2000-01-C028
`
`Future Automotive Multimedia Subsystem
`Interconnect Technologies
`Visteon Automotive Systems
`
`ABSTRACT
`
`the most
`between them. Table 1.0 lists some of
`important applications that are already presentorwill be
`soonintroduced into the automotive environment.
`
`Road-side
`assistance
`Mayday
`
`Radio
`(AM/FM/DAB)
`
`Internet access
`
`For the past decade or so, automotive entertainment
`subsystem architectures have consisted of a simple
`Human Machine Interface (HMI), AM-FM tuner, a tape
`Safety &|Entertainment Information and
`
`
`deck, an amplifier and a set of speakers. Over time, as
`Security
`Communication
`customer demand for more entertainment
`features
`increased, automotive entertainment
`integrators made
`room for new features by allowing for
`the vertical
`integration of analog audio and adding a digital control.
`The
`new digital
`control
`came
`to
`entertainment
`subsystems via a low speed multiplexing scheme
`Panic call Audio (cassette,|E-mail
`
`
`embedded
`into
`the
`entertainment
`subsystem
`CD player, MP3)
`S-DARS
`Collision
`Weatherforecast
`components,
`allowing remote control of
`these new
`avoidance
`Head-line News
`features. New features were typically incorporated into
`the
`entertainment
`subsystem by
`independently
`Stock quotes
`packaging functional modules. Examples of
`these
`modules are cellular telephone, Compact Disc Jockey
`(CDJ), rear-seat entertainment, Satellite Digital Audio
`Antitheft system|Video (TV, DVD)
`information
`Radio System (S-DARS)receiver, voice and navigation
`with its associated display and hardware. Figure 1.0 is a
`Traffic
`Navigation
`information
`block diagram of typical entertainment subsystem. This
`paper discussesalternatives to the module-expansion of
`|Tollingsystem|=|Cardiagnosis|
`entertainment subsystem via low speed digital control
`PoMobilephone|
`and analog audio. Moreover, the discussion is expanded
`to cover future multimedia and infotainment subsystem
`interconnects technologies.
`
`
`
`Games
`
`Table 1.0: Near-Future Vehicle’s Features
`
`INTRODUCTION
`
`Recently, great achievements have been reached in
`information,
`communication,
`entertainment,
`comfort,
`safety and security products. Moreover, new Intelligent
`Transport Systems (ITS) services, requiring state-of-the-
`art electronics, are appearing on the market to help
`drivers process
`information, make decisions,
`and
`Operate vehicles more safely and effectively.
`
`As a consequence,our cars will be equipped more and
`more with
`digital
`systems
`communicating
`and
`exchanging information. Wheneverpossible, the trend is
`
`This paper assesses the suitability of current mobile
`multimedia transport for the accommodation of these
`technological advancas.In addition, this paperidentifies
`system and functional requirements for future mobile
`multimedia transport as well as differences between
`existing networks such as Ethernet, IEEE 1394, Media
`Oriented System Transport (MOST).
`
`CURRENT MOBILE ARCHITECTURE
`
`the primary
`In the beginning of the automobile era,
`function of a vehicle was a reliable transport. Over a
`period of years, the desire for a basic transport has been
`
`Page 1 of 19
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`Daimler Exhibit 1015
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`

`

`features by allowing for the vertical integration of analog
`audio and adding a digital control. Despite the digital
`nature of most of
`the new added modules and the
`introduction of Digital Signal Processor (DSP) within the
`mobile multimedia system,
`the vertical
`integration of
`audio andits transport remained analog.
`
`The Current mobile architecture with its analog transport
`has the following limitations:
`
`¢
`
`life and is not
`the new subsystem had a short
`compatible with
`the
`digital
`trending of
`future
`entertainment features such as digital audio, digital
`video, Digital Audio Broadcast
`(DAB) and next
`generation of compact disc technology, Digital
`Versatile Disc (DVD).
