COMP 421 CMPET 401 COMMUNICATIONS and NETWORKING CLASS

COMP 421 /CMPET 401 COMMUNICATIONS and NETWORKING CLASS 6

Physical Layer Refers to transmission of unstructured bits over physical medium l Deals with characteristics of and access to the physical medium l

Data Link Layer Provides for reliable transfer of information across physical link l Includes: l – transmission of blocks of data (“frames”) – synchronization – error control – flow control

Asynchronous & Synchronous Transmission l l Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions exist – Asynchronous – Synchronous l l Both methods are concerned with timing issues How does the receiver know when the bit period begins and ends? Small timing difference becomes more significant over time if no synchronization takes place between sender and receiver Synchronization occurs on the data link layer

Asynchronous Transmission l l Used in serial communication Data transmitted 1 character at a time Character format is usually 1 start & 1+ stop bits, plus data of 5 -8 bits Character may include parity bit l l Timing needed only within each character Resynchronization is accomplished with each start bit Uses simple, cheap technology Wastes 20 -30% of bandwidth

Asynchronous Communications Asynchronous communications This is the method most widely used for PC or simple terminal serial communications. In asynch. serial communication, the electrical interface is held in the mark position between characters. The start of transmission of a character is signaled by a drop in signal level to the space level. At this point, the receiver starts its clock. After one bit time (the start bit) come 8 bits of true data followed by one or more stop bits at the mark level. The receiver tries to sample the signal in the middle of each bit time. The byte will be read correctly if the line is still in the intended state when the last stop bit is read. Thus the transmitter and receiver only have to have approximately the same clock rate. A little arithmetic will show that for a 10 bit sequence, the last bit will be interpreted correctly even if the sender and receiver clocks differ by as much as 5%. Asynch. is relatively simple, and therefore inexpensive. However, it has a high overhead, in that each byte carries at least two extra bits: a 25% loss of line bandwidth. A 56 kbps line can only carry 5600 bytes/second asynchronously, in ideal conditions.

Asynchronous Character Stream 5 to 8 data bits 1 0 Idle State Start Bit Odd Even None P Bit 1 to 2 Stop Bits Next Idle State • Parity bit is set so that the total number of 1’s will be even or odd, depending on which parity is set • The stop can be 1, 1. 5 or 2 2 bits. It is a binary 1 and is the same as the idle state level. • This data stream is called a frame and if the receive and transmit clocks are off by toomuch a framing error may occur.

Synchronous Transmission l l l Used in parallel communication Large blocks of bits transmitted without start/stop codes Synchronized by a clock signal or clocking data l l Data framed by preamble (opening)/ postamble (closing) bit patterns More efficient than asynchronous Overhead typically below 5% Used at higher speeds than asynchronous

Synchronous Frame 8 -bit flag Control fields Data Field Control fields 8 -bit flag • One side pulses the line regularly with one short pulse per bit time. the other uses these pulses as a clock • Each block begins with a preamble to help synchronize the frame other bits are added to convey control information. • The exact format of the frame depends on which data link procedure being used (such SDLC or HDLC, etc) • Less overhead than asynchronous, but over long distances data impairments and timing errors can become issues

Synchronization The synchronization problem Serial communication normally consists of transmitting binary data across an electrical or optical link such as RS 232 or V. 35. The data, being binary, is usually represented by two physical states. For example, +5 v may represent 1 and -5 v represent 0. The accurate decoding of the data at the remote end is dependent on the sender and receiver maintaining synchronization during decoding. The receiver must sample the signal in phase with the sender. If the sender and receiver were both supplied by exactly the same clock source, then transmission could take place forever with the assurance that signal sampling at the receiver was always in perfect synchronization with the transmitter. This is seldom the case, so in practice the receiver is periodically brought into synch. with the transmitter. It is left to the internal clocking accuracy of the transmitter and receiver to maintain sampling integrity between synchronization pulses.

