LAN and I/O Convergence:
`A Survey of the Issues
`
`Martin W. Sachs, Avraham Lefi', and Denise Sevigny
`International Business Machines
`
`ocal area networks (LANs) and computer l/O are both interconnects that
`move infonnatien from one location to another. Despite this shared purpose.
`LANs have traditionally connected independent and widely separated com-
`puters. In contrast, computer l/O has traditionally connected a host to peripheral de ,
`vices such as terminals. disks. and tape drives. Because these connection tasks were
`different, the architectures developed for one task were not suitable for the other.
`Consequently, the technologies used to implement one architecture could not ad-
`dress the issues laced by the other. and the technologies were seen as fundamentally
`dilt’erent.
`However, an examination of the architectural requirements of modern “0 and
`LANs shows that the differences between the two technologies are now disappear-
`ing. We believe that LAN and NO architectures are in fact converging. and that this
`convergence reflects significant changes in how — and where — computing resources
`are used. To illustrate this convergence and its implications. this article examines sev-
`eral modem LANs and channels,
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`— Environment and architecture convergence
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`Once tWO distinctly
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`separate technologies,
`LANS and I/O are
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`becoming more alike
`through similar
`distances, media, and
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`PHTPOSBS- What few
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`differences EXISt may
`disappear in the next
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`ecade.
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`Today’s I/O channels and LANs are characterized by a configuration size of less
`than 50 kilometers. Within this area. the environments under consideration include
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`I back-end networks (machine room environment),
`0 front-end networks (office environment).
`0 client-server networks. and
`0 campus backbone networks.
`Modern [/0 channels do not obviate the need for wide-area networks (WANs) or
`even large metropolitan—area networks (MANs). The generalpurpose [/0 channels
`that we discuss later in this article also do not lessen the need for optimized chan-
`nels for real-time applications such as embedded systems.
`Figure 1 (on page 26) depicts the evolution of the relationship between intercon-
`nect type and distance. Historically. I/O and communication network interconnects
`partitioned the space at the machine room boundary. In the 19805. the communica-
`tions space was further subdivided into LANs and WANs. followed by the introduc-
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`24
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`turts-ainzmmmo N94 IFEE
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`COMPUTER
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`Petitioner Valve - Ex. 1019, Page 24
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`Petitioner Riot Games, Inc. - Ex. 1019, p. 24
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`Petitioner Valve - Ex. 1019, Page 24
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`terriet Protocol) The network layer ofthe lntemet com-
`ions protocol. it defines the internet datagram as the ba-
`, sicinformation unit pasSed across theinternet and provides a
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`-V connecticnless delivery service.
`lligent Peripheral interface) —— An ANSl-standard l/O
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`primarily-used for attachment of data storage devices to
`into
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`Broadcastnetworkw A network in which dataa
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`3 processors
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`ously transmitted to all destinations
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`LAN (local area network) w A communications system typically
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`designed for use within a single organization, having a diameter
`Class-1 service —— A Fibre Channel service that establishes a
`greater than to m but less than several km.
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`circuit—switched connection between two communicating entities
`LLC (logical link control) ~ One of two sublayers of the OSl
`(NPorts).
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`model’s data link layer. it includes functions unique to the partic-
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`Class-2 service — A Fibre Channel service that multiplexes
`ular link control procedures associated with the attached node
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`frames to and from N_Ports with acknowledgment provided.
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`and are independent of the underlying communication medium.
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`Class-3 service —— A Fibre Channel service that multiplexes
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`The LLC sublayer uses services provided by the MAC sublayer
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`frames to and from N_Ports without acknowledgment.
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`and provides services to the network layer.
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`Connection-oriented —- A service in which a connection
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`MAC (medium access control) A A sublayer of the OSI
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`between source and destination must first be established before
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`model’s data link layer. It uses the services of the physical layer
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`communication can take place. Once the connection is
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`and supports topology-dependent functions, which it provides to
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`established, messages arrive at the destination in the order that
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`the LLC sublayer.
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`they are transmitted.
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`MAN (metropolitan area network) — A communications
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`Connectionless — A communication service in which every
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`system designed to cover city-wide areas (tens of km), using
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`message is transmitted independently of any other.