`
`The module level expansion strategy and vertical
`integration of analog audio resulted in
`costly
`subsystem architecture. Often, modules addedto the
`subsystem exhibited wasteful redundant hardware
`resources in order to achieve compatibility with an
`analog architecture. An example of this hardware
`wastefulness is the addition of a digital-to-analog
`converter to the output of CDJ or CD player to
`achieve compatibility with analog integration and
`processing. Moreover,
`the hardware
`resources
`available for each module are for that modules’ own
`use and can't be shared with other subsystem’s
`modules. This will add cost to each module and will
`contribute to the overall cost of the subsystem.
`
`subsystem
`the
`complicates
`transport
`e¢ Analog
`interconnects, decreasesreliability, adds weight and
`cost. Two twisted pairs are required for cabin media,
`one twisted pair is required for voice module, one
`twisted pair is required for cellular phone module,
`onetwisted pair is required for navigation module, 3-
`5 wires are required for low speed multiplex scheme
`and synchronization. Additionally, two more twisted
`pairs are required for an optional media player such
`as CDJ. In the case of rear seat entertainment, more
`wires or coaxial cables are required for video. The
`numberof wires or coaxial cables required for video
`applications is proportional
`to the number of rear
`seat occupants. Moreover, these wires require wide
`connectors with more pins at
`the analog power
`amplifiers input connector.
`
`e
`
`The module level expansion strategy and vertical
`integration of analog audio resulted in a closed
`architecture with limited expansion path. The number
`of pins available at
`the power amplifiers input
`connector bounded the expansion path.
`In addition,
`
`the module level expansion strategy and
`Presently,
`vertical integration of analog audio has reachedits upper
`integration limit for an HMI, AM-FM tuner, voice, cellular
`phone, CDJ, a media player, Steering Wheel Control
`(SWC), a Rear Integrated Control Panel
`(RICP) and
`navigation. The implementation of such a subsystem
`requires seven modules and a minimum of 31 wires. A
`saving of one module and four wires is possible if a
`media, HMI, tuner, and power amplifier is packaged in
`one module. However, a complicated heat dissipation
`
`Head-phone Module
`
`
`
`Navigation
`
`Steering Wheel
`
`
`Digital Control
`
`Rear
`
`SeatEntertainment
`
`Docking
`
`Figure 1.0: A Block Diagram of A Typical Entertainment System
`
`and overall power managementstrategy is required for
`the success of such integration, and maylimit the audio
`performance.
`
`REQUIREMENTS FOR NEW TRANSPORT
`
`Vehicles are running out of the real estate required to
`house new modules.
`Interconnect harness thickness
`and costs are ever
`increasing.
`This section is a
`discussion of both system and functional requirements
`for a new transport:
`
`e Open Standard: the new transport shall be an open
`standard to ensure that all vehicle and electronic
`product manufacturers have equal access to the
`standard and to the market.
`
`e Minimal Standard: the new transport standard shall
`be extensible and capable of migration to future
`technologies and different physical media with no
`impact on application software. A layered approach
`to the protocol, such as is used in the reference
`modal of Open System Interconnect (OSI) model,
`shall be used.
`
`e
`
`e¢
`
`to allow
`digital
`shall be
`The new transport
`multiplexing of data, control, audio and video over
`the same media. !n addition, digital transport enables
`open system architecture; a node can be added at
`anytime during the vehicle's life without modifying an
`existing node’s
`connector. Moreover,
`a digital
`transport enables the natural transport of digital data
`from one ever
`increasing digital application to
`another without
`exhibited wasteful
`redundant
`hardware resources such as Digital-to Analog (DAC)
`and Analog-to-Digital (ADC) converters.
`
`Safety & Security: the new transport shall provide an
`environment
`into which devices can be plugged,
`unplugged, and operated in a vehicle in a manner
`which doesnot threatenthe integrity of the vehicle or
`the devices that are being used. The new transport
`shall
`include
`suitable
`security for
`transactions
`involving the exchange of money and/or proprietary
`information.
