- Email: [email protected]
Overview of FFOL — FDDI Follow-On LAN
FFOL
O v e r v i e w of FFOL - - F D D I Follow-On LAN Floyd E Ross and Robert L Fink* look ahead to what will follow FDDI
FDDI has gained wide acceptance as the follow-on I_AN for Ethemet (IEEE802.3) and Token Ring (IEEE802.5) LANs as higher performance is required. FDDI, including FDDI-II isochronous services, will be widely deployed in the 1990s as both a backbone and deslctop distribution LAN. But what will follow FDDI? Task Group X3T9.5 of ANSI's Accredited Standards Committee (ASC) X3, the standards committee that developed and standardized FDDI, has initiated work on an FDDI Follow-On I_ANproject, called FFOL, to capitalize on the high degree of consensus that exists today for FDDI. FFOL will include both FDDI and FDDI-II services and will address the key issues for next generation networking and communications systems: interfacing to the SONET/Synchronous Digital Hierarchy, as well as ATM/BISDN services. This paper serves as an introduction to FFOL, focusing on FFOL requirements, project organization, and the architecture of the FFOL. Future papers in this continuing series will address specific components of the FFOL standard. Keywords: FDDI, FFOL, standards, Ethernet, token ring
PERSPECTIVE ON LANS Ethernet (IEEE 802.3) and Token Ring (IEEE 802.5) enjoyed wide use in the mid-to late-1980s. At first, single isolated local area networks (LAN) were the norm, but as interconnect technology (i.e. routers and bridges) developed, LANs were interconnected, usually with a single or multiple backbone LAN. As 1990 approached, many organizations were faced with the overload of their I_AN backbones as more and more LANs were interconnected. FDDI (Fibre Distributed Data Interface), with Timeplex Inc., Woodcliff Lake, NJ, USA *Lawrence Berkeley Laboratory, One Cyclotron Road, 50B-225B, Berkeley, CA 94720, USA
its 100Mbit/s performance, as compared to its predecessors (10 Mbit/s for Ethernet and 4 or 16 Mbit/s for token ring) rose to meet the challenge of a higherperformance backbone LAN. In this same late 1980 time frame, workstations evolved to higher performance levels through advances in RISC architectures and ever faster memory and disk drive technologies. As a consequence, networks became constrained by IEEE 802.3 and IEEE 802.5 LAN performance. LANs typically emerge as a LAN interconnect backbone, then move downward to service the higher performance systems served by the LANs they interconnect. Thus FDDI, widely accepted by the early 1990s as the backbone LAN for the heavily and widely used IEEE802.3 and IEEE 802.5 LANs, moved to the desktop to provide the higher bandwidth needed by these higher performance workstations. As the scenario repeats, it is now anticipated that a follow-on to FDDI will be needed in the 1995 timeframe, first as a backbone LAN, and later as a primary workstation attachment I_AN. FDDI has taken approximately eight years to become fully specified as a standard. It is expected that a next generation LAN standard, building on the expertise developed by the FDDI work, can be accomplished by 1995. It is widely accepted that networks have become so ubiquitous that no company can 'go it alone' with proprietary network standards. The consensus-based processes of the standards community seems required for a successful standard. The standards project underway in Task Group X3T9.5 to meet the projected need for a follow-on to FDDI is addressed here.
FFOL FFOL work was initiated in February 1990 by Task Group X3T9.5 of ASC X3 to establish requirements and formal
0140-3664/92/001005-06 © 1992 Butterworth-Heinemann Ltd vol 15 no I january/february 1992
5
FFOL projects for a next generation I_AN. In the following sections of this paper the background for startup of the FFOL project will be given, including its requirements and current status.
X3T9.5 background The American National Standards Institute (ANSI) has an Accredited Standards Committee (ASC) process to accredit national standards efforts in specific fields. ASC X3 is the accredited body for information processing standards. Within X3 are many Technical Committees to carry out specific standards work. Technical Committee X3T9 develops standards in the high-performance input/ output channel and LAN areas. X3T9 has three Task Groups: X3T9.2 working on SCSI, SCSI-2 and SCSl-3 standards; X3T9.3 working on IPI, HIPPI and Fibre Channel standards; and X3T9.5 working on FDDI and FFOI_ standards. Task Group X3T9.5 was formed in 1979 to provide a high-performance interconnection network for high-end systems, as it was perceived at that time that low-end systems did not require high-performance. This early work, Local Distributed Data Interface (LDDI), was not fibre optic-based, and did not gather a wide consensus among developers. By 1982, a new concept was introduced, a fibre optic-based FDDI I ring. Soon thereafter, most of X3T9.5's efforts were redirected to this exciting new fibre optic-based LAN project.
FDDI project success as a standards process FDDI was unique in several important ways other than its innovative technical characteristics. The design for FDDI was a product of consensus work directed by the X3T9.5 Task Group and a number of ad hoc working groups established as needed. The result was an unparalleled cooperative level of effort of the computer, the networking, and the communications industries. Key contributions to FDDI were made by individuals from a variety of disciplines. This not only improved FDDI - it broadened its base of support. This cooperative spirit, as well as acceptance of this process for progressing design work, has made X3T9.5 a leader of LAN standards, and with this leadership came the success of FDDI. To date there are five FDDI chip sets, with more to follow. At least three different SMT implementations are available. Many FDDI products are becoming available (from computer system and network vendors). X3T9.5 has a large body of expertise available, with over 800 individuals on the participation list and meeting attendance as high as 145 people from ten countries. The current active membership of X3T9.5 is approximately 75 organizations from six countries.