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`LAN technology.
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`CSMA/CD (carrier sense multiple access with collision
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`Native lIO — The I/O system designed as an intrinsic
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`detection) — A bus network in which the MAC protocol requires
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`component of a given computer architecture.
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`a station to detect whether another station is already transmitting
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`Node — A communications cntily.
`before transmitting its own frame and in which error conditions
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`resulting from simultaneous transmission by more than one sta-
`08! (Open Systems interconnection) architecture — A
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`tion are resolved through retransmission.
`framework for coordinating the development of standards for the
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`interconnection of computer systems. Network functions are di—
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`Cut-through — A technique used in frame buffering that permits
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`vided into a hierarchy of seven layers; each layer represents a
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`the beginning of a frame to be moved out of the buffer before the
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`collection of related communication functions.
`whole frame has arrived in the buffer.
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`Quality of service — Parameters characterizing communication
`Data link layer —— The OSl layer that controls data transfer over
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`service that a service user either desires or requires as minimum
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`a link between two nodes and performs error control for the link.
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`satisfactory service. Examples include specifications for through-
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`ESCON (Enterprise Systems Connection) — A fiber-optic l/O
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`put, delay, and error rates.
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`channel developed by lBM that transmits data at 17 Mbytes/sec.
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`SCSI (Small Computer Systems interface) v An ANSI-stan—
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`It provides point-topoint connections of up to 40 km and uses a
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`dard l/O intertace primarily used for attachment of data storage
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`nonblocking circuit switch.
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`devices to processors.
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`Fabric A The part of a network that transmits data from one
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`Shared-medium topology —— A communication network in
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`node to another, usually including routing function.
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`which a single communication channel is shared among all the
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`FDDI (Fiber Distributed Data Interface) ~ A high-
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`stations on the network. Examples are bus and ring topologies.
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`performance, ANSI-standard fiber-optic token ring LAN running
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`Station — A communication device attached to a network. The
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`at 10 Mbytes/sec over distances of up to 200 km with up to 1,000
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`connected stations.
`station is the component of a node that provides at least the
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`MAC and physical-layer function.
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`Fibre Channel — A proposed ANSI serial l/O channel standard
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`Switch-based topology — A communications network that is
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`capable of transmitting at gigabit rates. It provides both circuit
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`based on one or more discrete switches. which may be either cirr
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`and frame switching using space division switches or loops.
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`curl or packet switches,
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`HiPPl (High-Performance Parallel Interface) — A high-speed
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`TCP (Transmission Control Protocol) — The transport layer of
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`ANSI-standard parallel interface that transmits either 32 or 64
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`the Internet communications protocol. it provides reliable, full du-
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`bits in parallel and transmits data at up to 800 Mbits/sec.
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`plex service, and allows arbitrarily long streams of data to be
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`Hop count A A unit of distance in a communications network. A
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`transmitted. It provides a connection-oriented service and
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`hop count of 4 means that 3 nodes or gateways separate the
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`typically uses the lP protocol to transmit data.
`source from the destination.
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`WAN (wide area network) 7 A communications system
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`l/O channel A An l/O mechanism that manages the flow of data
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`typically designed to provide services to a geographical area that
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`between a processor memon/ and the link to attached l/O
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`is larger than the area served by a single LAN.
`devices.
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`.,
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`December 1994
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`’Ji
`
`Petitioner Valve - Ex. 1019, Page 25
`
`Petitioner Riot Games, Inc.
`
`- Ex. 1019, p. 25
`
`Petitioner Riot Games, Inc. - Ex. 1019, p. 25
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`Petitioner Valve - Ex. 1019, Page 25
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`

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` Device command set
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`"Hester. .'I‘.".9‘?E‘l’9_'___
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`Medium access control
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`Dam link
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`Figure 2. Layer
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`structures of
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`(a) LANs and
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`(b) channels.
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`(a)
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`Physical
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`Physical
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`('1)
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`Table 1. Interconnect requirements.
