connected detects it and pulses its laser at a low duty cycle that meets the safety requirements. The receiver of the other port (at the other end of the fiber) detects this pulsing signal and also pulses it's transmitter at a low duty cycle. When the open fiber path is restored both ports receive the pulsing signals, and after a double handshaking procedure the connection is automatically restored within a few seconds.
4.1.4 PORT BYPASS CIRCUIT
Fiber Channel provides port bypass circuitry which bypasses the drive if it is removed, incapable of providing loop services, unable to obtain a valid address, or otherwise incapable of providing loop services. These port bypass circuits (PBCs) are located external to the drive or other FC-AL device. When a device is disconnected from the loop it doesnâ„¢t break the loop. Fig: Relationship between the PBC and drive
4.2 FC-1: TRANSMISSION PROTOCOL
FC-1 defines the transmission protocol including serial encoding and decoding rules, special characters and error control. The information transmitted over a fiber is encoded 8 bits at a time into a 10-bit Transmission Character (IBM 8B/10B encoding). The primary rationale for use of a transmission code is to improve the transmission characteristic of information across a fiber. The layer also controls the media access, defining the receiving and transmitting ports and their operations. This layer is also responsible for synchronization, error recovery and retransmission mechanism.
4.2.1 FC-1 CHARACTER CONVERSION
An unencoded information byte is composed of eight information bits A, B, C, D, E, F, G, H and the control variable Z. This information is encoded by FC-1 into the bits a, b, c, d, e, i, f, g, h, j of a 10-bit Transmission Character. The control variable has either the value D (D-type) for Data characters or the value K (K-type) for special characters. Each valid Transmission Character has been given a name using the following convention: Zxx.y, where Z is the control variable of the unencoded FC-1 information byte, xx is the decimal value of the binary number composed of the bits E, D, C, B, and A, and y is the decimal value of the binary number composed of the bits H,G of the unencoded FC-1 information byte in that order. For example the name of the FC-1 Transmission Character composed of the hexadecimal "BC" special (K- type) code is K28.5. The information received is recovered 10 bits at a time and those Transmission Characters used for data (D-type) are decoded into the one of the 256 8-bit combinations. Some of the remaining Transmission Characters (K-type) referred to as special characters, are used for protocol management functions. Codes detected at the receiver that are not D- or K- type are signaled as code violation errors.
4.2.2 CODING RULES
Each data byte or special character has two (not necessarily different) transmission codes. The data bytes and special characters are encoded into these codes respectively, depending on the initial Running Disparity (RD). The RD is a binary parameter, which is calculated upon the balance of ones and zeros in the sub-blocks (the first six bits and the last four bits) of a transmission character. A new RD is calculated from the transmitted character at both the transmitter and the receiver. If the detected character has opposite RD the transmitter should have sent, (depending on the RD of the previous bit stream) the receiver indicates a disparity violation condition. A Transmission Word is composed of four contiguous transmission characters.
4.3 FC-AL: ARBITRATED LOOP PROTOCOL
FC-AL is the Fiber Channel Arbitrated Loop protocol. Here point-to- point unidirectional serial links interconnect neighboring ports to form a loop. The protocol allows up to 126 devices plus an optional fabric element attached to the loop. FC-AL protocol defines the procedures for acquiring loop access and for establishing a connection with the receiving port. Once a point-to- point connection is established between the sender and the receiver, the framing protocol defined in the FC-PH standard governs the transfer of data frames. Dual loop technology is typical in FC-AL storage networks.
4.4 FC-2: SIGNALLING PROTOCOL
The Signaling Protocol (FC-2) layer serves as the transport mechanism of Fibre Channel. The framing rules of the data to be transferred between ports, the different mechanisms for controlling the three service classes and the means of managing the sequence of a data transfer are defined by FC-2. The layer defines most of the Fibre Channel specific handling like flow control, encapsulation of data into frames and sequence exchanges. The layer provides Fibre Channel classes of service information, which allows a variety of different connection types and connection management options. The layer also defines the physical models for the Fibre Channel components, including the definition of nodes, ports, segmentation and reassembly, Login/Logout and Fibre Channel topologies.
