FIBRE CHANNEL

 FIBRE CHANNEL

As the speed and memory capacity of personal computers, workstations, and

servers have grown, and as applications have become ever more complex with

greater reliance on graphics and video, the requirement for greater speed in delivering

data to the processor has grown. This requirement affects two methods of data

communications with the processor: I10 channel and network communications.

An I10 channel is a direct point-to-point or multipoint communications link,

predominantly hardware-based and designed for high speed over very short distances.

The I10 channel transfers data between a buffer at the source device and a

buffer at the destination device, moving only the user contents from one device to

another, without regard for the format or meaning of the data. The logic associated

with the channel typically provides the minimum control necessary to manage the

transfer plus hardware error detection. I10 channels typically manage transfers

between processors and peripheral devices, such as disks, graphics equipment, CDROMs,

and video I10 devices.

A network is a collection of interconnected access points with a software protocol

structure that enables communication. The network typically allows many different

types of data transfer, using software to implement the networking protocols

and to provide flow control, error detection, and error recovery. As we have discussed

in this lesson, networks typically manage transfers between end systems over

local, metropolitan, or wide-area distances.

Fibre Channel is designed to combine the best features of both technologiesthe

simplicity and speed of channel communications with the flexibility and interconnectivity

that characterize protocol-based network communications. This fusion

of approaches allows system designers to combine traditional peripheral connection,

host-to-host internetworking, loosely-coupled processor clustering, and multi

media applications in a single multi-protocol interface. The types of channeloriented

facilities incorporated into the Fibre Channel protocol architecture include

Data-type qualifiers for routing frame payload into particular interface

buffers

Link-level constructs associated with individual I10 operations

Protocol interface specifications to allow support of existing I10 channel

architectures, such as the Small Computer System Interface (SCSI)

The types of network-oriented facilities incorporated into the Fibre Channel

protocol architecture include

a Full multiplexing of traffic between multiple destinations

e Peer-to-peer connectivity between any pair of ports on a Fiber Channel

network

Capabilities for internetworking to other connection technologies

Depending on the needs of the application, either channel or networking

approaches can be used for any data transfer. The Fibre Channel Association,

which is the industry consortium promoting Fibre Channel, lists the following ambitious

requirements that Fibre Channel is intended to satisfy [FCA94]:

Full duplex links with two fibers per link

Performance from 100 Mbps to 800 Mbps on a single link (200 Mbps to 1600

Mbps per link)

a Support for distances up to 10 km

Small connectors

High-capacity utilization with distance insensitivity

Greater connectivity than existing multidrop channels

Broad availability (i.e., standard components)

0 Support for multiple costlperformance levels, from small systems to supercomputers

0 Ability to carry multiple existing interface command sets for existing channel

and network protocols

The solution was to develop a simple generic transport mechanism based on

point-to-point links and a switching network. This underlying infrastructure supports

a simple encoding and framing scheme that in turn supports a variety of channel

and network protocols.

Fibre Channel Elements

The key elements of a Fibre Channel network are the end systems, called nodes,

and the network itself, which consists of one or more switching elements. The collection

of switching elements is referred to as a fabric. These elements are interconnected

by point-to-point links between ports on the individual nodes and

switches. Communication consists of the transmission of frames across the point-topoint

links.

Figure 13.15 illustrates these basic elements. Each node includes three or

more ports, called N-ports, for interconnection. Similarly, each fabric-switching

element includes one or more ports, called F-ports. Interconnection is by means of

bidirectional links between ports. Any node can communicate with any other node

connected to the same fabric using the services of the fabric. All routing of frames

between N-ports is done by the fabric. Frames may be buffered within the fabric,

making it possible for different nodes to connect to the fabric at different data rates.

A fabric can be implemented as a single fabric element, as depicted in Figure

13.15, or as a more general network of fabric elements, as shown in Figure 13.16. In

either case, the fabric is responsible for buffering and for routing frames between

source and destination nodes.