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`Requirement
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`Interconnect distance
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`Definition
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`Information model
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`Computation model
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`26
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`Maximum distance between any two points of the
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`configuration
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`Characteristics of the application information
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`carried by the interconnect
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`Relationship between the interconnected computers
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`or devices (for example, master-slave. peeHo-
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`peer)
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`generally optimized for long distances
`tion of MANs in the 1990s. In today‘s sys-
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`tems, distances served by 1/0 channels
`and multiple hops, and they have too
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`and networks now overlap in local and
`much overhead for high-performance
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`metropolitan areas.
`I/O. However, in intermediate configu—
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`rations for distances of l to 50 km, the is-
`Although the traditional dichotomy is
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`sues faced by LANs and NO are increas—
`still valid at opposite ends of the scale, it
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`blurs toward the middle. Some aspects of
`ingly similar.
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`computer I/O, such as flow control, can
`In the terminology of the 051 (Open
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`be optimized for short distances through
`Systems Interconnection) reference mod-
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`simple hardware protocols, but they do
`el,1 LAN5 are characterized by the phys—
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`not perform well at very long distances
`ical layer and by the medium-access-
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`Similarly. communications systems are
`control (MAC) sublayer of the data link
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`Petitioner Valve - Ex. 1019, Page 26
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`Petitioner Riot Games, Inc. - EX. 1019, p. 26
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`Metropolitan
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`area network
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`Local area network
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`II'O
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`(backplane)
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`110
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`l/O
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`Local area
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`network
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`Wide area
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`network
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`Communications
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`19903
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`5 19805
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`D.LLI
`8
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`19505-
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`19705
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`V0
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`‘———r——~i———i—r———l———1—>
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`1
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`Distance (meters)
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`Figure 1. Interconnect type and interconnect distance for various epochs.
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`link control
`layer. The same logical
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`(LLC) sublayer of the data link layer is ,
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`used by more than one type of LAN. l/O
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`channel architectures can be viewed as
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`consisting of three layers. The lowest two
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`are functionally equivalent to the OSI
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`physical and data link layers. The highest
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`layer specifies device command sets (for
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`example, disk and tape commands) to-
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`gether with their associated protocols.
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`The layer structures of LANs and
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`channels are illustrated in Figure 2. Typ»
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`ically, the software that supports a LAN
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`implements a layered architecture, which
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`may conform either to the higher layers
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`of the 081 model or to some other com-
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`munications architecture. In general,
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`these higher layers are not specific to
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`LANs and the same software may con-
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`currently support LAN, MAN, and
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`WAN communications. For a channel,
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`the supporting software consists of an I/O
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`supervisor and the device-specific and ap—
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`plication-specific software.
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`Interconnect media and bandwidths are
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`dealt with in the physical layer. The same
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`physical layer specifications can be used
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`for both LANs and I/O channels, inde»
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`pendent of functional convergence issucs.
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`Above the data link layer, protocols re»
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`flcct specific application types. Examples
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`are communications, such as the Internet
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`Protocol, and device architectures, such
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`as the American National Standards In—
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`stitute Intelligent Peripheral Interface
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`(IPI—3). These applications and their pro—
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`tocols are not converging. The data link
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`layer, however, is the focus of our dis—
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`cussion on architecture convergence.
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`At this point, it is worth briefly men—
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`tioning architecture openness. Tradition-
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`ally, a given No architecture was opti-
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`mized for a given system architecture for
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`the purpose of attaching I/O devices to a
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`particular processor. In contrast to LAN
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`architectures, l/O architectures were never
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`open in the sense of being able to incor-
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`porate the identical 1/0 architecture with
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`different system architectures. Early ex-
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`ceptions to this were the ANSI Small
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`Computer System Interface (SCSI) and
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`IPI architectures.