`
`the new transport compatible
`e Manufacturability:
`devices and software shail be easy to design,
`implement and integrate. This
`implies minimal
`specifications and unambiguous requirements. Ease
`of manufacture helps to assure the wide adoption of
`the standard and contributes to lowering the cost of
`the manufactured items.
`
`the incremental direct material cost to
`Low Cost:
`implement a new transport node shall be a fraction of
`the cost of the application supported.
`
`Graceful Degradation: the new transport's physical
`layer and its supported bus
`topology shall be
`designed such that any fault shall not cause any
`damage to the cable, the vehicle or any attached
`devices.
`Functional operation under any of these
`conditions may cease but shall resume within the
`boot/discovery time after the fault is removed. The
`new design shall be such that no single failure, other
`than a fault in the physical layer as described above,
`or the loss of primary power, shall cause the entire
`system to fail.
`
`it shall be possible to attach
`Hot Plug and Play:
`devices to, and remove devices from,
`the new
`transport system at any time, whether poweris on or
`off.
`
`Self-Identification: devices attached to the new
`transport shall be able to identify themselves to other
`system devices and shall self configure to obtain
`unique addresses on the new transport. No user
`intervention shall be required to complete this
`configuration other
`than the
`provision
`of
`the
`appropriate application software.
`
`Short Boot/Discovery Time: the new transport andall
`attached nodes shall complete self-configuration
`within one second after deviceinitialization. When a
`new node is added to the system while it
`is
`operating,
`detection
`of
`the
`new node
`and
`reconfiguration of the system to include it shall be
`completed within 2 seconds.
`
`the set of devices
`Peer-to-Peer Communications:
`that is likely to be attached to the new transport is
`unpredictable and no single device is guaranteed
`always to be present. Therefore, a device on the new
`transport shall be able to communicate directly with
`any other device in the system and the vehicle
`without need for any additional device. An application
`that
`is implemented across multiple devices may
`choose to implement a "master controller" for that
`application but this shall not be a requirementfor all
`applications.
`
`Automotive Physical and Electrical Specifications:
`the new transports physical
`layer
`shall meet
`automotive environmental
`(temperature, vibration,
`shock,
`EMI,
`etc.)
`and electrical
`specifications
`(reverse
`voltage,
`load
`dump,
`etc.)
`for
`the
`
`
`
`Page 2 of 19
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`

`

`In a thick Ethernet, all computers are connected to one
`coaxial cable. This cable is used for sending and
`raceiving messages. Whenthe busisidle, the voltage on
`it shail be possible for an
`Broadcast Messages:
`the coax cable ramains in a high impedance state at an
`application to generate a broadcast messageto all
`intermediate level. This level is not a one or a zero, so
`devices connected to the new transport without
`that all nodes can easily determine if the bus is idle.
`having to address each one explicitly. A broadcast
`Security and Authentication Services: The new
`When a nodeis transmitting, the voltage on the bus is
`transport
`protocol
`shall
`provide
`security
`and
`message may or may not require acknowledgment
`pulled to high and low voltages depending on the data to
`authentication,
`services
`for access
`to
`vehicle
`or
`confirmation of delivery. For
`example,
`an
`be transmitted. Only ona computer can send information
`functions. It is anticipated that additional services will
`application
`may
`require
`confirmation
`at any one time.
`If multiple nodes try to transmit at the
`require
`additional
`security
`measures
`to
`(acknowledgment) that at least one receiving device
`same time, a collision occurs,
`the data from both
`accommodate
`applications or
`transactions
`that
`capable of acting on the message has received the
`computers is corrupted, and both computers have to stop
`message.
`
`
`require billing,|authentication, confidentiality,
`transmitting and try again when the busisidle.
`Ethernet is a term commonly used to describe a variety
`confirmation, non-repudiation, etc.