4.4.1 ORDERED SET
The Ordered Sets are four byte transmission words containing data and special characters, which have a special meaning. Ordered Sets provide the availability to obtain bit and word synchronization, which also establishes word boundary alignment. An Ordered Set always begins with the special character K28.5. Three major types of Ordered Sets are defined by the signaling protocol. The Frame delimiters (the Start-of-Frame (SOF) and End-of-Frame (EOF) Ordered Sets) are Ordered Sets, which immediately precede or follow the contents of a Frame. There are multiple SOF and EOF delimiters defined for the Fabric and N_Port Sequence control. The two Primitive Signals: Idle and Receiver Ready (R_RDY) are Ordered Sets designated by the standard to have a special meaning. An Idle is a Primitive Signal transmitted on the link to indicate an operational Port facility ready for Frame transmission and reception. The R_RDY Primitive Signal indicates that the interface buffer is available for receiving further Frames. A Primitive Sequence is an Ordered Set that is transmitted and repeated continuously to indicate specific conditions within a Port or conditions encountered by the receiver logic of a Port. When a Primitive Sequence is received and recognized, a corresponding Primitive Sequence or Idle is transmitted in response. Recognition of a Primitive Sequence requires consecutive detection of 3 instances of the same Ordered Set. The Primitive Sequences supported by the standard are Offline (OLS), Not Operational (NOS), Link Reset (LR) and Link Reset Response (LRR).
4.4.2 FLOW CONTROL
The concept of flow control deals with the problem where a device receives frames faster than it can process them. When this happens, the result is that the device is forced to drop some of the frames. Fibre Channel has a built-in flow control solution to this problem. The idea is simple enough. A device can transmit frames to another device only when the other device is ready to accept them. Before the devices can send data to each other, they must login to each other. One of the things accomplished in login is establishing credit. Credit refers to the number of frames a device can receive at a time. This value is exchanged with another device during login, so each knows how many frames the other can receive. After enough frames have been transmitted and credit runs out, no more frames can be transmitted until the destination device indicates it has processed one or more frames and is ready to receive new ones. Thus, no device should ever be overrun with frames. Fibre Channel uses two types of flow control, buffer-to-buffer and end-to-end.
This type of flow control deals only with the link between an N_Port and an F_Port or between two N_Ports. Both ports on the link exchange values of how many frames it is willing to receive at a time from the other port. This value becomes the other port's BB_Credit value and remains constant as long as the ports are logged in. For example, when ports A and B log into each other, A may report that it is willing to handle 4 frames from B; B might report that it will accept 8 frames from A. Thus, B's BB_Credit is set to 4, and A's is set to 8. Each port also keeps track of BB_Credit_CNT, which is initialized to 0. For each frame transmitted, BB_Credit_CNT is incremented by 1. The value is decremented by 1 for each R_RDY Primitive Signal received from the other port. Transmission of an R_RDY indicates the port has processed a frame, freed a receive buffer, and is ready for one more. If BB_Credit_CNT reaches BB_Credit, the port cannot transmit another frame until it receives an R_RDY.
End-to-End flow control is not concerned with individual links, but rather the source and destination N_Ports. The concept is very similar to buffer-to-buffer flow control. When the two N_Ports log into each other, they report how many receive buffers are available for the other port. This value becomes EE_Credit. EE_Credit_CNT is set to 0 after login and increments by 1 for each frame transmitted to the other port. It is decremented upon reception of an ACK Link Control frame from that port. ACK frames can indicate the port has received and processed 1 frame, N frames, or an entire Sequence of frames. 4.4.3 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.
188.8.131.52 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.
184.108.40.206 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.