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`The SCSI and IPI standards both in-
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`clude a device model, which defines the
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`characteristics of the device (for example,
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`a disk drive) and its command set, and a
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`channel model, which describes the ar-
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`chitecture of the channel that connects
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`the device to the host. However, most im-
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`plementations of these standards were
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`made on processors that already had na—
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`
`
`
`
`
`
`
`tive I/O systems to which SCSI and IPI
`
`COMPUTER
`
`
`
`Petitioner Riot Games, Inc. - Ex. 1019, p. 26
`
`Petitioner Valve - Ex. 1019, Page 26
`
`

`

`adapters were attached. The emerging
`ANSI Fibre Channel2 standard goes fur-
`ther: It is a true open 1/0 architecture
`that is both processor and device indc‘
`pendent. Moreover, some implementa-
`tions in which Fibre Channel is the native
`l/O system are likely.
`
`Interconnect
`
`requirements
`
`Among the requirements that must be
`addressed by any interconnect architec-
`ture are the expected interconnect dis-
`tance, the information model, and the
`computation model (see Table 1). As
`shown in Table 2, both the interconnect
`distance and the computation model
`have traditionally differed for I/O and
`LANs. For short distances. l/O channels
`efficiently connected a smart host to a
`few dumb peripherals in a centralized.
`master-slave manner (Figure 3). Where
`longer distances were a factor. LAN ar-
`chitectures were needed to connect many
`autonomous. smart processors in a dis-
`tributed. peer-to-peer manner (Figure 4).
`With respect to the information model.
`both [/0 channels and LANs were pri-
`marily used for transferring data.
`Interconnect requirements are chang
`ing. Both LANs and channels have
`benefited from advances in fiber-optic
`technology that greatly extend the com-
`bination of bandwidth and interconnect
`distance. At the same time. the informa-
`tion model — driven by multimedia ap-
`plications using voice and video — is
`evolving to include information with very
`different characteristics
`In the evolving l/O computation model.
`data are increasingly off-loaded from the
`host to intelligent file servers that form the
`basis of client/server architectures. The re-
`sult is that a traditional LAN puma-peer
`interconnect quite naturally supports the
`current 110 computation model. There are
`few differences between LAN and [/0
`architectures developed to serve the de-
`mands of multimedia and client/server ap—'
`plications. and even these differences may
`disappear in the next decade.
`
`Technology
`Some of the classic differences be
`tween channels and LANs are indepen-
`dent of the interconnect's intrinsic archi-
`tecture. Instead. they‘re technological —
`
`December 1994
`
`Table 2. Traditional requirements for communication and U0.
`
`Requirement
`
`Interconnect distance
`Information model
`Computation model
`
`Master-slave
`
`> 102 meters
`Data
`Peer-to-peer
`
`< 102 meters
`Data
`
`
`
`Channel
`
`Channel
`
`g Channel
`I
`
`Channel
`
`
`
`
`
`
`
`
`
`
`
`Figure 3. Typical l/O interconnecl configuration.
`
`
`
`
`Figure 4.
`I Typical LAN inter-
`.
`connects—bus and
`i
`ring. Several work-
`i
`stations are shown
`interconnected to
`each other and to a
`file server.
`
`|
`
`27
`
`Petitioner Valve - Ex. 1019, Page 27
`
`Petitioner Riot Games, Inc. - Ex. 1019, p. 27
`
`Petitioner Valve - Ex. 1019, Page 27
`
`

`

`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Table 3. Two major technological differences between traditional LANs and chan-
`
`
`
`nels (late 1980s).
`
`
`
`
`
`
`
`
`
`
`such as carrier sense multiple access with
`
`
`
`
`collision detection (CSMA/CD) and
`
`
`
`
`
`token-passing. Moreover, in a shared-
`
`
`
`
`
`
`medium topology, the bandwidth of a sin-
`
`
`
`
`
`
`
`
`gle link is divided among all the stations.