`of network implementations that share the same basic
`technology. Some early varieties of Ethernet are
`10Base-2 and 10Base-5 which are also called ‘thin net’
`and ‘thick net’ respectively. All nodes on such network
`tap into a single cable. A later version of Ethemet,
`10Base-T, introduces the concept of a hub or a repeater.
`All nodes are connected directly to a single repeater,
`which simplifies cabling and provides buffering of
`electrical signals.
`
`new
`and
`technologies
`evolving
`accommodate
`applications. A layered approach to the protocol is
`required to guarantee minimum impact on existing
`designs as new physical layers and new applications
`are developed.
`
`addressto deliver this message andall other devices
`shall ignore it.
`
`Bit Error Rate:the bit error rate shall be less than 1 x
`10°. Applications requiring better than this shall be
`able to implement appropriate measures at higher
`layers of the protocol,
`
`Time-Critical Delivery of Packets- Deterministic
`Latency: Some
`applications may require
`that
`messages be transmitted within a given time period.
`The newtransport shall be deterministic and it shall
`be possible to detarmine the maximum latency for
`any messagein a given system configuration
`
`Private Message Service: Equipment manufacturers
`wish to be able to develop applications that span
`their own suite of products and pravide competitive
`functions and features not achievable when products
`from different manufacturers are interconnected. The
`new transport
`protocol.
`shall
`support
`the
`implementation of private messages that will allow
`such applications to be developed.
`
`Consumer-Friendly Device Connection (No Special
`Tools Required): in most cases,it shall be possible
`for a consumerto install new transport nodes with
`common hand tools. There will be cases where
`professional
`installation may be required, but. this
`should be the exception, nottherule.
`
`Wake/Sleep: any node shall have the ability to wake
`up the new transport system or put it to sleep.
`It
`shalt be possible to wake up tha nodes by sending a
`wake-up message, such as a pager.
`It shail be
`possible to put any node and all connected devices
`back to sleep with a sleep message. Absence of
`messagetraffic on the new transport for more than
`30 minutes shall cause the new transport and all
`attached devicesto go to sleep.
`
`Priority Sensitive Flow (Isochronous): consumer
`electronics devices such as video games, DVD and
`MP3 players, Dolby AC-3 audio components, etc.,
`may require support for high speed isochronous data
`communications (i.e., data packets delivered at a
`guaranteedrate in a guaranteed order).
`
`required: a
`if
`e Confirmation of Message Delivery,
`device sending a messageto another device shalt be
`able to explicitly request confirmation of error-free
`delivery of that message from the receiving device.
`
`EXISTSING PROTOCOLS: ETHERNET,IEEE 1394
`AND MOST
`
`Fast Ethernet
`
`A newer version of Ethernet, called Fast Ethernet,
`operates at 10 times the speed of 10Base-T or 100 Mbit
`per second, It has the samestar topology as 10Base-T
`and comesin a few different versions called 100Base-
`TX, 100Base-FX, and 100Base-T4. The difference
`between these versions is the physical layer which is
`electrical for TX and T4, and optical for FX. In this paper,
`Fast Ethernet will refer to the most common version
`100Base-TX.
`
`Ethernet Topology
`
`In Fast Ethemet, all nodes connect to a central repeater
`through two sets of
`twisted pairs. One pair
`is
`for
`transmitting and the other for receiving. Although each
`node hasits own cable, the network operates exactly like
`thick Ethemetat a higher level. When a computer sends
`a message to the central repeater, the repeater sends
`the data exactly as received to all other computers
`connectedto it. Again, if two computers try to send at the
`sametime, a collision occurs, and both computers must
`try again later.
`
`EthemetArbitration
`
`the computers on an Ethernet share the
`Since ail
`transmission media, only one computer can send
`information at any one time.
`If muitiple nodes try to
`transmit at the same time, they must arbitrate for use of
`the bus. The rules that every node follows is called
`Carrier Sense Multiple Access with Collision Detection
`(CSMA/CD). Before a node sends a message on the
`bus,
`It sends a stream of one’s and zero’s called a
`‘carrier’. All other nodes on the network sensethis carrier
`and do not attempt to send their own message until the
`original node completes its message.