The Fibre Channel network is quite different from the other LANs that we

have examined so far. Fibre Channel is more like a traditional circuit-switched or

packet-switched network, in contrast to the typical shared-medium LAN. Thus,

Fibre Channel need not be concerned with medium access control issues. Because

it is based on a switching network, the Fibre Channel scales easily in terms of

N-ports, data rate, and distance covered. This approach provides great flexibility.

Fibre Channel can readily accommodate new transmission media and data rates by

adding new switches and F-ports to an existing fabric. Thus, an existing investment

is not lost with an upgrade to new technologies and equipment. Further, as we shall

see, the layered protocol architecture accommodates existing I10 interface and networking

protocols, preserving the pre-existing investment.

Fibre Channel Protocol architecture

The Fibre Channel standard is organized into five levels. These are illustrated in

Figure 13.17, with brief definitions in Table 13.6. Each level defines a function or set

of related functions. The standard does not dictate a correspondence between levels

and actual implementations, with a specific interface between adjacent levels.

Rather, the standard refers to the level as a "document artifice" used to group

related functions.

Levels FC-0 through FC-2 of the Fibre Channel hierarchy are currently

defined in a standard referred to as Fiber Channel Physical and Signaling Interface

(FC-PH). Currently, there is no final standard for FC-3. At level FC-4, individual

standards have been produced for mapping a variety of channel and network protocols

onto lower levels.

We briefly examine each of these levels in turn in the remainder of this

lesson.

Physical Interface and Media

Fibre Channel level FC-0 allows a variety of physical media and data rates; this

is one of the strengths of the specification. Currently, data rates ranging from

100 Mbps to 800 Mbps per fiber are defined. The physical media are optical fiber,

coaxial cable, and shielded twisted pair. Depending on the data rate and medium

involved, maximum distances for individual point-to-point links range from

50 meters to 10 km.

Transmission Protocol

FC-1, the transmission protocol level, defines the signal encoding technique used

for transmission and for synchronization across the point-to-point link. The encoding

scheme used is 8B/10B, in which each 8 bits of data from level FC-2 is converted

into 10 bits for transmission. See Appendix 13A for a description.

Framing Protocol

Level FC-2, referred to as the Framing Protocol level, deals with the transmission

of data between N-ports in the form of frames. Among the concepts defined at this

level are

Node and N-port and their identifiers

Topologies

Classes of service provided by the fabric

Segmentation of data into frames and reassembly

Grouping of frames into logical entities called sequences and exchanges

Sequencing, flow control, and error control

Common Services

FC-3 provides a set of services that are common across multiple N-Ports of a node.

The functions so-far defined in the draft FC-3 documents include

@ Striping. Makes use of multiple N-Ports in parallel to transmit a single information

unit across multiple links simultaneously; this achieves higher aggregate

throughput. A likely use is for transferring large data sets in real time, as

in video-imaging applications.

Hunt Groups. A hunt group is a set of associated N-Ports at a single node.

This set is assigned an alias identifier that allows any frame sent to this alias

to be routed to any available N-Port within the set. This may decrease latency

by decreasing the chance of waiting for a busy N-Port.

(B Multicast. Delivers a transmission to multiple destinations. This includes

sending to all N-Ports on a fabric (broadcast) or to a subset of the N-Ports on

a fabric.

Mapping

FC-4 defines the mapping of various channel and network protocols to FC-PH. 110

channel interfaces include

Small Computer System Interface (SCSI). A widely used high-speed interface

typically implemented on personal computers, workstations, and server^.^

SCSI is used to support high-capacity and high-data-rate devices, such as disks

and graphics and video equipment.

High-Performance Parallel Interface (HIPPI). A high-speed channel standard

primarily used for mainframe/supercomputer environments. At one

time, HIPPI and extensions to HIPPI were viewed as a possible generalpurpose

high-speed LAN solution, but HIPPI has been superseded by Fibre

Channel.

Network interfaces include

IEEE 802. IEEE 802 MAC frames map onto Fibre Channel frames.