Synchronization Choices l Low-speed terminals and PCs commonly use asynchronous transmission – inexpensive l Large systems and networks commonly use synchronous transmission – overhead too expensive; efficiency necessary – error-checking more important

Isochronous Transmission • Isochronous data is synchronous data transmitted without a clocking source • Bits are sent continuously • Timing is recovered from transitions in the data stream • Isochronous transmission is transparent • Isochronous transmission does not recognize control characters • Used mostly for secure military applications • Some new LAN standards such as ISOEthernet (Isochronous Ethernet)

Pleisiochronous Transmission • Pleisiochronous data is synchronous data that carefully clocked usually through a GPS based time source

Digital Interfaces The point at which one device connects to another l Standards define what signals are sent, and how l Some standards also define the physical connector to be used l

Generic Communications Interface Illustration

DTE and DCE

RS-232 Overview RS-232 — Defines three types of connections: electrical, functional, and mechanical. The RS-232 interface is ideal for the data-transmission range of 0– 20 kbps/50 ft. (15. 2 m). It employs unbalanced signaling and is usually used with DB 25 connectors to interconnect DTEs (computers, controllers, etc. ) and DCEs (modems, converters, etc. ). Serial data exits through an RS-232 port via the Transmit Data (TD) lead and arrives at the destination device’s RS-232 port through its Receive Data (RD) lead. RS-232 is compatible with these standards: ITU V. 24, V. 28; ISO IS 2110.

RS-232 C (EIA 232 C) l EIA’s “Recommended Standard” (RS) l Specifies mechanical, electrical, functional, and procedural aspects of the interface l Used for connections between DTEs and voice-grade modems, and many other applications BAUD 1200 2400 4800 9600 DISTANCE (ft) 1000 500 250 150

EIA-232 -D l Newer version of RS-232 -C adopted in 1987 l Improvements in grounding shield, test and loop-back signals l The popularity of RS-232 -C in use made it difficult for EIA-232 -D to enter into the marketplace

V. 24/EIA-232 -F l l ITU-T v. 24 Only specifies functional and procedural – References other standards for electrical and mechanical l EIA-232 -F (USA) – – – Based on RS-232 Mechanical aspects are defined by ISO 2110 Electrical v. 28 Functional v. 24 Procedural v. 24 EIA-Electronics Industries Association ITU-International Telecommunication Union ISO-International Standards Organization

Limits The standards for RS-232 and similar interfaces usually restrict RS-232 to 20 kbps or less and line lengths of 15 m (50 ft) or less. These restrictions are mostly throwbacks to the days when 20 kbps was considered a very high line speed, and cables were thick, with high capacitance. However, in practice, RS-232 is far more robust than the traditional specified limits of 20 kbps over a 15 m line would imply. Most 56 kbps DSUs are supplied with both V. 35 and RS-232 ports because RS-232 is perfectly adequate at speeds up to 200 kbps.

DTE / DCE If the full EIA 232 standard is implemented as defined, the equipment at the far end of the connection is named the DTE device (Data Terminal Equipment, usually a computer or terminal), has a male DB 25 connector, and utilizes 22 of the 25 available pins for signals or ground. Equipment at the near end of the connection (the telephone line interface) is named the DCE device (Data Circuit-terminating Equipment, usually a modem), has a female DB 25 connector, and utilizes the same 22 available pins for signals and ground.

The DTE Connector

The DCE Connector

Mechanical Specifications l 25 -pin connector with a specific arrangement of leads l DTE devices usually have male DB 25 connectors while DCE devices have female l In practice, fewer than 25 wires are generally used in applications

RS 232 DB 25 Connector RS-232 Serial PC Port Connector DB-25 M Function Abbreviation Pin #1 Chassis/Frame Ground GND Pin #2 Transmitted Data TD Pin #3 Receive Data RD Pin #4 Request To Send RTS Pin #5 Clear To Send CTS Pin #6 Data Set Ready DSR Pin #7 Signal Ground GND Pin #8 Data Carrier Detect DCD or CD Pin #9 Transmit + (Current Loop) TD+ Pin #11 Transmit - (Current Loop) TD- Pin #18 Receive + (Current Loop) RD+ Pin #20 Data Terminal Ready DTR Pin #22 Ring Indicator RI Pin #25 Receive - (Current Loop) RD-

V. 24/EIA-232 -F l l l ITU-T v. 24 Only specifies functional and procedural – References other standards for electrical and mechanical EIA-232 -F (USA) – Based on RS-232 – Mechanical aspects are defined by ISO 2110 – Electrical v. 28 – Functional v. 24 – Procedural v. 24 EIA-Electronics Industries Association ITU-International Telecommunication Union ISO-International Standards Organization