220.127.116.11 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.
18.104.22.168 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.
22.214.171.124 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.
126.96.36.199 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.
Unlike many LAN technologies that use a fixed six-byte Media Access Control (MAC) address, Fibre Channel uses a three-byte address identifier, which is dynamically assigned during Login. N_Ports transmit frames from their own Source_ID (S_ID) to a Destination_ID (D_ID). Addresses in the range of hex'FFFFF0' to hex'FFFFFE' are special, well-known addresses uses for such things as the Fabric, Alias Server, or the Multicast Server. Before Fabric Login, the N_Port's S_ID is undefined: hex'000000'. Hex'FFFFFF' is reserved for broadcast. In a point-to-point topology, Fabric Login will fail of course, and the two ports will simply chose two unique addresses. Arbitrated Loop devices still use the three byte address identifiers, but also use an Arbitrated Loop Physical Address (AL_PA). AL_PAs are one-byte values dynamically assigned each time the Loop is initialized. Once the Loop is initialized and (hopefully) each L_Port has selected an AL_PA, public NL_Ports will attempt Fabric Login. If there is an FL_Port, the Fabric will assign the upper two bytes of the NL_Port's address identifier and usually allow the low byte to be the NL_Port's AL_PA. (If not, the Loop will need to be re-initialized so the NL_Port can select the Fabric assigned AL_PA). If no Fabric exists or if an NL_Port is a private NL_Port (does not login with the Fabric), the upper two bytes of the address identifier will remain '0000', and the lower byte will simply be the NL_Port's AL_PA. But there still needs to be a way of uniquely identifying a port - even for much of the above initialization to take place. This is accomplished using Name_Identifiers, a fixed 64-bit value. Name_Identifiers are used to uniquely identify nodes (Node_Name), a Port (Port_Name), and a Fabric (Fabric_Name). Name Identifiers are not used to route frames, but are used in mapping to upper layer protocols.
Fibre Channel defines two types of login procedures, Fabric and N_Port. With the exception of private NL_Ports, all other node ports must attempt to log in with the Fabric. This is typically done right after the link or the Loop has been initialized. It consists of the node port transmitting an Fabric Login (FLOGI) frame to the well-known Fabric address hex'FFFFFE'. The normal response is an Accept (ACC) frame from the Fabric back to the node port. Fabric Login accomplishes the following things: It determines the presence or absence of a Fabric. If a Fabric is present, it provides a specific set of operating characteristics associated with the entire Fabric, including which Classes of service are supported. If a Fabric is present, it shall optionally assign or shall confirm the native N_Port Identifier of the N_Port that initiated the Login. If a Fabric is not present, an ACC from an N_Port indicates that the requesting N_Port is attached in a point-to-point topology. If a Fabric is present, it initializes the buffer-to-buffer credit. Before a node port can communicate with another node port, it must first perform N_Port Login with that node port. Similar to Fabric Login, the process entails transmitting a PLOGI frame to the destination node port. Again, the normal response is an ACC frame. N_Port Login accomplishes the following things: It provides a specific set of operating characteristics associated with the destination N_Port, including which Classes of service are supported. Initializes the destination end-to-end credit. In point-to-point topology, buffer-to-buffer credit is initialized. Both Fabric Login and N_Port Login are intended to be long-lived. Once logged in, a device can stay logged in indefinitely, even if it has no further data to transmit at that time.
4.5 FC-3: COMMON SERVICES
The FC-3 layer is intended to provide the common services required for advanced features such as: 4.5.1 STRIPING
Stripping multiplies bandwidth using multiple N_Ports in parallel to transmit a single information unit across multiple links.
4.5.2 HUNT GROUPS
Hunt grouping is the ability for more than one Port to respond to the same alias address. This improves efficiency by decreasing the chance of reaching a busy N_Port.
Multicast delivers a single transmission to multiple destination ports. This includes sending to all N_Ports on a Fabric (broadcast) or to only a subset of the N_Ports on a Fabric.