FFOL project startup By 1989 there was discussion by X3T9.5 about what the next step following FDDI should be. Participants well
6
knew the length of time it took to specify FDDI, and even though there was agreement that work could progress faster in the future, there was a growing impatience to get started. By the end of 1989 there was pressure for an ad hoc working group to look into a new direction and prepare a report. In February 1990 the first working group meeting was held, with agreement reached to start work on a Requirements and Design Considerations document for what is now known as FFOL. By June 1990 the Requirements and Design Considerations document 2 was completed, and discussion started on an acceptable architectural structure for the work to follow. This led to the development of six project proposals 3 for portions of FFOL work which were presented to the ASC X3 Standards Planning and Requirements Committee (SPARC) in October of 1990. In January 1991, the first six FFOL projects were approved by X3 and assigned to X3T9.5.
Why not a scaled-up FDDI? A natural question to ask is why not just design a faster, i.e. scaled-up, FDDI ('tweak-up the clock' so to speak). There had been general discussion of this concept for several years, with an initial disbelief this was reasonable to do for simple technical reasons, e.g. clocking issues with the current physical layer design. However, more recently there has been a general consensus that a scaled-up version of FDDI is technically possible. Then the issue became that of whether a scaled-up FDDI would be right choice? As more designers looked towards their experience with FDDI, and their needs for the future, other issues emerged which spoke against this. For one, a next generation architecture should be scalable beyond Gbit/s. A scaled up FDDI would probably 'topout' at about Gbit/s due to limitations of the token ring access protocol. In addition, there appeared to be a need for multiple speeds and access methods that enhanced bandwidth utilization. There is also growing demand for payload rate matching with common carrier offerings, in particular the SONET/Synchronous Digital Hierarchy (SDH). The current FDDI encoding structure is not efficient for mapping into SONET services, and the design of the physical layer of FDDI does not lend itself to replacement with different encoding structures. Otherwise, the emergence of cell-based switching architectures, in the form of ATM-based and Broadband ISDN (ATM/BISDN) services, led to a feeling that interfacing to these structures was important for the future. FDDI did not readily lend itself to this. These conclusions were not reached explicitly by the X3T9.5 community, but rather are an 'after the fact' reading of a process that led to the requirements and design guidelines document for FFOL that would rather seem to preclude a simple scaling up of FDDI as the solution.
computer communications
FFOL REQUIREMENTS AND DESIGN GUIDELINES FOR FFOL As stated previously, work started in early 1990 on a Requirements and Design Considerations document 2 for FFOL. Results of this work are summarized here to provide an understanding of the current directions of the FFOL project. (Whenever Requirements is used in the following summary the entire Requirements and Design Considerations document is being referred to.)
Services The Requirements state that FFOL should be capable of providing sufficient bandwidth to act as a backbone for multiple FDDI networks, including FDDI packets, FDDI-II channels and Wideband Channels (WBC), and also allow efficient interconnection to wide area networks (WAN) such as SMDS and BISDN. FFOL should support many and varied applications; for example, remote file access, image transfer, video, videoconferencing, voice, multimedia applications, transaction processing, and low latency real time applications. These varied applications (services) would be supported through various media access modes (discussed below).
recovery, though simplex links are not ruled out. Physical topologies of dual rings and trees would be supported, thus allowing the use of existing FDDI cable plants, including existing star-wired cabling from wiring closets to offices. FFOI_ stations could have either single or dual data paths, with segments of public networks being included in FFOLtopologies. FFOLwould have recovery mechanisms to provide complete connectivity, despite a single fault in the dual dng. Multiple faults are to be recoverable, but without complete connectivity. The Requirements state that FDDI fibre types will be considered for FFOL, including single- and multi-mode. For FDDI specified multi-mode fibre, FFOL might be limited to 200 meters at 1.25 Gbit/s rates, but this would still allow the use of most all wiring closet to office wiring. For FDDI specified single-mode fibre, FFOL would be capable of distances up to tens of kilometers between nodes. FFOL would support the use of the FDDI standard MIC receptacle. If FFOL is to utilize the FDDI dual ring of trees cable plant it must be designed to work efficiently both on a dual ring and in the single path found in the trees. In particular, FFOL would have to function when only a single data path, in effect creating a single ring, passes through a station, using that ring independent of the other ring.
Data rate considerations Scalability The Requirements state that FFOL should have a data rate in excess of FDDI, with an initial physical medium layer that supports a rate that is less than 1.25 Gbit/s on FDDI multi-mode fibre. The FFOL payload rate would be selected to match that of the SON ET/SDH, thus allowing efficient interfacing with both private fibre and public network links, while minimizing interface buffering requirements over the public network. SONET/SDH public network and payload rates discussed in the FFOL Requirements are listed in Table I.