RS-232 DB-25 Connectors DB-25 Female DB-25 Male DB Connector-Data Bus Connector

RS-232 DB-25 Pinouts

Important Pins

See Table 6. 1, Page 184 For the older RS-232 -C standard, some of the pin definitions are: Pin Number Name (function) 2 TD (Transmitted Data) 3 RD (Received Data) 4 RS (Request to Send) 5 CS (Clear to Send) 6 DSR (Data Set Ready) 20 DTR (Data Terminal Ready) 8 CD (Carrier Detect) 21 SQ (Signal Quality detector)

Limited Distance Modem Example (Point-to-Point) l Only a few circuits are necessary: – – – – l Additional circuits necessary sometimes: Signal Ground (7) – DTE Ready(20) Transmitted Data (2) – Ring Indicator (22) Received Data (3) Request to Send (4) Clear to Send (5) DCE Ready (6) Received Line Signal Detector [Carrier Detect] (8)

RS-232 DB-9 Connectors l Limited RS-232

Electrical Specifications Specifies signaling between DTE and DCE l Uses NRZ-L encoding l – Voltage < -3 V = binary 1 – Voltage > +3 V = binary 0 – Voltage could be as high as 25 volts l Rated for >20 Kbps and <15 M – greater distances and rates are theoretically possible, but not necessarily wise

Functional Specifications l Specifies the role of the individual circuits l Data circuits in both directions allow full-duplex communication l Timing signals allow for synchronous transmission (although asynchronous transmission is more common)

Procedural Specifications Multiple procedures are specified l Simple example: exchange of asynchronous data on private line l – Provides means of attachment between computer and modem – Specifies method of transmitting asynchronous data between devices – Specifies method of cooperation for exchange of data between devices

Control Lines The essential feature of RS-232 is that the signals are carried as single voltages referred to a common earth on pin 7. Data is transmitted and received on pins 2 and 3 respectively. Data set ready (DSR) is an indication from the Dataset (i. e. , the modem or DSU/CSU) that it is on. Similarly, DTR indicates to the Dataset that the DTE is on. Data Carrier Detect (DCD) indicates that carrier for the transmit data is on.

Control Lines Pins 4 and 5 carry the RTS and CTS signals. In most situations, RTS and CTS are constantly on throughout the communication session. However where the DTE is connected to a multipoint line, RTS is used to turn carrier on the modem on and off. On a multipoint line, it is imperative that only one station is transmitting at a time. When a station wants to transmit, it raises RTS. The modem turns on carrier, typically waits a few milliseconds for carrier to stabilize, and raises CTS. The DTE transmits when it sees CTS up. When the station has finished its transmission, it drops RTS and the modem drops CTS and carrier together.

Clocks The clock signals are only used for synchronous communications. The modem or DSU extracts the clock from the data stream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals do not have to be the same, or even at the same baud rate. The auxiliary clock signal on pin 24 is supplied on in order to allow local connections without the need for a modem eliminator. The baud rate of the auxiliary clock is programmable. By jumpering this signal to pins 15 and 17 each side, you can use a simple null-modem cable for synchronous connections. This arrangement is much less expensive that using Modem Eliminator boxes to provide the cable crossover and clocking

Signal Timing An acceptable pulse (top) moves through the transition region quickly and without hesitation or reversal. Defective pulses (bottom) could cause data errors. 4 - The slope of the rising and falling edges of a transition should not exceed 30 v/µS. Rates higher than this may induce crosstalk in adjacent conductors of a cable.

RS-232 Signals (Asynch) Even Parity Odd Parity No Parity See ASCII Table 3. 1, Page 83

Connection Establishment

Dial Up Operation (1)

Dial Up Operation (2)

Dial Up Operation (3)

Voltage Levels Signal State Voltage Assignments - Voltages of -3 v to -25 v with respect to signal ground (pin 7) are considered logic '1' (the marking condition), whereas voltages of +3 v to +25 v are considered logic '0' (the spacing condition). The range of voltages between -3 v and +3 v is considered a transition region for which a signal state is not assigned.

Voltage Levels The truth table for RS 232 is: Signal > +3 v = 0 Signal < -3 v = 1 <-3 v> The output signal level usually swings between +12 v and -12 v. The "dead area" between +3 v and -3 v is designed to absorb line noise. In the various RS 232 -like definitions this dead area may vary. For instance, the definition for V. 10 has a dead area from +0. 3 v to -0. 3 v. Many receivers designed for RS-232 are sensitive to differentials of 1 volt or less.

