FC Q & A

1. What is FC ?

Fibre Channel is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical. It t has become the standard connection type for storage area networks (SAN) in enterprise storage. Despite common connotations of its name, Fibre Channel signaling can run on both twisted pair copper wire and fiber-optic cables.

Fibre Channel Protocol (FCP) is the interface protocol of SCSI on the Fibre Channel.

2. What are the advantages of FC ?

Highest bandwidth 4Gbps,10Gbps

Better signal quality since it is optical

Longer media support 1 Km,10Km & more with expanders or switches

Support to maximum no. of device connectivity ( in switched mode upto 16 million devices)

3. Mention the differences between FC & SCSI ?

FC is not a shared bus while SCSI is .

FC has support to maximum 16 million devices – SCSI maximum 16 devices

FC has maximum bandwidth of 10Gbps while SCSI max ~3 Gbps

FC is optical(generally) while SCSI is electrical transmission

FC has minimum latency & does not have problems of SCSI like clock skewness,noise collision,cross talk

4. Explain the layers in FC Protocol ?

Fibre Channel is a layered protocol. It consists of 5 layers, namely:

 FC0 The physical layer, which includes cables, fiber optics, connectors, pinouts etc.

 FC1 The data link layer, which implements the 8b/10b encoding and decoding of signals.

 FC2 The network layer, defined by the FC-PI-2 standard, consists of the core of Fibre Channel, and defines the main protocols.

 FC3 The common services layer, a thin layer that could eventually implement functions like encryption or RAID.

 FC4 The Protocol Mapping layer. Layer in which other protocols, such as SCSI, are encapsulated into an information unit for delivery to FC2.

5. What are the functions of layer2 and layer3 in FC ?

Above answer is a brief one (plz find attachment FCLayers.pdf which goes in detail)

6. What are different FC topologies ?

Point to Point , Arbitrated Loop (FC-AL) , Switched

7. What is the difference between point to point  & Switched in FC ?

Point-to-Point is limited to two devices but they can talk at greater distances than SCSI

allows.

Switched fabric can theoretically allow 16 million nodes to talk (16^6 – There are 6

Port Identifier (PID) slots with 16 hex choices per slot)). The committee reserves ½

million of these addresses for well known addresses and testing purposes.

8. Explain FC-AL ?

Fibre Channel - Arbitrated Loop (FC-AL) is limited to 126 devices in a blocking architecture (plus one for FL_Port) . Without a switch only two of these devices can talk at a time, all others are “blocked” until those two are done. An arbitrated loop attached to a switch allows

queuing into and out of the port where the loop is attached. The embedded port will take

one AL_PA, so on a Brocade switch port there are 125 available AL_PAs.

28. What is AL_PA

Arbitrated Loop Physical Address. A 1-byte value used in the Arbitrated Loop topology used to identify L_Ports. This value will then also become the last byte of the address identifier for each public L_Port on the loop

9. How does FC Switch maintain the addresses?

                FC Switch uses simple name server to maintain the mapping table

10.Explain these terms : LIFA,LIPA,LIHA,LISA

LIFA : Loop Initialized Fabric Assigned (Address)

LIPA : Loop Initialized Previously Aquired (Address)

LIHA : Loop Initialized Hard Assigned (Address)

LISA : Loop Initialized Soft Assigned (Address)

These are different phases in which an FC node or device acquires its address

11.What are the different FC ports ?

N_port is a port on the node (e.g. host or storage device) used with both FC-P2P or FC-SW topologies. Also known as Node port.

NL_port is a port on the node used with an FC-AL topology. Also known as Node Loop port.

F_port is a port on the switch that connects to a node point-to-point (i.e. connects to an N_port). Also known as Fabric port. An F_port is not loop capable.

FL_port is a port on the switch that connects to a FC-AL loop (i.e. to NL_ports). Also known as Fabric Loop port.

E_port is the connection between two fibre channel switches. Also known as an Expansion port. When E_ports between two switches form a link, that link is referred to as an inter-switch link (ISL).

D_port is a diagnostic port, used solely for the purpose of running link-level diagnostics between two switches and to isolate link level fault on the port, in the SFP, or in the cable.

EX_port is the connection between a fibre channel router and a fibre channel switch. On the side of the switch it looks like a normal E_port, but on the side of the router it is a EX_port.

12.What is a Fabric and what are its components ?

The hardware/network that connects workstations and servers to storage devices in a SAN is referred to as a "fabric." The SAN fabric or FC Fabric enables any-server-to-any-storage device connectivity through the use of Fibre Channel switching technology.

Its components are :

FC Network

FC Switch

FC HBA (Host Bus adaptor) cards

Servers/Hosts

Storage subsystems

13.Mention different types of flow controls in FC?

buffer to buffer   and End to End flow control

14.What is the difference between buffer to buffer   and End to End flow control ?