The Requirements lay out a design goal that FFOL would have an access and recovery protocol that is relatively insensitive to network size, with a default parameter for total network distance greater than 100 km. In addition, the access and recovery protocols are to be relatively insensitive to the network bit rate.
Physical encoding The Requirements discuss several different encodings that are to be considered for FFOL (note that FDDI is a 4/5 code):
Topology and cabling considerations The Requirements state that FFOL's basic links shall be duplex for providing a back-channel for link failure
Table1. Public network and payload rates for SON ET/SDH (*Note that public carriers have indicated their intent to support $T5-3, $T$-12 and $T$-48)
SDH service
Network rate (Mbit/s)
Payload rate (Mbit/s)
STS-3c* STS-12c" STS-24c STS-48c
155.52 622.08 1244.16 2488.322
149.760 600.768 1202.112 2404.800
• 4/6 for a balanced code with no DC baseline wander; • 8/10 to provide more control codes for signalling and framing delimiters as well as possible fault recovery (note 8/10 code also has no DC baseline wander); • larger encodings for possible greater parallelism on the electrical interface; • other encodings will be considered as well as public network encodings.
Media accessing modes and methods
vol 15 no 1 january/february 1992
The Requirements discuss accessing modes and methods, stating that the range of varied applications could be supported through the provision of three media access modes:
7
FFOL • Asynchronous mode, using an ATM-like protocol, that is easily bridged to ATM/BISDN, for connection oriented and for connectionless transport services. • Isochronous mode, for circuit switched services, providing FDDI-II type services. • Packet mode, for providing FDDl-like packet services. Then several accessing methods are outlined that are to be examined for use in FFOL: • Slotted ring or bus: a shared medium access method based on a slotted protocol might be used to provide isochronous, ATM and packet services. The slots provide cells to contain the various size frames or portions of frames. Slots may be either of a fixed size or of a variable size and number to meet requests or the current services load balance. • Insertion ring or bus: the sourcing of a frame is started by a station only when no frames are entering the station. An insertion protocol allows for a high ring utilization, but the variable latency may produce buffering problems for isochronous service. • Token ring or bus: a token ring allows a scaling of the current FDDI Media Access Control (MAC) protocols, simplifying the standardization process. • Combination: a protocol that utilizes combinations of the above access methods.
Logical topology At the logical level the Requirements state that both ring and bus protocols will be examined, as well as others. Both ring and bus protocols allow the assignment of a centralized control point for the generation of slots or allocation of resources. A ring protocol may provide advantages for distributed recovery mechanisms, while a bus protocol may use the head end to generate slots, and does not have to address the stripping issue unless spatial reuse is performed.
Media access properties A number of media access properties and issues are discussed in the Requirements. FFOL would provide mechanisms for reservations, preallocations and priorities to allow different classes and priorities of service. It is expected that from two to eight levels would be supported, sufficiently specified to be consistently usable across stations. FFOL would have to carefully address frame stripping for ring accessing methods. Stripping of frames based on the destination address would allow slots to be reused, increasing the available bandwidth. Broadcast frames would, however, have to be stripped by the sourcing station. Stripping might also have to be performed at the sourcing station to provide consistency with FDDI. FFOL would provide an efficient mechanism for supporting multicast addressing. Multicast addressing allows the location of resources and an efficient utilization
8
of the media. Multicast frames must be removed by either the last receiving station or the sourcing station. It should be designed to provide consistency in a multi-vendor environment and allow a large numbers of addresses to be supported in a straightforward manner. For FFOL, bridging options would include defining control indicators to provide useful status for bridged frames. ATM frames, as well as FDDI and IEEE802 packets could be bridged onto FFOL. Also, FDDI-II channels and Wideband Channels (WBC) are to be supported by FFOL.
Recovery The Requirements discuss redundancy, stating that it will be allowed in FFOL. The design of FFOL would be optimized for the normal condition of no faults, with upper bounds being placed on the recovery and reconfiguration times.
ARCHITECTURE FOR AN FFOL PROJECT Upon conclusion of work on the FFOL Requirements and Design Considerations, it became clear that there was strong consensus to establish an FFOL project. In theANSI ASC X3 community projects are started by the approval of one or more project proposals, which was done, as mentioned earlier in this paper, by going through the specified ASC X3 process. Without an approved project or projects, there would be no authorization to continue meeting beyond the first few ad hoc working group meetings. However, to make a project proposal there has to be enough consensus for an architecture for the resultant standards to allow specific projects to be started. Although this has its disadvantages in that a potentially premature architecture may be selected, it has the advantage of providing an early focus for the work. It is also the case that the architecture may be changed, with a potential change in the scope and number of projects. This is, in fact, specifically noted in the FFOL Project Proposal documents. Discussion of possible architectures was started in the X3T9.5 FFOL working group, with much initial focus on the current FDDI 'family' architecture (see Figure 1), and the requirements and design guidelines previously developed. It must again be emphasized that this was not meant to be an end-all discussion of an FFOLarchitecture, but rather a choice for purposes of making project proposals and moving ahead. The resultant architecture (see Figure 2) was one that looks remarkably like the current FDDI architecture. Though there was some sentiment to not break portions of the architecture up at this time, e.g. leave SMUX, AMAC and IMAC as one standard, there was consensus that it was important to allow independent efforts to proceed on these parts of FFOL in the interest of parallel work effort.
computer communications
FFOL ~4rcu4I
S~lc.~tt~ f
LO(jNCal Llt~k n
mode fibre. The signalling rate would be significantly higher than FDDI, capable of supporting data rates in the 600 Mbit/s to 1 + Gbit/s range. FDDI-compatible cabling plant would be used where feasible.