4.6 FC-4: APPLICATION INTERFACE
FC-4, the highest level in the FC structure defines the application interfaces that can execute over Fiber Channel. It specifies the mapping of upper layer protocols using the FC levels below. Fiber Channel is equally adept at transporting both network and channel information and allows both protocol types to be concurrently transported over the same physical interface.
The following network and channel Upper Layer Protocols (ULP)
mapping are currently specified or proposed as FC-4s.
Small Computer System Interface (SCSI)
Internet Protocol (IP)
High Performance Parallel Interface (HIPPI) Framing Protocol
Link Encapsulation (FC-LE)
IEEE 802.2 (Logical Link Control)
Asynchronous Transfer Mode - Adaption Layer 5 (ATM-AAL5)
Intelligent Peripheral Interface - 3 (IPI-3) (disk and tape)
Single Byte Command Code Sets (SBCCS)
5. TRANSMISSION HIERARCHY
The easiest way to understand the methods by which information is transmitted over Fiber Channel is by looking at the problem in the form of a hierarchy.
5.1 8B/10B TRANSMISSION CHARACTER
At the lowest level, Fiber Channel uses the IBM 8B/10B-encoding scheme. Prior to encoding the data is converted to an internal FC-1 format. The hexadecimal to FC-1 decimal value translation is accomplished as shown below.
y = The decimal value of the 3 MSB bits
xx = The decimal value of the 5 LSB bits
= 10111100 = 101 11100
A decimal value is assigned to each bit combination with the range of 0 to 31 for xx and 0 to 7 for y. This means the range of valid data characters using the FC-1 naming conversion is D00.0 to D31.7 Basically, every byte of data that is to be transmitted is first converted into a 10-bit value called a Transmission Character. Using this encoding scheme has the following benefits: Improvement of the transmission characteristics of information to be transferred Provides enough transitions to make clock recovery possible at the receiver Improves the detection of single and multiple bit errors Some Transmission Characters contain a unique bit pattern (comma) to aid in achieving word alignment. Fiber Channel defines a "1" to simply be the state with more optical power (for optical links), or the state where the "+" pin is more positive than the "-" pin (in the case of copper). The 8B/10B encoding uses the idea of running disparity, which is concerned with the number of 1s and 0s in each Transmission Character. Running disparity is evaluated after the first 6 bits of each Transmission Character and again after the last 4 bits and can be either positive (more 1s than 0s) or negative (more 0s than 1s). It is desirable to try and equalize the number of 1s and 0s over time. Thus, every byte to be transmitted is encoded into one of two 10-bit representations depending on the current running disparity. As stated, every byte to be transmitted is first converted into a 10-bit Transmission Character. But there are many more possible 10-bit Transmission Characters than are needed to map to particular bytes. Most of the remaining 10-bit encoding are not defined, and only one is used. This is the special K28.5 Transmission Character, which contains the "comma", a 7-bit string that cannot occur in any Data Transmission Character. Because of this, the K28.5 is used as a special control character.