Fibre Channel devices transfer information from an output buffer in the transmitting node to an input buffer of the receiving node. This is called a buffer-to-buffer transfer. Each node may have from 1 to n buffers. The number of buffers in each node does not have to be equal. Each buffer is the size a frame may transfer in its payload.

The Fibre Channel standard does not define the actual length of the buffer or the method used to store the bytes in the buffer. Figure below shows how data is sent from the transmit buffer and received by the receive buffer.

End-to-end credit (EE_Credit).

This type of credit or flow control is negotiated between a source N_Port and a destination N_Port. This credit is managed using an acknowledgment (ACK) frame sent from the destination back to the source N_Port.

15.What are the different classes of Services and explain them ?

To ensure efficient transmission of different types of traffic, FC defines three classes of service. Users select service classes based on the characteristics of their applications, like packet length and transmission duration, and allocate the services by the Fabric Login protocol.
Class 1 is a service which provides dedicated connections, in effect providing the equivalent of a dedicated physical connection. Once established, a Class 1 connection is retained and guaranteed by the Fabric. This service guarantees the maximum bandwidth between two N_Ports, so this is the best for sustained, high throughput transactions. In Class 1, Frames are delivered to the destination Port in the same order as they are transmitted. Figure below shows the flow control management of a Class 1 connection.



Class 2 is a Frame-switched, connectionless service that allows bandwidth to be shared by multiplexing Frames from multiple sources onto the same channel or channels. The Fabric may not guarantee the order of the delivery and Frames may be delivered out of order. This service class can be used, when the connection setup time is greater than the latency of a short message. Both Class 1 and Class 2 send acknowledgment Frames confirming Frame delivery. If delivery cannot be made due to congestion, a Busy frame  is returned and the sender tries again. 



Class 3 service is identical to Class 2, except that the Frame delivery is not confirmed. (Flow control is managed only on buffer level, see Figure 7) This type of transfer, known as datagram provides the quickest transmission by not sending confirmation. This service is useful for real- time broadcasts, where timeliness is key and information not received in time is valueless.
The FC standard also defines an optional service mode called intermix. Intermix is an option of Class 1 service, in which Class 1 Frames are guaranteed a special amount of bandwidth, but Class 2 and Class 3 Frames are multiplexed onto the channel, only when sufficient bandwidth is available to share the link .



ANOTHER KIND OF ANSWER  for above question

Class 1: dedicated connection between two communicators with acknowledgement of frame delivery.

Class 2: is connection less but provides acknowledgement

Class 3: is connection less and provides no notification of delivery

Class 4: allows fractional bandwidth for virtual circuits

            Class 5: Provides multicast with acknowledgment

            Class F: Is used for switch to switch communication in the fabric.

16. Describe in brief the composition of FC Frame?

                Start of the Frame locator

                Frame header (includes destination id and source id, 24 bytes/6 words)

                Data Payload (encapsulate SCSI instruction can be 0-2112 bytes in length)

                CRC (error checking, 4 bytes)

                                End of Frame (1 byte)

17.Explain FC login process ?

fabric login (FLOGI):    The process by which a Fibre Channel node establishes a logical connection to a fabric switch.

18.What is transmission character and transmission word ?

Transmission Character

A (valid or invalid) 10-bit character transmitted serially over the fibre. Valid Transmission Characters are determined by the 8B/10B encoding specification.

Transmission Word

A string of four consecutive Transmission Characters.

19.What is a frame, sequence and exchange ?

Frame

The basic unit of communication between two N_Ports. Frames are composed of a starting delimiter (SOF), a header, the payload, the Cyclic Redundancy Check (CRC), and an ending delimiter (EOF). The SOF and EOF contain the Special Character and are used to indicate where the frame begins and ends. The 24-byte header contains information about the frame, including the S_ID, D_ID, routing information, the type of data contained in the payload, and sequence/exchange management information. The payload contains the actual data to be transmitted, and may be 0-2112 bytes in length. The CRC is a 4-byte field used for detecting bit errors in the received frame.\

Sequence

A group of related frames transmitted unidirectionally from one N_Port to another.

Exchange

The highest level Fibre Channel mechanism used for communication between N_Ports. Exchanges are composed of one or more related sequences. Exchanges may be bidirectional or unidirectional.

20. What are the services provided by Fabric to all the nodes?

            a) Fabric Login

            b) SNS

            c) Fabric Address Notification

            d) Registered state change notification

            e) Broadcast Servers

21. What are the 5 states of Arbitrary Loop in FC?

            a) Loop Initialization

            b) Loop Monitoring

            c) Loop arbitration

            d) Open Loop

e) Close Loop

22.How many connections  are possible in FC ?

16 million devices connection in switched topology

23.What are the maximum devices supported in each topologies of FC ?