Scope of PHY standard A Physical Layer Protocol standard which specifies the encode/decode, clocking and data framing over PMD and SONET links. It would be scalable from STS-3 to at least STS-48 data rates in the Synchronous Digital Hierarchy (SDH).
Data t
Lay__er Physil Layer
SOb
Scope of SMUX standard
STS
Figure 1. FDDI standards architecture (* = HRC requires the use of PHY-2 and MAC-2; otherwise, any combination of MAC or MAC-2 and PHY or PHY-2 is allowed)
A Service Multiplexer standard which specifies the multiplexing of isochronous and asynchronous (circuit switched and packet) data. It would make efficient use of PHY links and support both open and closed loop topologies. It would be scalable from STS-3 to at least STS48 data rates in the Synchronous Digital Hierarchy (SDH).
Scope of AMAC standard An Asynchronous Media Access Control standard which specifies access to the medium, addressing` data checking, and packet generation/reception for asynchronous (packet switched) service, over an SMUX service.
Data Unk Layer
Physical Layer
Scope of IMAC standard An Isochronous Media Access Control standard which specifies access to the medium, and transmission channel generation/reception for isochronous (circuit switched) service, over an SMUX service.
Figure 2.
FFOL standards architecture
Scope of SMIT standard Also, several possible architectural enhancements were left out for the first generation of FFOL work. These primarily centred around the relaying and interconnection of multiple portions of an FFOL network operating over different topologies and links (e.g. SONET/SDH), at different speeds and for different levels of service. The resulting six approved FFOL projects 3 are summarized here, as they were approved as project by X3. The anticipated completion of these initial FFOLstandards is in the 1993-1995 timeframe.
Scope of PMD standard A Physical Layer Medium Dependent standard which specifies the private optical fibre links and related optical components. It would include single-mode and multi-
vol 15 no 1 january/february 1992
A Station Management standard which provides control for PMD, PHY, SMUX, AMAC and IMAC, and specifies the FFOL station configurations, topology configurations, and the control required for proper operation of stations as a member of the FFOL network. SMT also provides the interface to the FFOL network management agent.
Critical issues and goals for FFOL Several critical issues face the FFOL working group. For example, it will soon have to choose whether or not to go in the direction of ATM/BISDN based structures for AMAC services and, if so, how to proceed effectively. It is strongly believed that the viability of FFOL will, to a large extent, rely on the cost effectiveness of a low-cost
9
FFOL PMD that will operate over FDDI multi-mode fibre from the communications closet to the office/desktop at the 600 MbitJs payload rate (possibly 750 Mbaud if an 8/10 encoding is used). Several goals or expectations for an FFOL PMD, and other parts of the protocol, are noted below. The use of Bi-CMOS for the serial I/O portion of the FFOL PMD operating at 750 Mbaud is a practical target in 1995. CMOS is an outside possibility if multi-level signalling can be used over the fibre. If a 32-bit structure for FFOL protocols can be maintained, CMOS for the parallel portions of the protocol operating at 750 Mbaud is a practical target in 1995. There is also a reasonable prospect for low-cost laser diodes by 1995 operating at 750 Mbaud. Another reasonable goal is to operate in the I to 2 km range over FDDI multi-mode fibre operating at 750 Mbaud. At the 2.4Gbit/s payload rate (3Gbaud with 8/10 encoding), Bi-CMOS for the parallel portions of the protocol is quite possible by 1995, with an outside chance for CMOS.
CONCLUSION FFOL is the natural choice as a follow-on I_AN to FDDI. FFOL's higher performance and ability to handle all FDDI data formats will allow its use as a backbone to existing FDDI installations. FFOL will also be used as a replacement for FDDI and other LANs when its higher performance is required. In addition, the enhanced services offered by
I0
FFOL, e.g. ATM/BISDN-Iike services, will allow it to address applications beyond the scope of current-day LANs. The FFOL project is off to a strong start. While there are obviously critical issues that will need resolution, a forum for this work has been established in the X3T9.5 Task Group. The authors, along with other participants in the FDDI/FFOI_ work, believe that the possibility for success for the FFOI_project is best assured in an organization with X3Tg.5's expertise and proven track record. FFOL will be a success because the time is right and the process is right!
ACKNOWLEDGEMENTS This work was supported in part by the US Department of Energy under Contract Number DE-AC03-76F00098.