`
`
`
`
`
`Future applications will require LANs
`
`
`
`
`
`
`
`with more bandwidth per station than is
`
`
`
`
`now feasible through shared-medium
`
`
`
`topologies. Switch-based LANs i such
`
`
`
`as asynchronous—transfer-mode (ATM)5
`
`
`
`
`
`LANs and switched Ethernet — are at-
`
`
`
`
`
`tractive because they provide aggregate
`
`
`
`
`
`
`
`bandwidth, which is a multiple of the
`
`
`
`
`
`
`
`bandwidth of a single link. ATM tech-
`
`
`
`
`
`
`
`
`
`nology is even being applied as an I/O in-
`
`
`
`
`
`terconnect.6 In the extreme, a nonblock-
`
`
`
`
`
`
`
`ing circuit switch, similar to the IBM
`
`
`
`
`
`
`ESCON Director3 or a Fibre Channel cir-
`
`
`
`
`
`
`cuit switch, provides the full bandwidth
`
`
`
`
`
`
`
`
`of a single link concurrently to every sta—
`
`
`
`
`
`tion engaged in a connection. Channels
`
`
`
`
`
`
`
`and LANs are clearly evolving from their
`
`
`
`
`original shared-medium topologies to
`
`
`switch»based topologies.
`
`
`
`
`
`Transmission latency. The intrinsic la-
`
`
`
`
`
`
`
`
`tency of all channels and LANs is similar
`
`
`
`
`
`for a given transmission medium, being
`
`
`
`
`
`
`
`determined by the signal speed in the
`
`
`
`
`
`
`transmission medium. For optical fiber,
`
`
`
`
`this is approximately 2 x 103 meters/sec,
`
`
`
`
`
`corresponding to a propagation delay of
`
`
`
`
`
`
`5 nanoseconds/meter, that is, 50 micro—
`
`
`
`
`
`seconds at 10 kilometers. However,
`
`
`
`
`
`
`whereas channel latency is measured in
`
`
`
`
`
`microseconds, LAN latency is measured
`
`
`
`
`
`
`in milliseconds. This difference relates to
`
`
`
`
`
`aspects of architecture and implementa-
`
`
`
`
`
`
`tion above the OSI physical layer.
`
`
`
`
`Interconnect architecture affects latency
`
`
`
`
`
`
`
`because of the access protocols needed to
`
`
`
`
`mediate simultaneous transmission re-
`
`
`
`
`
`quests. For example, the medium-access—
`
`
`
`
`
`
`control protocol used in CSMA/CD and
`
`
`
`
`token-ring LANs causes considerable
`
`
`
`
`
`
`transmission latency, compared to the sim-
`
`
`
`
`
`
`pler switch-based access protocols of chan-
`
`
`
`
`
`
`
`nels. In addition, the end-to-end latency of
`
`
`
`
`
`
`
`
`both channels and LANs may, in fact, be
`
`
`
`
`
`dominated by software that implements
`
`
`
`
`
`
`
`the applications and higher layers of the
`
`
`
`
`
`
`communications protocol stack. In the fu~
`
`
`
`
`
`
`
`ture, as both channels and LANs adopt
`
`
`
`
`
`similar switch—based topologies, the laten-
`
`
`
`
`
`
`
`cies in the MAC-layer function will be-
`
`
`
`
`
`
`come similar. Further, improvements in ar—
`
`
`
`
`
`
`chitecture will reduce the effect of software
`
`
`on latency.
`
`
`
`
`
`Single-hop versus multihop. An inter-
`
`
`
`
`
`
`connect must ensure that data are trans-
`
`
`
`
`
`
`mitted from the source to the destination.
`
`
`
`
`
`COMPUTER
`
`
`
`Attribute
`
`Interconnect distance
`
`Bandwidth
`
`> 102 meters
`
`
`1 Mbyte/sec
`
`10 Mbytes/sec
`
`Channels
`
`< 102 meters
`
`
`
`
`
`
`
`
`
`
`
`
`for example, the use of parallel versus se-
`
`
`
`
`
`
`
`rial links or copper cable versus optical
`
`
`
`
`
`
`fiber. These design decisions depend on
`
`
`
`
`product-specific cost, performance, and
`
`
`
`
`
`
`distance trade-offs. In theory, both inter-
`
`
`
`
`
`
`connects could use the same technology.
`
`
`
`
`Table 3 compares two technologically
`
`
`
`determined attributes — interconnect
`
`
`
`
`distance and bandwidth — that distin-
`
`
`
`
`
`
`
`guished channels from LANs in the late
`
`
`
`
`
`19805. Compared to channels, LANs
`
`
`
`
`
`
`have had lower bandwidth and a longer
`
`
`interconnection distance.