5.2 TRANSMISSION WORD
All information in Fiber Channel is transmitted in groups of four Transmission Characters called Transmission Words. Fig: Formation of a transmission word Some Transmission Words have K28.5 as the first Transmission Character. These are special Transmission Characters called Ordered Sets. Some Ordered Sets mark the beginning and end of frames (frame delimiters). Others convey information in between frames in the form of Primitive Signals (a single Ordered Set) and Primitive Sequences (a stream of the same Ordered
Transmission Word (40 Bits â€œ 4 10-Bit Encoded Bytes 8B/10B Encoded byte 1 (10) 8B/10B Encoded Byte 4 (10)
Byte (8 bits)
Byte (8 bits)
Set). Examples of Ordered Sets are: Start of Frame (SOF), End of Frame (EOF), Idle, Receiver Ready (R_RDY), Loop Initialization Primitive (LIP), Arbitrate (ARB), Open (OPN), Close (CLS), and several others. 5.3 FRAME
Fiber Channel defines a variable length frame consisting of 36 bytes of overhead and up to 2112 bytes of payload for a total maximum size of 2148 bytes. The total size of the frame must be an even multiple of four bytes so that partial Transmission Words are not sent. Between 0 and 3 pad bytes are appended to the end of the payload to satisfy this rule. A Start of Frame (SOF) delimiter and End of Frame (EOF) delimiter mark the beginning and end of each Fiber Channel frame. The Cyclic Redundancy Check (CRC) is the same 32-bit CRC used in FDDI
A Fiber Channel Sequence is a series of one or more related frames transmitted unidirectionally from one port to another. All frames must be part of a Sequence. Frames within the same Sequence have the same SEQ_ID field in the header. The SEQ_CNT field identifies individual frames within a Sequence. For each frame transmitted in a Sequence, SEQ_CNT is incremented by 1. This provides a means for the recipient to arrange the frames in the order in which they were transmitted and to verify that all expected frames have been received. Multiple Sequences to multiple ports may be active at a time.
A Fiber Channel Exchange is a series or one or more nonconcurrent Sequences between two ports. The Sequences may be in either direction. All Sequences (and therefore all frames) must be part of an Exchange. The originator of the Exchange assigns the OX_ID field. The responder assigns the RX_ID field. As another perspective, one can use the following analogy: Frame Word Sequence Sentence Exchange Conversation Of course, one main difference is that a Fibre Channel device can "speak" more than one sentence and hold more than one "conversation" at a time.
6. FC ARBITRATED LOOP CONCEPTS
This section describes some basic Fibre Channel Arbitrated Loop (FC- AL) concepts. FC-AL is a topology for connecting devices in Fibre Channel environments. Fibre Channel is a serial data channel that provides logical point-to-point service to two communicating devices. With FC-AL, you can have a maximum of one point-to-point circuit at any one time. When this circuit is active, only two L_Ports are communicating, but up to 127 devices may be attached to and participating on the same loop. All of the L_Ports that are on the loop but that are not one of the two communicating L_Ports see all data transferred across the loop and retransmit this information so that the data reaches its intended destination.
6.1 ARBITRATED LOOP PHYSICAL ADDRESS (AL_PA)
Each device communicating on an arbitrated loop must have an Arbitrated Loop Physical Address (AL_PA). The AL_PA is an 8-bit (1-byte) 8B/10B encoded value that is a valid data character. This 8-bit character, when encoded to 10 bits, must have an equal number of 1â„¢s and 0â„¢s in the address to maintain neutral running disparity. Neutral-running disparity is required so that the AL_PA data character does not change the current running disparity of the current transmission word. There are 134 characters that result in neutral disparity. Seven of the 134 neutral disparity characters are reserved. This leaves 127 valid addresses.