REFERENCES 1 Ross, F E 'An overview of FDDI: the Fiber Distributed Data Interface', IEEE J. Selected Areas in Commun., Vol 7 No 7 (September 1989) 20chellree, K, Horvalh, S and Mityko, G Requirements and Design Considerations for the FDDI Follow-On LAN, X3T9.5 Working Group Document FFOL-007 (17 May 1990) 3 X3T9, Project Proposals for a New Family of X3 Standards - FDDI Follow-On Local Area Network (FFOL), X3T9 Technical Committee Document X3T9/ 90-110 (24 August 1990)
computer communications
O v e r v i e w of FFOL - - F D D I Follow-On LAN Floyd E Ross and Robert L Fink* look ahead to what will follow FDDI
FDDI has gained wide acceptance as the follow-on I_AN for Ethemet (IEEE802.3) and Token Ring (IEEE802.5) LANs as higher performance is required. FDDI, including FDDI-II isochronous services, will be widely deployed in the 1990s as both a backbone and deslctop distribution LAN. But what will follow FDDI? Task Group X3T9.5 of ANSI's Accredited Standards Committee (ASC) X3, the standards committee that developed and standardized FDDI, has initiated work on an FDDI Follow-On I_ANproject, called FFOL, to capitalize on the high degree of consensus that exists today for FDDI. FFOL will include both FDDI and FDDI-II services and will address the key issues for next generation networking and communications systems: interfacing to the SONET/Synchronous Digital Hierarchy, as well as ATM/BISDN services. This paper serves as an introduction to FFOL, focusing on FFOL requirements, project organization, and the architecture of the FFOL. Future papers in this continuing series will address specific components of the FFOL standard. Keywords: FDDI, FFOL, standards, Ethernet, token ring
PERSPECTIVE ON LANS Ethernet (IEEE 802.3) and Token Ring (IEEE 802.5) enjoyed wide use in the mid-to late-1980s. At first, single isolated local area networks (LAN) were the norm, but as interconnect technology (i.e. routers and bridges) developed, LANs were interconnected, usually with a single or multiple backbone LAN. As 1990 approached, many organizations were faced with the overload of their I_AN backbones as more and more LANs were interconnected. FDDI (Fibre Distributed Data Interface), with Timeplex Inc., Woodcliff Lake, NJ, USA *Lawrence Berkeley Laboratory, One Cyclotron Road, 50B-225B, Berkeley, CA 94720, USA
its 100Mbit/s performance, as compared to its predecessors (10 Mbit/s for Ethernet and 4 or 16 Mbit/s for token ring) rose to meet the challenge of a higherperformance backbone LAN. In this same late 1980 time frame, workstations evolved to higher performance levels through advances in RISC architectures and ever faster memory and disk drive technologies. As a consequence, networks became constrained by IEEE 802.3 and IEEE 802.5 LAN performance. LANs typically emerge as a LAN interconnect backbone, then move downward to service the higher performance systems served by the LANs they interconnect. Thus FDDI, widely accepted by the early 1990s as the backbone LAN for the heavily and widely used IEEE802.3 and IEEE 802.5 LANs, moved to the desktop to provide the higher bandwidth needed by these higher performance workstations. As the scenario repeats, it is now anticipated that a follow-on to FDDI will be needed in the 1995 timeframe, first as a backbone LAN, and later as a primary workstation attachment I_AN. FDDI has taken approximately eight years to become fully specified as a standard. It is expected that a next generation LAN standard, building on the expertise developed by the FDDI work, can be accomplished by 1995. It is widely accepted that networks have become so ubiquitous that no company can 'go it alone' with proprietary network standards. The consensus-based processes of the standards community seems required for a successful standard. The standards project underway in Task Group X3T9.5 to meet the projected need for a follow-on to FDDI is addressed here.
FFOL FFOL work was initiated in February 1990 by Task Group X3T9.5 of ASC X3 to establish requirements and formal
0140-3664/92/001005-06 © 1992 Butterworth-Heinemann Ltd vol 15 no I january/february 1992
5
FFOL projects for a next generation I_AN. In the following sections of this paper the background for startup of the FFOL project will be given, including its requirements and current status.
X3T9.5 background The American National Standards Institute (ANSI) has an Accredited Standards Committee (ASC) process to accredit national standards efforts in specific fields. ASC X3 is the accredited body for information processing standards. Within X3 are many Technical Committees to carry out specific standards work. Technical Committee X3T9 develops standards in the high-performance input/ output channel and LAN areas. X3T9 has three Task Groups: X3T9.2 working on SCSI, SCSI-2 and SCSl-3 standards; X3T9.3 working on IPI, HIPPI and Fibre Channel standards; and X3T9.5 working on FDDI and FFOI_ standards. Task Group X3T9.5 was formed in 1979 to provide a high-performance interconnection network for high-end systems, as it was perceived at that time that low-end systems did not require high-performance. This early work, Local Distributed Data Interface (LDDI), was not fibre optic-based, and did not gather a wide consensus among developers. By 1982, a new concept was introduced, a fibre optic-based FDDI I ring. Soon thereafter, most of X3T9.5's efforts were redirected to this exciting new fibre optic-based LAN project.
FDDI project success as a standards process FDDI was unique in several important ways other than its innovative technical characteristics. The design for FDDI was a product of consensus work directed by the X3T9.5 Task Group and a number of ad hoc working groups established as needed. The result was an unparalleled cooperative level of effort of the computer, the networking, and the communications industries. Key contributions to FDDI were made by individuals from a variety of disciplines. This not only improved FDDI - it broadened its base of support. This cooperative spirit, as well as acceptance of this process for progressing design work, has made X3T9.5 a leader of LAN standards, and with this leadership came the success of FDDI. To date there are five FDDI chip sets, with more to follow. At least three different SMT implementations are available. Many FDDI products are becoming available (from computer system and network vendors). X3T9.5 has a large body of expertise available, with over 800 individuals on the participation list and meeting attendance as high as 145 people from ten countries. The current active membership of X3T9.5 is approximately 75 organizations from six countries.