`
`
`
`
`
`Table 4 compares channel and LAN
`
`
`
`
`
`bandwidth over three generations of
`
`
`
`
`
`technology. Through the mid-1980s, both
`
`
`
`
`
`
`used the same basic copper—wire trans—
`
`
`
`
`
`mission medium. Channels were de—
`
`
`
`
`
`
`
`signed to move large volumes of data
`
`
`
`
`
`
`rapidly between a host and its attached
`
`
`
`high-performance data-storage devices,
`
`
`
`
`
`
`while LANs were designed as low—cost,
`
`
`
`
`
`
`higher bandwidth alternatives to the ex-
`
`
`
`
`isting communication networks over
`
`
`
`
`shorter distances. Therefore, channels
`
`
`
`
`
`
`
`were optimized for high speed and very
`
`
`
`
`
`
`low error rates, using parallel transmis-
`
`
`
`
`
`
`sion for relatively short distances. LANs
`
`
`
`
`
`
`were optimized for economy and rela~
`
`
`
`
`
`tively long interconnect distances, and
`
`
`
`
`
`
`they featured serial transmission and tol»
`
`
`
`
`
`
`erated higher error rates. LAN specifi-
`
`
`
`
`
`
`cations also permitted repeaters to ex-
`
`
`
`
`
`
`tend distance; indeed, repeating is an
`
`
`
`
`
`
`
`
`
`essential part of a ring, one of the com—
`
`
`
`mon LAN topologies.
`
`
`
`
`
`Recent developments in serial fiber
`
`
`
`
`optics permit multikilometer attachment
`
`
`
`
`
`
`
`of peripheral devices to channels. For ex-
`
`
`
`
`
`
`ample, the IBM Enterprise Systems Con-
`
`
`
`
`
`nection (ESCON)3 channel has 3—km
`
`
`
`
`
`point-to-point connections and a laser
`
`
`
`
`
`
`
`option that enables up to 20-km point-to-
`
`
`
`
`
`point connections. Fibre Channel speci-
`
`
`
`
`
`
`
`fies a gigabit/sec option with a lO-km
`
`
`
`
`point—to-point connection. Moreover, the
`
`
`
`
`
`
`actual maximum distance for both the
`
`
`
`
`
`
`ESCON channel and Fibre Channel can
`
`
`
`
`
`be greatly extended through repeaters
`
`
`
`
`
`
`and complex switching fabrics. At the
`
`
`28
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`same time, emerging LANs such as the
`
`
`
`
`
`ANSI Fiber Distributed Data Interface
`
`
`
`
`
`(FDDI) have applied fiber-optic tech—
`
`
`
`
`
`
`nology to greatly extend the combination
`
`
`
`
`
`of bandwidth and interconnect distance
`
`
`
`
`
`
`
`available with earlier LANs, such as Eth-
`
`
`
`
`
`
`ernet and token-ring topologies. Thus, in-
`
`
`
`
`
`terconnect distance and bandwidth no
`
`
`
`
`
`longer distinguish channels from LANs.
`
`
`Architectural aspects
`
`
`
`
`
`To compare I/O and LAN architec-
`
`
`
`
`
`
`tures, we identify and discuss nine key
`
`
`
`features: topology, transmission latency,
`
`
`
`single-hop versus multihop configura-
`
`
`
`tions, connection—oriented versus con-
`
`
`
`nectionless service, real-time constraints,
`
`
`
`
`fair access, priorities, multiplexing, and
`
`
`bandwidth management.
`
`
`
`
`Topology. Channel topology originally
`
`
`
`
`reflected the host/peripheral relationship
`
`
`
`
`
`
`very strongly. A host communicated in
`
`
`
`
`
`
`master-slave fashion to its attached de-
`
`
`
`
`
`
`vices. Devices could share the medium
`
`
`
`
`
`because the master—slave relationship al-
`
`
`
`
`
`
`
`
`lowed them to use a very simple access-
`
`
`
`
`
`control protocol to mediate contention.