`
`Ethernet Switches
`
`Ethernet switches have become more popular than
`repeaters in recent years. An Ethernet switch is a more
`
`Thick net Ethernet uses a single cable as a backbone for
`the new transport gateway shail
`Power Loading:
`the network. Each node in the network taps into this
`make power available for all devices connected to
`cable through what is called a T connector. In an office
`Data Types: The new transport protocol: shall allow
`the new transport system. The total operating current
`environment,
`this cable could be routed through the
`drain of all devices connected to the new transport
`any data type (ASCII, binary, bulk, etc.)
`to be
`ceiling with taps dropping into each office. This. works
`transmitted in a message without need for any
`shall not exceed the capacity of the gateway unless
`There isafinite amount of time from when a node begins
`well exceptit is difficult to add new users to the network
`poweris routed directly to the device from another
`special escape characters or other similar artifacts
`sending a carrier to when all nodes detect this carrier.
`and signalquality is sometimesdifficult to control.
`source or the device is self-powered (e.g.,
`internal
`added by the application.
`During this time, other nodes may attempt to send a
`batteries).
`carrier.
`If this happens, a ‘collision’ will occur which
`corrupts the data on the bus. Transmitting nodes will
`both ‘detect’ this condition and stop transmitting. The
`rules then specify that each node must wait a random
`amount of time before attempting to transmit again. The
`probability that anothercollision will occuris low.
`
`Intemetworking:it is anticipated that wireless access
`to and from the tnternet will be required for many
`devices attached to the new transport system. The
`new protocol
`shall
`not
`preclude
`the
`future
`implementation of internetworking services across
`multiple gateways or bridges between the new
`transport and other subnets such as_
`Intelligent
`Transportation Data Bus (IDB).
`
`Fair Access to the new transport system: no single
`device shall be allowed to monopolize the new
`transport system.
`
`the new transport
`Priority Flagging:
`Message
`protocol shall provide a means to specify that the
`current message is a high or normal priority
`message.
`The
`protocol
`simply
`provides
`a
`mechanismto identify the message as a highpriority
`message.
`It
`is up to the device manufacturer to
`
`A Fast Ethemet uses a central repeater which connects
`directly to each node. This star topology enables the
`signal quality on the transmission line between a
`computer and the repeater to be well controlled and
`provides a relatively simple means to add users. The
`repeater has a number of ports which connect to each
`computer on the network. To add another computer, a
`wire is run from a free port on the repeater to the new
`computer.
`If there are no free ports,
`typically, another
`hub (or switch) can be connected to an uplink port to
`expand the network to virtually any size.
`
`Page 3 of 19
`
`

`

`If
`operates just like the coaxial cable in thick Ethernet.
`two computers try to send messagesat the same time,
`the messagescollide.
`
`switch divides the network into many
`An Ethernet
`collision domains. Each port on a switch is a different
`collision domain. For example,
`if one computer
`is
`connected to a port on an Ethernet switch, the collision
`domain consists of two nodes;
`the computer and the
`switch. If a port on a repeater is connected to a port on a
`switch, the collision domain consists of the switch and all
`computers connectedto the repeater.
`
`particular Wavfile from the server can be played on the
`sound card in the following way. The client software on
`the PC manages a FIFO (First-In First-Out) which
`continually outputs audio data to the soundcard. When
`the FIFO gets close to being empty, the client sends a
`message to the server to send more data. The server
`sends another packet to the client to fill the FIFO up
`again. As long as the server sends the new packet
`before the FIFO empties, audio can be heard.
`If the
`server responds slowly or the network is very busy, the
`packet may notarrive in time, the FIFO will empty and
`sounds will momentarily stop. This is unacceptable for
`most audio applications.