@ Asynchronous Transfer Mode

Fi Internet Protocol (IP). This protocol is described in Lesson 16.

The FC-4 mapping protocols make use of the FC-PH capabilities to transfer

upper-layer protocol (ULP) information. Each FC-4 specification defines the for-

mats and procedures for ULP.

Fibre Channel Physical Media and Topologies

One of the major strengths of the Fibre Channel standard is that it provides a range

of options for the physical medium, the data rate on that medium, and the topology

of the network.

Transmission Media

Table 13.7 summarizes the options that are available under Fibre Channel for physical

transmission medium and data rate. Each entry specifies the maximum pointto-

point link distance (between ports) that is defined for a given transmission

medium at a given data rate. These media may be mixed in an overall configuration.

For example, a single-mode optical link could be used to connect switches in different

buildings, with multimode optical links used for vertical distribution inside, and

shielded twisted pair or coaxial cable links to individual workstations.

Topologies

The most general topology supported by Fibre Channel is referred to as a fabric or

switched topology. This is an arbitrary topology that includes at least one switch to

interconnect a number of N-ports, as shown in Figure 13.18a. The fabric topology

may also consist of a number of switches forming a switched network, with some or

all of these switches also supporting end nodes (Figure 13.16).

Routing in the fabric topology is transparent to the nodes. Each port in the

configuration has a unique address. When data from a node are transmitted into the

fabric, the edge switch to which the node is attached uses the destination port

address in the incoming data frame to determine the destination port location. The

switch then either delivers the frame to another node attached to the same switch

or transfers the frame to an adjacent switch to begin the routing of the frame to a

remote destination.

The fabric topology provides scalability of capacity: As additional ports are

added, the aggregate capacity of the network increases, thus minimizing congestion

and contention, and increasing throughput. The fabric is protocol-independent and

largely distance-insensitive. The technology of the switch itself and of the transmis-

sion links connecting the switch to nodes may be changed without affecting the

overall configuration. Another advantage of the fabric topology is that the burden

on nodes is minimized. An individual Fibre Channel node (end systems) is only

responsible for managing a simple point-to-point connection between itself and the

fabric; the fabric is responsible for routing between N-ports and error detection.

In addition to the fabric topology, the Fibre Channel standard defines two

other topologies. With the point-to-point topology (Figure 13.18b) there are only

two N-ports, and these are directly connected, with no intervening fabric switches.

In this case, there is no routing.

Finally, the arbitrated loop topology (Figure 13.18~)is a simple, low-cost

topology for connecting up to 126 nodes in a loop. The ports on an arbitrated loop

must contain the functions of both N-ports and F-ports; these are called NL-ports.

The arbitrated loop operates in a manner roughly equivalent to the token ring protocols

that we have seen. Each port sees all frames and passes and ignores those not

addressed to itself. There is a token acquisition protocol to control access to the

loop.

The fabric and arbitrated loop topologies may be combined in one configuration

to optimize the cost of the configuration. In this case, one of the nodes on the

arbitrated loop must be a fabric-loop (FL-port) node so that it participates in routing

with the other switches in the fabric configuration.

The type of topology need not be configured manually by a network manager.

Rather, the type of topology is discovered early in the link initialization process.

13.6 WIRELESS LANS

A set of wireless LAN standards has been developed by the IEEE 802.11 committee.

The terminology and some of the specific features of 802.11 are unique to this

standard and are not reflected in all commercial products. However, it is useful to

be familiar with the standard as its features are representative of required wireless

LAN capabilities.

Figure 13.19 indicates the model developed by the 802.11 working group. The

smallest building block of a wireless LAN is a basic service set (BSS), which consists

of some number of stations executing the same MAC protocol and competing for

access to the same shared medium. A basic service set may be isolated, or it may

connect to a backbone distribution system through an access point. The access point

functions as a bridge. The MAC protocol may be fully distributed or controlled by

a central coordination function housed in the access point. The basic service set generally

corresponds to what is referred to as a cell in the literature.