`
`
`
`
`
`
`Device sharing among multiple hosts was
`
`
`
`
`
`
`provided by the device’s controller. Each
`
`
`
`
`
`
`controller had multiple ports for channel
`
`
`
`
`
`
`connections and a switch that resolved
`
`
`
`
`
`contention among hosts. Newer channels,
`
`
`
`
`
`
`
`such as the ESCON channel, Fibre Chan-
`
`
`
`
`
`nel, and ANSI High—Performance Paral—
`
`
`
`
`
`
`lel Interface (HiPPI),4 have moved to a
`
`
`
`
`
`switch-based topology that is intrinsically
`
`
`
`
`
`
`
`peer to peer. Hosts set up connections
`
`
`
`
`
`
`through the switch to share devices.
`
`
`
`
`
`
`In contrast to the master-slave topol-
`
`
`
`
`
`
`
`ogy of early channels, LANs evolved in a
`
`
`
`distributed, peer-to-peer environment.
`
`
`
`
`Shared-medium topologies (for example,
`
`
`
`
`
`
`rings and buses) were attractive because
`
`
`
`
`
`
`they provided sufficient bandwidth at low
`
`
`
`
`
`
`cost. However, the combination of peer—
`
`
`
`
`
`to-peer relationship and shared medium
`
`
`
`
`
`required more complex access protocols,
`
`
`'l
`
`, n
`
`
`
`i
`
`
`
`
`
`Petitioner Valve - Ex. 1019, Page 28
`
`Petitioner Riot Games, Inc. - Ex. 1019, p. 28
`
`Petitioner Riot Games, Inc. - Ex. 1019, p. 28
`
`Petitioner Valve - Ex. 1019, Page 28
`
`

`

`
`
`
`ll.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Table 4. A comparison of LAN and channel bandwidth over several generations of technology.
`
`
`
`
`
`
`Time period
`
`1970s—mid 19805
`
`
`
`
`Early 19905
`Mid 19905—late 19905
`
`
`
`
`
`
`
`Local area networks
`
`
`
`
`
`1-2 Mbytes/sec (CSMA/CD)
`
`
`
`10 Mbytes/sec (FDDI)
`100 Mbytes/sec (ATM)
`
`
`
`
`
`100 Mbytes/sec (Fibre Channel)
`
`
`
`Channels
`
`
`
`
`
`
`>4 Mbytes/sec (IBM 370)
`
`
`
`17 Mbytes/sec (ESCON 1/0)
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`This task is complicated when interme-
`
`
`
`
`
`diate nodes separate the destination from
`
`
`
`
`
`the source. Such multihop configurations
`
`
`
`
`
`usually require the interconnect
`to
`
`
`
`
`
`
`choose among several potential trans-
`
`
`
`
`
`mission routes. It’s important to avoid
`
`
`
`
`
`
`overwhelming the interconnect with traf—
`
`
`
`
`
`
`fic interaction from multiple nodes. This
`
`
`
`
`
`
`task, termed congestion control, is inde-
`
`
`
`
`
`
`
`pendent from flow control; the latter re-
`
`
`
`
`
`lates only to the point-to-point traffic be-
`
`
`
`
`
`
`
`tween source and destination. Routing
`
`
`
`
`
`
`
`and congestion control are dealt with in
`
`
`
`
`
`the network layer of the 081 model.
`
`
`
`
`
`
`
`Channels have traditionally been sin-
`
`
`
`
`
`
`
`gle-hop: A single path connected the host
`
`
`
`
`
`
`to the device, with no intervening node.
`Even in the switch—based ESCON archi-
`
`
`
`
`
`
`
`
`
`
`
`
`tecture, the ESCON Director is a switch,
`
`
`
`
`not an intermediate node. Fibre Channel
`
`
`
`
`
`
`also provides a circuit-switched (known
`
`
`
`
`
`as class-l) service. Consequently, few tra-
`
`
`
`
`
`
`ditional network-layer issues and algo-
`
`
`
`rithms are present in channel architec—
`
`
`
`
`
`
`ture and implementations.