FFOL project startup By 1989 there was discussion by X3T9.5 about what the next step following FDDI should be. Participants well
6
knew the length of time it took to specify FDDI, and even though there was agreement that work could progress faster in the future, there was a growing impatience to get started. By the end of 1989 there was pressure for an ad hoc working group to look into a new direction and prepare a report. In February 1990 the first working group meeting was held, with agreement reached to start work on a Requirements and Design Considerations document for what is now known as FFOL. By June 1990 the Requirements and Design Considerations document 2 was completed, and discussion started on an acceptable architectural structure for the work to follow. This led to the development of six project proposals 3 for portions of FFOL work which were presented to the ASC X3 Standards Planning and Requirements Committee (SPARC) in October of 1990. In January 1991, the first six FFOL projects were approved by X3 and assigned to X3T9.5.
Why not a scaled-up FDDI? A natural question to ask is why not just design a faster, i.e. scaled-up, FDDI ('tweak-up the clock' so to speak). There had been general discussion of this concept for several years, with an initial disbelief this was reasonable to do for simple technical reasons, e.g. clocking issues with the current physical layer design. However, more recently there has been a general consensus that a scaled-up version of FDDI is technically possible. Then the issue became that of whether a scaled-up FDDI would be right choice? As more designers looked towards their experience with FDDI, and their needs for the future, other issues emerged which spoke against this. For one, a next generation architecture should be scalable beyond Gbit/s. A scaled up FDDI would probably 'topout' at about Gbit/s due to limitations of the token ring access protocol. In addition, there appeared to be a need for multiple speeds and access methods that enhanced bandwidth utilization. There is also growing demand for payload rate matching with common carrier offerings, in particular the SONET/Synchronous Digital Hierarchy (SDH). The current FDDI encoding structure is not efficient for mapping into SONET services, and the design of the physical layer of FDDI does not lend itself to replacement with different encoding structures. Otherwise, the emergence of cell-based switching architectures, in the form of ATM-based and Broadband ISDN (ATM/BISDN) services, led to a feeling that interfacing to these structures was important for the future. FDDI did not readily lend itself to this. These conclusions were not reached explicitly by the X3T9.5 community, but rather are an 'after the fact' reading of a process that led to the requirements and design guidelines document for FFOL that would rather seem to preclude a simple scaling up of FDDI as the solution.
computer communications
FFOL REQUIREMENTS AND DESIGN GUIDELINES FOR FFOL As stated previously, work started in early 1990 on a Requirements and Design Considerations document 2 for FFOL. Results of this work are summarized here to provide an understanding of the current directions of the FFOL project. (Whenever Requirements is used in the following summary the entire Requirements and Design Considerations document is being referred to.)
Services The Requirements state that FFOL should be capable of providing sufficient bandwidth to act as a backbone for multiple FDDI networks, including FDDI packets, FDDI-II channels and Wideband Channels (WBC), and also allow efficient interconnection to wide area networks (WAN) such as SMDS and BISDN. FFOL should support many and varied applications; for example, remote file access, image transfer, video, videoconferencing, voice, multimedia applications, transaction processing, and low latency real time applications. These varied applications (services) would be supported through various media access modes (discussed below).
recovery, though simplex links are not ruled out. Physical topologies of dual rings and trees would be supported, thus allowing the use of existing FDDI cable plants, including existing star-wired cabling from wiring closets to offices. FFOI_ stations could have either single or dual data paths, with segments of public networks being included in FFOLtopologies. FFOLwould have recovery mechanisms to provide complete connectivity, despite a single fault in the dual dng. Multiple faults are to be recoverable, but without complete connectivity. The Requirements state that FDDI fibre types will be considered for FFOL, including single- and multi-mode. For FDDI specified multi-mode fibre, FFOL might be limited to 200 meters at 1.25 Gbit/s rates, but this would still allow the use of most all wiring closet to office wiring. For FDDI specified single-mode fibre, FFOL would be capable of distances up to tens of kilometers between nodes. FFOL would support the use of the FDDI standard MIC receptacle. If FFOL is to utilize the FDDI dual ring of trees cable plant it must be designed to work efficiently both on a dual ring and in the single path found in the trees. In particular, FFOL would have to function when only a single data path, in effect creating a single ring, passes through a station, using that ring independent of the other ring.
Data rate considerations Scalability The Requirements state that FFOL should have a data rate in excess of FDDI, with an initial physical medium layer that supports a rate that is less than 1.25 Gbit/s on FDDI multi-mode fibre. The FFOL payload rate would be selected to match that of the SON ET/SDH, thus allowing efficient interfacing with both private fibre and public network links, while minimizing interface buffering requirements over the public network. SONET/SDH public network and payload rates discussed in the FFOL Requirements are listed in Table I.
The Requirements lay out a design goal that FFOL would have an access and recovery protocol that is relatively insensitive to network size, with a default parameter for total network distance greater than 100 km. In addition, the access and recovery protocols are to be relatively insensitive to the network bit rate.