6.2 LOOP INITIALIZATION
Loop initialization is the process used to obtain or verify AL_PAs. Loop initialization occurs at power-up, when a new node is inserted in the loop, or for error recovery. During loop initialization, each NL_Port discovers that it is connected in an arbitrated loop topology and that there may be multiple NL_Ports on the arbitrated loop. The procedure used for this discovery is called the loop port state machine (LPSM). The FC-2 protocol is also used. A buffer on the drive receives each of the following loop initialization frames: LISM, LIFA, LIPA, LIHA, LISA, LIRP, and LILP. All other frames may be discarded if the L_Portâ„¢s buffer is full. If the NL_Ports finds that there are not any more NL_Ports attached, but that there is only an F_Port or N_Port attached, the NL_Port configures itself to operate as an N_Port by going into N_Port mode. An L_Port begins the loop initialization procedure in the initializing state at the request of the node. In most cases, the back panel will be set to provide a hard assigned physical address which will not be changed by the initialization process, unless duplicate hard assigned physical addresses are found. The AL_PA of the one FL_Port (if present) is always 00h. All other AL_PA values will range from 01 h
The loop initialization process begins when any NL_Port forces the loop into the INITIALIZING state. This initializing port begins transmitting loop initialization primitive sequences (LIPs). Any NL_Port can cause this by sending any of the LIPS. All LIPs cause the 4-step initialization process to occur. A loss of signal will also force the loop to initialize. There are five Loop Initialization Primitive (LIP) sequences
6.2.1 LOOP INITIALIZATION SEQUENCES (LIP)
1. No valid AL_PA
: BC 15 F7 F7
Used when a port wants to acquire an AL_PA (Initializing LIP)
2. Loop failure, No valid AL_PA
: BC 15 F8 F7
The originating port has detected an input failure but the port does
not have an AL_PA (Loop Failure)
3. Valid AL_PA
: BC 15 F7 AL_PS
The originating port has detected poor loop performance, Example:
The port is unable to win the arbitration. (Performance degradation)
4. Loop failure, Valid AL_PA
: BC 15 F8 AL_PS
The originating port has detected an input failure. The AL_PS is the
AL_PA of the port. (Loop Failure)
5. Loop Initialization Reset
: BC 15 AL_PD AL_PS
The source port (AL_PS) wants to reset the destination port (AL_PD).
All ports complete loop initialization. The selected port performs a reset after loop initialization. (Selective Reset) The next port in the loop receives the LIPs and transitions to the OPEN_INIT state and transmits LIPs to the next port on the loop. This cycle continues until the port that started the initialization process receives the LIPs. It then also transitions to the OPEN_INIT state. This means all ports on the loop are in the OPEN_INIT state at this point. Each port transmits a minimum of 12 LIPs and then transmits Idles for the AL_TIME (15 ms). The ports then transmit Loop Initialization Select Master (LISM) frames to select a loop master who will then control the initialization process.
6.2.2 LOOP INITIALIZATION STEPS
There are four primary steps involved in initializing the loop.
1. Select a loop master based on the 8-byte port name (Loop Initialization
2. Assign each port an arbitrated loop physical address (AL_PA). There are four ways a port can acquire an AL_PA. These are listed below.
• Fabric assigned (Loop Initialization Fabric Assignedâ€LIFA).
• Previously acquired (Loop Initialization Previously Acquiredâ€LIPA).
• Hard assigned (Loop Initialization Hardware Assignedâ€LIHA).
• Soft assigned (Loop Initialization Soft Assignedâ€LISA).
3. Generate AL_PA position map (Loop Initialization Report Positionâ€LIRP).
4. Transmit completed AL_PA position map around the loop (Loop Initialization Loop Positionâ€LILP).
6.2.3. LOOP INITIALIZATION SELECT MASTER (LISM)
12-byte payload (with 8-byte port name)
The loop master is determined as follows:
• Each NL_Port selects an initial AL_PA of ËœEFâ„¢. The FL_Port (if present) selects an AL_PA of Ëœ0â„¢.
• Each port transmits LISM with the D_ID and S_ID fields of the header set to its AL_PA. The payload is set to the port name, which includes the world wide name (WWN).
• Each port examines the payload in the inbound LISM. If it is greater than its own, the port transmits a new LISM with its own port name in the payload. If it is less than its own, the port retransmits the received LISM.
If it is the same as its own, it becomes the loop master. Â¢ The loop master sends ARB (F0) to purge the loop and to inform all other ports that a master has been selected. When the master receives ARB(F0) on the inbound fiber, initialization proceeds to LIFA.
6.2.4 LOOP INITIALIZATION FABRIC ASSIGNED (LIFA)
20-byte payload (with 16-byte bit map of AL_PAs) The loop master primes the bit map with zeroes (0) then sends the bit map around the loop in the LIFA payload. Any port which has an AL_PA assigned by the fabric will set a Ëœ1â„¢ in the bit map at the position corresponding to its AL_PA. If a Ëœ1â„¢ already exists, no map entry is made.