`
`
`
`
`
`
`Today, both channels and LANs are
`
`
`
`
`
`
`beginning to provide some multihop ser-
`
`
`
`
`
`vices. In Fibre Channel, for example,
`
`
`
`
`
`
`frame-switched service is multihop since
`the interconnect medium is a fabric of
`
`
`
`
`
`
`
`
`
`
`
`
`store—and—forward nodes. Each station
`
`
`
`
`
`has a direct link to the fabric; the fabric is
`
`
`
`
`
`
`responsible for buffering and delivering
`frames to their destinations. For effi-
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`ciency, taking advantage of the low error
`
`
`
`
`
`
`
`rates on the optical fiber link, addressing
`
`
`
`
`
`
`
`and error checking are done only end—to-
`
`
`
`
`
`
`end: No checking is done at intermediate
`
`
`
`
`
`
`nodes. Fibre Channel performs the end-
`
`
`
`
`
`
`to-end addressing and error checking in
`
`
`
`
`
`
`
`its FC-2 level (this includes functions
`
`
`
`
`
`
`equivalent to the data link layer), which
`
`
`
`
`is usually implemented in adapter mi—
`
`
`
`
`
`
`crocode and hardware. Communications
`
`
`
`
`
`
`architectures, on the other hand, perform
`
`
`
`
`
`
`these functions in higher level software
`
`
`(081 network and transport layers or
`
`
`
`
`
`their equivalent).
`
`
`
`
`
`
`As broadcast networks, the main con-
`cern for traditional LANs has been de-
`
`
`December 1994
`
`
`
`
`
`
`
`
`
`
`
`
`
`termining interconnect usage among
`
`
`
`
`
`
`competing stations. Once contention is
`
`
`
`
`
`resolved, LANs behave essentially as sin-
`
`
`
`
`
`
`gle-hop configurations. When LANs are
`
`
`
`
`
`
`
`
`bridged, however, the resulting set of
`
`
`
`
`
`bridged LANs takes on some of the char-
`acteristics of a multihop network.
`
`
`
`
`
`
`
`
`Connection-oriented versus connec-
`tionless service. From an end-to-end
`
`
`
`
`
`
`
`
`interconnects can provide
`viewpoint,
`either connection-oriented or connec—
`
`
`
`
`
`
`
`Longer distances for
`
`
`
`channels mean
`
`
`increased bit rates
`
`
`and switch
`
`fabrics with
`multihop properties.
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`tionless service (see Tanenbaurn7 for an
`
`
`
`
`
`extended discussion). In connection-
`
`
`
`oriented service, data cannot be trans—
`
`
`
`
`
`mitted before establishing a connection
`
`
`
`
`
`between the source and destination. Con-
`
`
`
`
`
`
`nections deliver data in sequence; error
`
`
`
`
`
`
`control and flow control are provided as
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`part of the service. ln connectionless ser—
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`vice, as the phrase implies, no connection
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`is established between the source and
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`destination. Instead, each unit of data
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`contains the destination address and is
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`transmitted independently. The inter-
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`connect makes a “best effort” attempt to
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`deliver the data. Connectionless service
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`does not guarantee sequential delivery of
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`data, nor does it provide error and flow
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`control. A common example of a con-
`nectionless service is Internet Protocol
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`(IP), in which each message (11’ data
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`gram) is sent separately to the destina~
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`tion. An example of a connection service
`is Transmission Control Protocol (TCP),
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`which provides the connection services
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`on top of IP.
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`LANs typically support both connec»
`tionless and connection-oriented service
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`at the logical link control sublayer. Al-
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`though LANs are basically single-hop
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`topologies, and hence provide ordered
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`delivery, the use of bridges to intercon-
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`nect multiple LANs introduces the pos-
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`sibility of multiple routes between two
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`endpoints. The distinction between con-
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`nectionless and connection-oriented ser-
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`vice is thus meaningful for LANs. Emerg—
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`ing switch-based LANs (for example,
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`ATM) illustrate a trend toward connec—
`tion-oriented services.
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`In the past, the issue of connectionless
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`versus connection-oriented service never
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`arose for channels. The reason is that
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`channels have always had single-hop bus
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`or circuit-switched topologies, which are
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`inherently connection oriented. Further,
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`the time to establish a circuit-switched
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`connection in a channel interconnect is
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