Physical encoding The Requirements discuss several different encodings that are to be considered for FFOL (note that FDDI is a 4/5 code):
Topology and cabling considerations The Requirements state that FFOL's basic links shall be duplex for providing a back-channel for link failure
Table1. Public network and payload rates for SON ET/SDH (*Note that public carriers have indicated their intent to support $T5-3, $T$-12 and $T$-48)
SDH service
Network rate (Mbit/s)
Payload rate (Mbit/s)
STS-3c* STS-12c" STS-24c STS-48c
155.52 622.08 1244.16 2488.322
149.760 600.768 1202.112 2404.800
• 4/6 for a balanced code with no DC baseline wander; • 8/10 to provide more control codes for signalling and framing delimiters as well as possible fault recovery (note 8/10 code also has no DC baseline wander); • larger encodings for possible greater parallelism on the electrical interface; • other encodings will be considered as well as public network encodings.
Media accessing modes and methods
vol 15 no 1 january/february 1992
The Requirements discuss accessing modes and methods, stating that the range of varied applications could be supported through the provision of three media access modes:
7
FFOL • Asynchronous mode, using an ATM-like protocol, that is easily bridged to ATM/BISDN, for connection oriented and for connectionless transport services. • Isochronous mode, for circuit switched services, providing FDDI-II type services. • Packet mode, for providing FDDl-like packet services. Then several accessing methods are outlined that are to be examined for use in FFOL: • Slotted ring or bus: a shared medium access method based on a slotted protocol might be used to provide isochronous, ATM and packet services. The slots provide cells to contain the various size frames or portions of frames. Slots may be either of a fixed size or of a variable size and number to meet requests or the current services load balance. • Insertion ring or bus: the sourcing of a frame is started by a station only when no frames are entering the station. An insertion protocol allows for a high ring utilization, but the variable latency may produce buffering problems for isochronous service. • Token ring or bus: a token ring allows a scaling of the current FDDI Media Access Control (MAC) protocols, simplifying the standardization process. • Combination: a protocol that utilizes combinations of the above access methods.
Logical topology At the logical level the Requirements state that both ring and bus protocols will be examined, as well as others. Both ring and bus protocols allow the assignment of a centralized control point for the generation of slots or allocation of resources. A ring protocol may provide advantages for distributed recovery mechanisms, while a bus protocol may use the head end to generate slots, and does not have to address the stripping issue unless spatial reuse is performed.
Media access properties A number of media access properties and issues are discussed in the Requirements. FFOL would provide mechanisms for reservations, preallocations and priorities to allow different classes and priorities of service. It is expected that from two to eight levels would be supported, sufficiently specified to be consistently usable across stations. FFOL would have to carefully address frame stripping for ring accessing methods. Stripping of frames based on the destination address would allow slots to be reused, increasing the available bandwidth. Broadcast frames would, however, have to be stripped by the sourcing station. Stripping might also have to be performed at the sourcing station to provide consistency with FDDI. FFOL would provide an efficient mechanism for supporting multicast addressing. Multicast addressing allows the location of resources and an efficient utilization
8
of the media. Multicast frames must be removed by either the last receiving station or the sourcing station. It should be designed to provide consistency in a multi-vendor environment and allow a large numbers of addresses to be supported in a straightforward manner. For FFOL, bridging options would include defining control indicators to provide useful status for bridged frames. ATM frames, as well as FDDI and IEEE802 packets could be bridged onto FFOL. Also, FDDI-II channels and Wideband Channels (WBC) are to be supported by FFOL.
Recovery The Requirements discuss redundancy, stating that it will be allowed in FFOL. The design of FFOL would be optimized for the normal condition of no faults, with upper bounds being placed on the recovery and reconfiguration times.
ARCHITECTURE FOR AN FFOL PROJECT Upon conclusion of work on the FFOL Requirements and Design Considerations, it became clear that there was strong consensus to establish an FFOL project. In theANSI ASC X3 community projects are started by the approval of one or more project proposals, which was done, as mentioned earlier in this paper, by going through the specified ASC X3 process. Without an approved project or projects, there would be no authorization to continue meeting beyond the first few ad hoc working group meetings. However, to make a project proposal there has to be enough consensus for an architecture for the resultant standards to allow specific projects to be started. Although this has its disadvantages in that a potentially premature architecture may be selected, it has the advantage of providing an early focus for the work. It is also the case that the architecture may be changed, with a potential change in the scope and number of projects. This is, in fact, specifically noted in the FFOL Project Proposal documents. Discussion of possible architectures was started in the X3T9.5 FFOL working group, with much initial focus on the current FDDI 'family' architecture (see Figure 1), and the requirements and design guidelines previously developed. It must again be emphasized that this was not meant to be an end-all discussion of an FFOLarchitecture, but rather a choice for purposes of making project proposals and moving ahead. The resultant architecture (see Figure 2) was one that looks remarkably like the current FDDI architecture. Though there was some sentiment to not break portions of the architecture up at this time, e.g. leave SMUX, AMAC and IMAC as one standard, there was consensus that it was important to allow independent efforts to proceed on these parts of FFOL in the interest of parallel work effort.
computer communications
FFOL ~4rcu4I
S~lc.~tt~ f
LO(jNCal Llt~k n
mode fibre. The signalling rate would be significantly higher than FDDI, capable of supporting data rates in the 600 Mbit/s to 1 + Gbit/s range. FDDI-compatible cabling plant would be used where feasible.