`
`The switch has intelligence which learns the addresses
`of
`the computers connected to each port. When a
`messageis received at oneport, the destination addrass
`ig determined and the message is
`sent out
`the
`appropriate port. Switches are typically much more
`efficient than repeaters. However, they cost more.
`
`(CSR) Architecture for Microcomputer Buses formally
`adopted as ISO/IEC 13213 (ANSI/IEEE 1212).
`
`This architecture defines a set of core features such as
`node architecture, address space, common transaction
`types, Control and Status Registers (CSR), configuration
`ROM format
`and
`content, message
`broadcast
`mechanism to all nodes and interrupt broadcast to all
`nodes.
`IEEE 1394 specifies how units attached to a
`serial bus can talk to each other, but does not define the
`protocols used to communicate between the nodes.
`
`IEEE 1394 is similar to Fast Ethernet in many ways. Data
`is always communicated between nodes in packets.
`If
`multiple nodestry to send packets at the sametime, they
`must arbitrate for the bus. The information in the packet
`headers, the packetsizes andthe arbitration method, are
`different. However,
`the fundamental mechanisms are
`similar.
`
`that the transmission line can be longer. The maximum
`cable length for Fast Ethernet is 100meters, while it
`is
`only 4.5 meters for IEEE 1394.
`
`Each node on an IEEE 1394 bus(or in a Fast Ethernet)
`has its own timing source which is typically a crystal
`oscillator. This timing source is used by a node to
`transmit data and is used by a node to over sample the
`received data and strobelines to recover the data. This
`meansthat all IEEE 1394 nodes are asynchronousat the
`lowest
`level. The accuracy of the timing reference in
`IEEE 1394 is specified to be + 100 PPM which is
`typically the frequency tolerance of widely available
`crystal oscillators.
`
`There is a trade off between FIFO size, the frequency of
`requests for more data and packet size. Since the large
`The’ nominal data rate in 100 Mbit IEEE 1394 is 98.304
`packets are more efficient
`than small packets
`(%
`Mbit.
`If the crystal oscillator at a particular node is
`overhead from header,etc), let’s assume we will use the
`operating at the high endofits frequency tolerance,it will
`largest packet size; 1500 bytes of user data. If the audio
`be able to transmit data at 98.304 Mbit +100 PPM or
`sample rate is 48 kHz, and the audio is 16 bits/sample
`98.314 Mbit/sec.If it is operating at the low end of the
`stereo, then we need an average of 192K bytes/second
`Ethernet Communication Mechanism
`The most significant feature that IEEE 1394 provides
`frequency tolerance,
`it will
`transmit data at 98.294
`or 128 packets/sec. The overhead for
`the header,
`(which Fast Ethernet does not) is guaranteed bandwidth
`Mbit/sec. This may seem likeatrivial issue. However,
`checksum and the required idle between packets is 38
`for
`real
`time
`applications. These applications are
`Information in Ethernet networks is communicated in
`the section on system timing will
`illustrate
`some
`allocated isochronous bandwidth which enable real time
`bytes. Since the client software on the PC with the sound
`important consequencesforreal time applications.
`packets. Each packet consists of a header, usable data
`card must inform the server when the FIFO is nearly
`and a checksum. The header contains information such
`data to be communicated in packets sent at regular time
`empty, there are another 84 bytes of overhead to send
`intervals. This is an improvement over Fast Ethernet.
`as source and destination address, the length of usable
`this messageto the server.
`However,
`it will be shown that there are still serious
`data and possibly information about the message type.
`limitations.
`The checksum is a code sentat the end of the message
`so that the receiving node can determine if the packet
`was corrupted during transmission.
`
` =:
`
`The minimum total bandwidth required for one audio
`channelis:
`
`Since the header and checksum are only used to send
`the packet safely from the transmitting node to the
`receiving node,
`it is considered network overhead.
`It is
`not
`information usable by the application.
`In Fast
`Ethernet,
`this consumes 18 bytes.
`If you include the
`arbitration time,
`the total overhead is 38 bytes.