Asynchronous Operation

Signal Timing The EIA 232 standard is applicable to data rates of up to 20, 000 bits per second (the usual upper limit is 19, 200 baud). Fixed baud rates are not set by the EIA 232 standard. However, the commonly used values are 300, 1200, 2400, 9600, and 19, 200 baud. Other accepted values that are not often used are 110 (mechanical teletype machines), 600, and 4800 baud. Changes in signal state from logic '1' to logic '0' or vice versa must abide by several requirements, as follows: 1 - Signals that enter the transition region during a change of state must move through the transition region to the opposite signal state without reversing direction or reentering. 2 - For control signals, the transit time through the transition region should be less than 1 ms. 3 - For Data and Timing signals, the transit time through the transition region should be a - less than 1 ms for bit periods greater than 25 ms, b - 4% of the bit period for bit periods between 25 ms and 125µs, c - less than 5µs for bit periods less than 125µs. The rise and fall times of data and timing signals ideally should be equal, but in any case vary by no more than a factor of three.

Limited Distance Modem Example (Point-to-Point) l Only a few circuits are necessary: – – – – l Additional circuits necessary sometimes: Signal Ground (7) – DTE Ready(20) Transmitted Data (2) – Ring Indicator (22) Received Data (3) Request to Send (4) Clear to Send (5) DCE Ready (6) Received Line Signal Detector [Carrier Detect] (8)

Null Modem Cable l Allows DTE to DTE direct communication SG DTR DSR RTS CTS CD TD RD

Balanced Interfaces RS-422, RS-485, V. 11 and other balanced interfaces. The limitations of RS-232 are largely eliminated by the balanced line interface. A pair of wires is used to carry each signal. The data is encoded and decoded as a differential voltage between the two lines. A typical truth table for a balanced interface is as follows: VA-VB < -0. 2 v =0 VA-VB > +0. 2 v=1 As a differential voltage, in principle the interface is unaffected by differences in ground voltage between sender and receiver.

RS-232 and RS-449 l It is a physical protocol to interface computers with modems – specify mechanical, electrical, functional, and procedural interface Protective Ground (1) Transmit (2) Receive (3) Computer or Terminal Request to Send (4) Clear to Send (5) Data Set Ready (6) Common Return (7) Carrier Detect (8) Date Terminal Ready (20) Modem

RS-449 An EIA standard that improves on the capabilities of RS-232 -C l Provides for a 37 -pin connection, cable lengths up to 200 feet, and data transmission rates up to 2 million bps l Equates with the functional and procedural portions of R-232 -C l – the electrical and mechanical specifications are covered by RS-422 and RS-423

RS-449 — Defines functional/mechanical interfaces for DTEs/DCEs that employ serial binary data interchange and is usually used with synchronous transmissions. It identifies signals (TD, RD, etc. ) that correspond with the pin numbers for a balanced interface on DB 37 and DB 9 connectors. RS 449 was originally intended to replace RS-232, but RS-232 and RS-449 are completely incompatible in mechanical and electrical specifications

RS-449 Pins Pin A B EIA CKT From DCE Description To DCE 1 2 SI Shield Signaling Rate Indicator *C 4 5 22 23 SD ST Send Data Send Timing *T 6 7 24 25 RD RS Receive Data Request to Send 8 9 26 27 RT CS Receive Timing Clear to Send *T 10 11 29 LL DM Local Loopback Data Mode *C 12 13 30 31 TR RR Terminal Ready Receiver Ready *C 14 15 RL IC Remote Loopback Incoming Call *C SR TT Signaling Rate Selector Incoming Call *C *T 16 17 35 *D *C *C *C 18 19 TM SG Test Mode Signal Ground *C 20 28 RC IS Receive Common Terminal in Service *C 32 33 SS SQ Select Standby Signal Quality 34 36 37 NS SB SC New Signal Standby Indicator Send Common *C *C

RS-530 — Supersedes RS-449 and complements RS-232. Based on a 25 -pin connection, it works in conjunction with either electrical interface RS-422 (balanced electrical circuits) or RS-423 (unbalanced electrical circuits). RS-530 defines the mechanical/electrical interfaces between DTEs and DCEs that transmit serial binary data, sync or async, at rates from 20 kbps to 2 Mbps. (Maximum distance depends on the electrical interface. ) RS-530 takes advantage of higher data rates with the same mechanical connector used for RS-232. Though RS-530 and RS-232 are not compatible, RS-530 is compatible with these standards: ITU V. 10, V. 11, X. 26; MIL-188/114; RS-449.