Scope of PHY standard A Physical Layer Protocol standard which specifies the encode/decode, clocking and data framing over PMD and SONET links. It would be scalable from STS-3 to at least STS-48 data rates in the Synchronous Digital Hierarchy (SDH).
Data t
Lay__er Physil Layer
SOb
Scope of SMUX standard
STS
Figure 1. FDDI standards architecture (* = HRC requires the use of PHY-2 and MAC-2; otherwise, any combination of MAC or MAC-2 and PHY or PHY-2 is allowed)
A Service Multiplexer standard which specifies the multiplexing of isochronous and asynchronous (circuit switched and packet) data. It would make efficient use of PHY links and support both open and closed loop topologies. It would be scalable from STS-3 to at least STS48 data rates in the Synchronous Digital Hierarchy (SDH).
Scope of AMAC standard An Asynchronous Media Access Control standard which specifies access to the medium, addressing` data checking, and packet generation/reception for asynchronous (packet switched) service, over an SMUX service.
Data Unk Layer
Physical Layer
Scope of IMAC standard An Isochronous Media Access Control standard which specifies access to the medium, and transmission channel generation/reception for isochronous (circuit switched) service, over an SMUX service.
Figure 2.
FFOL standards architecture
Scope of SMIT standard Also, several possible architectural enhancements were left out for the first generation of FFOL work. These primarily centred around the relaying and interconnection of multiple portions of an FFOL network operating over different topologies and links (e.g. SONET/SDH), at different speeds and for different levels of service. The resulting six approved FFOL projects 3 are summarized here, as they were approved as project by X3. The anticipated completion of these initial FFOLstandards is in the 1993-1995 timeframe.
Scope of PMD standard A Physical Layer Medium Dependent standard which specifies the private optical fibre links and related optical components. It would include single-mode and multi-
vol 15 no 1 january/february 1992
A Station Management standard which provides control for PMD, PHY, SMUX, AMAC and IMAC, and specifies the FFOL station configurations, topology configurations, and the control required for proper operation of stations as a member of the FFOL network. SMT also provides the interface to the FFOL network management agent.
Critical issues and goals for FFOL Several critical issues face the FFOL working group. For example, it will soon have to choose whether or not to go in the direction of ATM/BISDN based structures for AMAC services and, if so, how to proceed effectively. It is strongly believed that the viability of FFOL will, to a large extent, rely on the cost effectiveness of a low-cost
9
FFOL PMD that will operate over FDDI multi-mode fibre from the communications closet to the office/desktop at the 600 MbitJs payload rate (possibly 750 Mbaud if an 8/10 encoding is used). Several goals or expectations for an FFOL PMD, and other parts of the protocol, are noted below. The use of Bi-CMOS for the serial I/O portion of the FFOL PMD operating at 750 Mbaud is a practical target in 1995. CMOS is an outside possibility if multi-level signalling can be used over the fibre. If a 32-bit structure for FFOL protocols can be maintained, CMOS for the parallel portions of the protocol operating at 750 Mbaud is a practical target in 1995. There is also a reasonable prospect for low-cost laser diodes by 1995 operating at 750 Mbaud. Another reasonable goal is to operate in the I to 2 km range over FDDI multi-mode fibre operating at 750 Mbaud. At the 2.4Gbit/s payload rate (3Gbaud with 8/10 encoding), Bi-CMOS for the parallel portions of the protocol is quite possible by 1995, with an outside chance for CMOS.
CONCLUSION FFOL is the natural choice as a follow-on I_AN to FDDI. FFOL's higher performance and ability to handle all FDDI data formats will allow its use as a backbone to existing FDDI installations. FFOL will also be used as a replacement for FDDI and other LANs when its higher performance is required. In addition, the enhanced services offered by
I0
FFOL, e.g. ATM/BISDN-Iike services, will allow it to address applications beyond the scope of current-day LANs. The FFOL project is off to a strong start. While there are obviously critical issues that will need resolution, a forum for this work has been established in the X3T9.5 Task Group. The authors, along with other participants in the FDDI/FFOI_ work, believe that the possibility for success for the FFOI_project is best assured in an organization with X3Tg.5's expertise and proven track record. FFOL will be a success because the time is right and the process is right!
ACKNOWLEDGEMENTS This work was supported in part by the US Department of Energy under Contract Number DE-AC03-76F00098.
REFERENCES 1 Ross, F E 'An overview of FDDI: the Fiber Distributed Data Interface', IEEE J. Selected Areas in Commun., Vol 7 No 7 (September 1989) 20chellree, K, Horvalh, S and Mityko, G Requirements and Design Considerations for the FDDI Follow-On LAN, X3T9.5 Working Group Document FFOL-007 (17 May 1990) 3 X3T9, Project Proposals for a New Family of X3 Standards - FDDI Follow-On Local Area Network (FFOL), X3T9 Technical Committee Document X3T9/ 90-110 (24 August 1990)
computer communications