`In
`addition,
`the minimum usable data is 46 bytes per
`packet. Even if you only wish to send one byte, you still
`must send the 18-byte header, 46 bytes of data, and wait
`20 bytes worth oftime for the bus.
`
`The efficiency of the network can be defined as the
`number of user data bytes per packet divided by the
`number of bytes in the packet plus the overhead of
`waiting for the bus. If only one byte of user data is sent
`per packet the efficiency is 1/(64+20) X 100% = 1.2%.
`Since the maximum user data per packet is 1500 bytes
`in 100BaseT,
`the theoretic maximum efficiency is
`1500/(1518+20) X 100% = 97.5%.
`
`1500 + 38 + 84 = 1622 bytes/packet
`
`1622 bytes/packet * 128 packets/sec = 207616 bytes/sec
`
`207616 bytes/sec * 8 Bits/bytes = 1.66 Mbit/sec
`
`Since the packet size in this example is the largest
`allowed by Fast Ethernet, the network overhead is small
`compared
`to
`the
`audio
`data
`throughput. The
`disadvantage of the large packet size is the buffer size
`requirement in the Client.
`ft must be 1500 bytes deep
`plus more for handshaking.If the extra depthis not large
`enough for the network to guarantee another packetwill
`arrive prior to the buffer emptying,a loss of audio quality
`may occur.If the buffer empties, the audio stops. Ina
`Fast Ethernet,
`it
`is
`impossible to guarantee any
`bandwidth. If the network haslots of traffic, you may not
`evenbe able to gatthe 1.66 Mbit/sec average throughput
`that is required. More commonly,at times of high traffic
`the buffer may empty no matter how large it may be.
`
`The maximum efficiency is never achieved since many
`collisions will occurif there is a lot of activity on the bus.
`Theeffect on efficiencyis difficult to predict.
`
`IEEE 1394
`
`The raw bit rate for IEEE 1394 is defined to be selectable
`between approximately 100, 200, and 400 Mbit/sec.
`Workis currently being done on an 800 Mbit specification
`as well. Silicon is currently being advertised to run at
`both 100 and 200 Mbit. However, most implementations
`are now at 100 Mbit/sec which is the same data rate as
`the large installed base of 100BaseT. Gigabit Ethernetis
`currently under developmentas well.
`
`IEEE 1394 Physical Layer
`
`The physical layer for IEEE 1394 consists of two sets of
`twisted pair wire for signals and two wires for power and
`ground connected between each pair of nodes. One set
`of twisted pairs is called data and the other is called
`strobe. When one node begins to send a packet, it sends
`Nonreturn-to-Zero (NRZ) data on the data line and
`transitions the strobe line only between consecutive 1's
`or O's. Both sets of twisted pairs are bi-directional. Each
`node sends and receives data on the samesetsof wires.
`Whenneither node is sending data, the twisted pairs are
`held in a high impedancestate.
`
`IEEE 1394 Topology
`
`The physical topology for a typical IEEE 1394 networkis
`a tree structure. Typically, a node will have a least two
`ports which enables multiple nodes to be daisy chained
`together.
`If a node has more than two ports, multiple
`branches can be created. During initialization, one node
`is defined to be the root node with all nodes extending
`down different branches. The topology can have any
`numberof branchesif no loops are created.
`
`The tree topology, with the ability to daisy chain nodes,
`has the advantage of simplicity for small networks.If you
`have a few devices,it is easy to plug them togetherin a
`daisy
`chain. However,
`large
`networks
`can
`be
`cumbersome, particularly if network performance is
`optimized. To improve performance,
`it
`is desirable to
`minimize the propagation delay of data between any two
`nodesin the network. Long daisy chains can besplit into
`many branches to reduce the delay; however, care
`should be taken to balancethe length of the branches.
`
`In a Fast Ethernet, a repeater or switch is required for a
`network with more than two nodes. This makes small
`networks complicated. However,it makes large networks
`simpler.