RS-530 Speed and Distance Terminated Circuits 10 MHz 10 Meters 6 MHz 17 Meters 2 MHz 40 Meters 1 MHz 100 Meters 100 KHz 1000 Meters 10 KHz 1000 Meters Non- Terminated Circuits 1 MHz 10 Meters 100 KHz 100 Meters 56 KHz 110 Meters 10 KHz 1000 Meters

RS-530 25 pin D-SUB MALE connector at the DTE (Computer) 25 pin D-SUB FEMALE connector at the DCE (Modem)

RS-530 PINS Pin Name Dir Description Circuit Paired with 1 2 3 4 5 6 Tx. D RTS CTS DSR Shield Transmitted Data Received Data Request To Send Clear To Send Data Set Ready BA BB CA CB CC 18 14 16 19 13 22 7 SGND Signal Ground 21 8 9 10 DCD Data Carrier Detect Rtrn Receive Sig. Elmnt Timing Rtrn DCD CF DD CF 10 17 8 11 Rtrn Transmit Sig. Elmnt Timing DA 24 12 Rtrn Transmit Sig. Elmnt Timing DB 15 13 14 Rtrn CTS Rtrn Tx. D CB BA 5 2 15 Transmit Signal Element Timing DB 12 16 17 18 19 20 21 22 23 24 25 Rtrn Rx. D Receive Signal Element Timing Local Loopback Rtrn RTS Data Terminal Ready Remote Loopback Rtrn DSR Rtrn DTR Transmit Signal Element timing Test Mode BB DD LL CA CD RL CC CD DA TM 3 9 1 4 23 7 6 20 11 LL DTR RL

RS-422 — Defines a balanced interface with no accompanying physical connector. Manufacturers who adhere to this standard use many different connectors, including screw terminals, DB 9, DB 25 with nonstandard pinning, DB 25 following RS-530, and DB 37 following RS -449. RS-422 is commonly used in point-to-point communications conducted with a dual-state driver This is accomplished by splitting each signal across two separate wires in opposite states, one inverted and one not inverted. The difference in voltage between the two lines is compared by the receiver to determine the logical state of the signal. This wire configuration, called differential data transmission or balanced transmission

RS-485 — Resembles RS-422. It may be used in multipoint applications where one computer controls many different devices. Up to 64 devices may be interconnected with RS-485.

A Comparison RS-232 RS-422 RS-485 single ended differential Drivers per Line 1 1 32 Receivers per Line 1 10 32 Maximum Cable Length 50 feet 4000 feet Maximum Data Rate 20 kbps 10 Mbps ± 25 V -0. 25 to +6 V -7 to +12 V ± 5 V ± 2 V ± 1. 5 V ± 15 V ± 5 V 3 k. W to 7 k. W 100 k. W 54 k. W Max. Driver Output Current (Power on) n/a ± 100 m. A Max. Driver Output Current (Power off) VMAX/300 W ± 100 m. A 30 V/ms max. n/a ± 15 V -7 V to +7 V -7 V to +12 V Receiver Input Sensitivity ± 3 V ± 200 m. V Receiver Input Resistance 3 k. W to 7 k. W 4 k. W 12 k. W Mode of Operation Driver Output Maximum Voltage Driver Output Signal Level (loaded) Driver Output Signal Level (unloaded) Driver Load Impedance Slew Rate Receiver Input Voltage Range

V. 35 — V. 35 has been around for quite some time and was originally designed for a 48 K bps modem, that's right officially it's top speed is 48 Kbps. However, it has been shown if implemented correctly 2. 048 Mhz and faster is possible. In 1989 CCITT BLUE BOOK (UIT) recommended the interface to become obsolete, however it hasn't, but most vendors are using the specifications from V. 11 for the differential part of the V. 35 interface as recommended by the CCITT

V. 35 Connector / Pins Pin Signal Pin Signal A Chassis Ground B Signal Ground C Request to Send D Clear to Send E Data Set Ready F Receive Line Signal Detect H Data Terminal Ready J Ring Indicator P Transmitted Data (Signal A) R Recieved Data (Signal A) S Transmitted Data (Signal B) T Received Data (Signal B) U Terminal Timing V Receive Timing A W Terminal Timing X Receive Timing Y Transmit Timing AA Transmit Timing