Point to Point : 2 devices

FC-AL : 126 devices

Switched : 16 million

24.What is the difference between FC & iSCSI ?

FC is designed to transmit data between computer devices at data rates of up to 4 gigabits per second (Gbps) (and 10 Gbps in the near future). It is designed to connect computer servers to shared storage devices and for interconnecting storage controllers and drives. It's a high-performance, high-cost technology.

iSCSI (or Internet Small Computer System Interface) is an IP-based storage networking standard that has been touted for the wide range of choices it offers in both performance and price. Some analysts say iSCSI is reliable enough for important data and is frequently used for Microsoft Exchange storage. And, because iSCSI uses TCP/IP and SCSI, which have existed for decades and are well understood in the storage world, it's an appealing option for many in the industry.

 25. Why do we need Login in FC?

            Port Login: To exchange service parameters between N_Ports and N_Ports

Process Login: To establish the SCSI operating environment between two N_PORTS

27. Explain the below terms

LIP

Loop Initialization Primitive Sequence. This Primitive Sequence applies only to the Arbitrated Loop topology. It is transmitted by an L_Port to (re)initialize the Loop.

LIFA

Loop Initialization Fabric Assigned Frame. This is the first Frame transmitted in the Loop initialization process after a temporary Loop master has been selected. L_Ports which have been assigned their AL_PA by the Fabric will select their AL_PA's in this frame as it makes its way around the Loop.

LIHA

Loop Initialization Hard Assigned Frame. This is the third Frame transmitted in the Loop initialization process after a temporary Loop master has been selected. L_Ports which have been programmed to select a particular AL_PA (if available) by the manufacturer will select their AL_PA's in this frame as it makes its way around the Loop.

LILP

Loop Initialization Loop Position Frame. This is the second Frame transmitted in the Loop initialization process after all L_Ports have selected an AL_PA (after LISA has been around the loop). This Frame is transmitted around the Loop so that all L_Ports may know the relative position of all other L_Ports around the Loop. Support for this Frame by an L_Port is optional.

LIPA

Loop Initialization Previously Assigned Frame. This is the second Frame transmitted in the Loop initialization process after a temporary Loop master has been selected. L_Ports which had an AL_PA prior to the Loop initialization will select their AL_PA's in this frame as it makes its way around the Loop.

LIRP

Loop Initialization Report Position Frame. This is the first Frame transmitted in the Loop initialization process after all L_Ports have selected an AL_PA (after LISA has been around the loop). This Frame is transmitted around the Loop so that all L_Ports report their relative physical position on the loop. Support for this Frame by an L_Port is optional.

LISA

Loop Initialization Soft Assigned Frame. This is the fourth Frame transmitted in the Loop initialization process after a temporary Loop master has been selected. L_Ports which did not select an AL_PA in any of the previous Loop Initialization Frames (LIFA, LIPA, or LIHA) will select their AL_PA's in this frame as it makes its way around the Loop.

LISM

Loop Initialization Select Master Frame. This Frame applies only to the Arbitrated Loop topology. It is the first frame transmitted in the initialization process in which L_Ports select an AL_PA. It is used to select a temporary Loop master, or the L_Port that will subsequently initiate transmission of the remaining initialization frames (LIFA, LIPA, LIHA, LISA, LIRP, and LILP).

29. Explain the below terms

Participating Mode

The normal operating mode for an L_Port on a Loop. An L_Port in this mode has acquired an AL_PA and is capable of communicating on the Loop.

Primitive Sequence

An Ordered Set transmitted repeatedly and used to establish and maintain a link. LR, LRR, NOS, and OLS are Primitive Sequences used to establish an active link in a connection between two N_Ports or an N_Port and an F_Port. LIP, LPB, and LPE are Primitive Sequences used in the Arbitrated Loop topology for initializing the Loop and enabling or disabling an L_Port.

Primitive Signal

An Ordered Set used to indicate an event. Idle and R_RDY are used in all three topologies. ARB, OPN, CLS, and MRK are used only in the Arbitrated Loop topology.

Private Loop

An Arbitrated Loop which stands on its own, i.e., it is not connected to a Fabric.

Private NL_Port

An NL_Port which only communicates with other ports on the loop, not with the Fabric. Note that a Private NL_Port may exist on either a Private Loop or a Public Loop.

Public Loop

An Arbitrated Loop which is connected to a Fabric.

Public NL_Port

An NL_Port which may communicate with other ports on the Loop as well as through an FL_Port to other N_Ports connected to the Fabric.

30. Name some of FC components & their vendors which u might have used ?

FC Cables : Single Mode , Multi Mode

FC HBA cards : Qlogic , Emulex , Brocade

FC Switches : Brocade , McData , HP , Cisco

FC Storage arrays  : HP, Netapp , EMC2 , IBM , Hitachi , Dell