`
`In contrast, the physical layer for Fast Ethernet consists
`of
`two sets of
`twisted pairs; one pair
`is used for
`
`IEEE 1394Arbitration
`
`Page 4 of 19
`
`

`

`this
`the same time, the transmitted data is corrupted,
`condition is detected, and the nodes bagin to arbitrate for
`the bus. Likewise, all nodes on an IEEE 1394 network
`have the same collision domain. Only one node can send
`a message at one time.
`If multiple nodes try to send
`messages at the same time, only one node will gain
`control of the bus.
`
`IEEE
`While Ethernet treats all packet data the same,
`1394 provides different types of packets. The primary
`packet
`types are
`asynchronous
`and_
`isochronous.
`Asynchronous packets are functionally equivalent
`to
`Ethernet packets. Isochronaus packets are only available
`in IEEE 1394, and provide guaranteed bandwidth to time
`critical applications.
`
`which is clocked by a local 24.576 MHz clock and counts
`up to 3072. When the counter
`reaches 3072,
`the
`Isochronous Resource Managerresets its counter and
`broadcasts a messagetoall the nodesin the network to
`reset
`their
`local
`counters. The frequency of
`this
`synchronization messageis 24.576 MHz / 3072 or 8 kHz.
`
` The Isachronous Resource Managerhas a local counter
`
`nodes,the arbitration time is approximately equivalent to
`20 bytes of data.
`
`In summary, to send the 24 bytes of audio data, a 1394
`network consumesup to 35 bytes of overhead. Thetotal
`time required sending such as message is about 4.7
`psec and the efficiency is about 40%. Since the
`maximum amount of
`time that can be allocated to
`isochronousis 100 pisec/Frame, the maximum numberof
`audio channels that a 100 Mbit IEEE 1394 network can
`support is about 20.
`
`If 100 psec per frame are consumed by isochronous
`packets, 25 psecwill be left for asynchronous messages.
`As shown in the previous section, the time to send a
`minimum size packet
`is 9.2 psec. Consequently, a
`maximum of two asynchronous packets can be sent per
`isochronousframe.In other words, 16000relatively small
`asynchronous packets can be sent per second.
`
`As shownearlier, a 512-byte packet(largest possible in
`a100 Mbit IEEE 1394 network) takes 50 jsec to send.
`Since only 25 wusec are available per
`frame for
`asynchronous messages,
`it takes two frames to send
`such a message. The message rate will be 4000
`Bytes/second.It is possible to send such message, since
`the isochronous packets on the second frame are
`delayed until after the large asynchronous packet has
`been sent. If the transmit time of a packet were allowed
`to be larger than 50 psec orif less than 25 psec per
`frame were reserved for asynchronous packets, delivery
`ofall the isochronous packets could not be guaranteed.
`
`IEEE 1394 System Timing
`
`The timing source for each node in an IEEE 1394
`network is typically a crystal oscillator. As described in
`the physical
`layer section,
`the frequency of crystal
`oscillators can vary by + 100 PPM from their nominal
`value. At every node a local 24.576 MHz clock is
`generated to clock the modulo 3072 counters used to
`create the 8 kHz isochronous frames. The Isochronous
`Resource Manager must
`send a_
`synchronization
`message every 125 sec to resynchronize the counters
`in all nodessince they are all clockedbyslightly different
`
`
`
`After the synchronization massage, all nodes that have
`been allocated isochronous bandwidth are allowed to
`send their packets. After all isochronous packets have
`An asynchronouspacket consists of a header, a header
`been sent,
`then nodes that need to send normal
`checksum, user data and a user data checksum. The
`asynchronous packets are allowed to do so. The time
`header contains information such as source address,
`Voiceis typically sampled at 8 kHz with 8-bit resolution.
`between the 8 kHz sync messages is 125 psec. The
`destination address, message length, message type, etc.
`The minimum data field in an isochronous packet is 4
`maximum time that can be allocated to isoc