An extended service set (ESS) consists of two or more basic service sets interconnected

by a distribution system. Typically, the distribution system is a wired

backbone LAN. The extended service set appears as a single logical LAN to the

logical link control (LLC) level.

The standard defines three types of stations, based on mobility:

a No-transition. A station of this type is either stationary or moves only within

the direct communication range of the communicating stations of a single

BSS.

BSS-transition. This is defined as a station movement from one BSS to

another BSS within the same ESS. In this case, delivery of data to the station

requires that the addressing capability be able to recognize the new location

of the station.

a ESS-transition. This is defined as a station movement from a BSS in one ESS

to a BSS within another ESS. This case is supported only in the sense that the

station can move. Maintenance of upper-layer connections supported by

802.11 cannot be guaranteed. In fact, disruption of service is likely to occur.

Physical Medium Specification

Three physical media are defined in the current 802.11 standard:

Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and

950 nm.

@ Direct-sequence spread spectrum operating in the 2.4-GHz ISM band. Up to

7 channels, each with a data rate of 1 Mbps or 2 Mbps, can be used.

Frequency-hopping spread spectrum operating in the 2.4-GHz ISM band. The

details of this option are for further study.

Medium Acces Control

The 802.11 working group considered two types of proposals for a MAC algorithm:

distributed-access protocols which, like CSMAICD, distributed the decision to

transmit over all the nodes using a carrier-sense mechanism; and centralized access

protocols, which involve regulation of transmission by a centralized decision maker.

A distributed access protocol makes sense of an ad hoc network of peer workstations

and may also be attractive in other wireless LAN configurations that consist

primarily of bursty traffic. A centralized access protocol is natural for configurations

in which a number of wireless stations are interconnected with each other

and with some sort of base station that attaches to a backbone wired LAN; it is

especially useful if some of the data is time-sensitive or high priority.

The end result of the 802.11 is a MAC algorithm called DFWMAC (distributed

foundation wireless MAC) that provides a distributed access-control mechanism

with an optional centralized control built on top of that. Figure 13.20 illustrates

the architecture. The lower sublayer of the MAC layer is the distributed coordination

function (DCF). DCF uses a contention algorithm to provide access to all traffic.

Ordinary asynchronous traffic directly uses DCF. The point coordination function

(PCF) is a centralized MAC algorithm used to provide contention-free service.

PCF is built on top of DCF and exploits features of DCF to assure access for its

users. Let us consider these two sublayers in turn.

Distributed Coordination Function

The DCF sublayer makes use of a simple CSMA algorithm. If a station has a MAC

frame to transmit, it listens to the medium. If the medium is idle, the station may

transmit; otherwise, the station must wait until the current transmission is complete

before transmitting. The DCF does not include a collision-detection function (i.e.,

CSMAICD) because collision detection is not practical on a wireless network. The

dynamic range of the signals on the medium is very large, so that a transmitting station

cannot effectively distinguish incoming weak signals from noise and the effects

of its own transmission.

To ensure the smooth and fair functioning of this algorithm, DCF includes a

set of delays that amounts to a priority scheme. Let us start by considering a single

delay known as an interframe space (IFS). In fact, there are three different IFS values,

but the algorithm is best explained by initially ignoring this detail. Using an

IFS, the rules for CSMA access are as follows:

I. A station with a frame to transmit senses the medium. If the medium is idle,

the station waits to see if the medium remains idle for a time equal to IFS, and,

if this is so, the station may immediately transmit.

2. If the medium is busy (either because the station initially finds the medium

busy or because the medium becomes busy during the IFS idle time), the station

defers transmission and continues to monitor the medium until the current

transmission is over.

3. Once the current transmission is over, the station delays another IFS. If the

medium remains idle for this period, then the station backs off using a binary

exponential backoff scheme and again senses the medium. If the medium is

still idle, the station may transmit.

As with Ethernet, the binary exponential backoff provides a means of handling

a heavy load. If a station attempts to transmit and finds the medium busy, it

backs off a certain amount and tries again. Repeated failed attempts to transmit

result in longer and longer backoff times.