6.2.5.LOOP INITIALIZATION PREVIOUSLY ASSIGNED (LIPA)
20-byte payload (16-byte bit map of AL_PAs) The loop master transmits the bit map resulting from the LIFA. The L_Port checks to see if the bit that corresponds to its previously acquired AL_PA is set. If not, the L_Port sets it to Ëœ1â„¢. If the L_Portâ„¢s bit has already been set by another port, it will attempt to assume a soft-assigned AL_PA. The L_Port then retransmits the LIPA frame.
6.2.6 LOOP INITIALIZATION HARD ASSIGNED (LIHA)
20-byte payload (16-byte bit map of AL_PAs) The loop master transmits the bit map resulting from the LIPA. The L_Port checks to see if the bit that corresponds to its hard address is set (usually using switches or jumpers). If not, the L_Port sets it to Ëœ1â„¢. If the L_Portâ„¢s bit has already been set by another port, it will attempt to assume a soft-assigned AL_PA (LISA). The L_Port then retransmits the LIHA frame.
6.2.7 LOOP INITIALIZATION SOFT ASSIGNED (LISA)
20-byte payload The loop master transmits the bit map resulting from the LIHA. The L_Port checks to see if there are any free addresses left in the bit map by checking for the first available Ëœ0â„¢. The L_Port sets it to Ëœ1â„¢. If there are no zeroes in the bit map, all 126 NL_Port addresses have been taken and the port is not allowed to actively participate in the loop. The L_Port then retransmits the LISA frame.
6.2.8 LOOP INITIALIZATION REPORT POSITION (LIRP)
The LIRP and LILP sequences build a mapping between AL_PA values and participating L_Port positions. This allows all L_Ports to know where all of the other AL_PA addresses are physically located on the loop. To build this address map, the loop master primes the 128-byte position map with ËœFFâ„¢ in each position. The loop master then sets byte 0 (the counter byte) of the position map to Ëœ01â„¢ and places its own AL_PA in byte 1 and then sends the position map around the loop. Each port increments the position counter (byte 0) by one and places its AL_PA at the map position indicated by the counter. For example, the second device on the loop (with the loop master counted as the first device) increments byte 0 by one to make byte 0 has a value of Ëœ2â„¢ and then places its AL_PA in position 2 of the position map. Each port retransmits the LIPA frame until the map completes its journey to each of the devices on the loop and gets back to the loop master. At this point, the LIRP process results in a position map containing the physical location and AL_PA of every device on the loop.
6.2.9 LOOP INITIALIZATION LOOP POSITION (LILP)
When the loop master gets the LIRP frame back from the loop, it contains the AL_PAs and physical location of each participating port on the loop. The loop master retransmits the completed position map (now called the LILP) to the next port on the loop. This informs all ports of the loopâ„¢s physical make-up and also makes it possible for each port to make a copy of the map before retransmitting it to the next port. When the loop master gets the LILP back from the loop, it sends a CLS followed by Idles. All of the other ports retransmit the CLS and transition to the Monitoring state. When the loop master gets the CLS back from the loop, it removes it and then transitions to the Monitoring state. This signals the end of the initialization process.
6.3 PRIMITIVE SIGNALS CUSTOM MADE FOR FC-AL
There are eight primitive signals used exclusively within arbitrated loop (FC-AL) topologies. These primitives are listed below. 6.3.1 ARBITRATE (ARBx) ARBx is transmitted to request access to the loop. Each ARBx contains the Physical (port) Address (AL_PA) of the requestor.
6.3.2 ARBITRATE (ARB (F0))
ARB (F0) is transmitted to manage access fairness. It is also used to assign a loop master during initialization.
188.8.131.52 ACCESS FAIRNESS
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