The above scheme is refined for DCF to provide priority-based access by the

simple expedient of using three values for IFS:

SIFS (short IFS). The shortest IFS, used for all immediate response actions,

as explained below.

PIFS (point coordination function IFS). A mid-length IFS, used by the centralized

controller in the PCF scheme when issuing polls.

DIFS (distributed coordination function IFS). The longest IFS, used as a minimum

delay for asynchronous frames contending for access.

Figure 13.21a illustrates the use of these time values. Consider first the SIFS.

Any station using SIFS to determine transmission opportunity has, in effect, the

highest priority, because it will always gain access in preference to a station waiting

an amount of time equal to PIFS or DIFS. The SIFS is used in the following circumstances:

" Acknowledgment (ACK). When a station receives a frame addressed only to

itself (not multicast or broadcast) it responds with an ACK frame after waiting

only for an SIFS gap; this has two desirable effects. First, because collision

detection is not used, the likelihood of collisions is greater than with

CSMAICD, and the MAC-level ACK provides for efficient collision recovery.

Second, the SIFS can be used to provide efficient delivery of an LLC protocol

data unit (PDU) that requires multiple MAC frames. In this case, the following

scenario occurs. A station with a multiframe LLC PDU to transmit

sends out the MAC frames one at a time. Each frame is acknowledged by the

recipient after SIFS. When the source receives an ACK, it immediately (after

SIFS) sends the next frame in the sequence. The result is that once a station

has contended for the channel, it will maintain control of the channel until it

has sent all of the fragments of an LLC PDU.

" Clear to Send (CTS). A station can ensure that its data frame will get through

by first issuing a small Request to Send (RTS) frame. The station to which this

frame is addressed should immediately respond with a CTS frame if it is ready

to receive. All other stations receive the RTS and defer using the medium

until they see a corresponding CTS, or until a timeout occurs.

Poll response. This is explained in the discussion of PCF, below.

The next longest IFS interval is the PIFS; this is used by the centralized controller

in issuing polls and takes precedence over normal-contention traffic. However,

those frames transmitted using SIFS have precedence over a PCF poll.

Finally, the DIFS interval is used for all ordinary asynchronous traffic.

Point Coordination Function

PCF is an alternative access method implemented on top of the DCF. The operation

consists of polling with the centralized polling master (point coordinator). The

point coordinator makes use of PIFS when issuing polls. Because PIFS is smaller

than DIFS, the point coordinator can seize the medium and lock out all asynchronous

traffic while it issues polls and receives responses.

As an extreme, consider the following possible scenario. A wireless network

is configured so that a number of stations with time-sensitive traffic are controlled

by the point coordinator while remaining traffic, using CSMA, contends for access.

The point coordinator could issue polls in a round-robin fashion to all stations configured

for polling. When a poll is issued, the polled station may respond using

SIFS. If the point coordinator receives a response, it issues another poll using PIFS.

If no response is received during the expected turnaround time, the coordinator

issues a poll.

If the discipline of the preceding paragraph were implemented, the point coordinator

would lock out all asynchronous traffic by repeatedly issuing polls. To prevent

this situation, an interval known as the superframe is defined. During the first

part of this interval, the point coordinator issues polls in a round-robin fashion to

all stations configured for polling. The point coordinator then idles for the remainder

of the superframe, allowing a contention period for asynchronous access.

Figure 13.21b illustrates the use of the superframe. At the beginning of a

superframe, the point coordinator may optionally seize control and issue polls for

a give period of time. This interval varies because of the variable frame size issued

by responding stations. The remainder of the superframe is available for contention-

based access. At the end of the superframe interval, the point coordinator

contends for access to the medium using PIFS. If the medium is idle, the point coordinator

gains immediate access, and a full superframe period follows. However, the

medium may be busy at the end of a superframe. In this case, the point coordinator

must wait until the medium is idle to gain access; this results in a foreshortened

superframe period for the next cycle.