V. 35 Cable Recommendations V. 35/RS 449 Data Rate: Max cable length recommended: (feet) Max cable length recommended: ( meters) 2 Mb/sec 1 Mb/sec 512 Kb/sec 50 ft. 100 ft. 200 ft. 15. 24 m 30. 48 m 60. 96 m 256 Kb/sec 400 ft. 121. 92 m 128 Kb/sec 800 ft. 243. 84 m 56 K 1. 2 Kb/sec 1600 ft. 3000 ft. 487. 68 m 914. 40 m

HSSI Characteristics The High-Speed Serial Interface (HSSI) is a DTE/DCE interface that was developed by Cisco Systems and T 3 plus Networking to address the need for high-speed communication over WAN links HSSI defines both electrical and physical interfaces on DTE and DCE devices. It operates at the physical layer of the OSI reference model Characteristic Value Maximum signaling rate 52 Mbps Maximum cable length 50 feet Number of connector points 50 Interface DTE-DCE Electrical technology Differential ECL Typical power consumption 610 m. W Topology Point-to-point Cable type Shielded twisted-pair wire

HSSI CABLE SPECIFICATION • Cable type: multi-conductor cable, consisting of 25 twisted pairs cabled together with an overall double shield and PVC jacket • Gauge: 28 AWG, 7 strands of 36 AWG, tinned annealed copper, nominal 0. 015 in. diameter • Insulation: polyethylene or polypropylene; 0. 24 mm, . 0095 in. nominal wall • Thickness; 0. 86 mm +/- 0. 025 mm, . 034 in. +/- 0. 001 in. out-side diameter • Foil shield: 0. 051 mm, 0. 002 in. nominal aluminum/polyester/aluminum laminated tape spiral wrapped around the cable core with a 25% minimum overlap • Braid shield: braided 36 AWG, tinned plated copper in accordance with 80% minimum coverage • Jacket: 75 degrees C flexible polyvinylchloride • Jacket wall: 0. 51 mm, 0. 020 in. minimum thickness • Dielectic strength: 1000 VAC for 1 minute • Outside diameter: 10. 41 mm +/- 0. 18 mm, 0. 405 in. +/- 0. 015 in. • Plug type: 2 row, 50 pin, shielded tab connectors AMP plug part number • Receptacle type: 2 row, 50 pin, receptical header with rails and latch blocks.

USB The standard defines three different devices: hosts, hubs and functions. Hosts are the initiating devices, like PCs, and only 1 host may exist in a network. Functions are dumb devices, like keyboards, mice, printers. And hubs are multi-port repeaters which act like distributing devices in the serial network. 12 Mbps 1. 5 Mbps Cable STP UTP Max. Cable length 5 meter 3 meter Connector A-Series or B-Series Max. amount of HUBs 5 Max. amount of units 127

USB Cable There are two types of cables. The standard USB cable which is used for 12 Mbps and has an A-series connector consists of one pair 20 -28 AWG wire for power and one 28 AWG twisted pair for data. The cable has a shield an overall jacket which makes it a STP-cable. The alternative cable is used for the 1. 5 Mbps version and has a Btype connector. This cable has one pair of 28 AWG wire stranded copper for data and one pair 20 -28 AWG for power. This cable is only used in sub-channel applications. Signal Color Pin +Data Green 3 -data White 2 VCC Red 1 GND Black 4

Electrical Specifications A differential "1" is defined as (D+) - (D-) > 200 m. V and a "0" is defined as (D+) - (D-) < -200 m. V. The line encoding used is always NRZI. This is independent of the low or high speed version. The maximum end-to-end signal delay is 70 ns, which gives us a maximum configuration of 5 hubs per link between function and host. If all cables are high -speed cables, the max. distance between a function and a host is 30 meters.

IEEE 1394 A very fast external bus standard that supports data transfer rates of up to 400 Mbps(in 1394 a) and 800 Mbps (in 1394 b). Products supporting the 1394 standard go under different names, depending on the company. Apple, which originally developed the technology, uses the trademarked name Fire. Wire. Other companies use other names, such as i. link and Lynx, to describe their 1394 products. A single 1394 port can be used to connect up 63 external devices. In addition to its high speed, 1394 also supports isochroous data -- delivering data at a guaranteed rate. This makes it ideal for devices that need to transfer high levels of data in real-time, such as video devices. Although extremely fast and flexible, 1394 is also expensive. Like USB, 1394 also provides power to peripheral devices.

SCSI Acronym for small computer system interface. Pronounced "scuzzy, " SCSI is a parallel interface standard used by for attaching peripheral devices to computers. SCSI ports are used for attaching devices such as disk drives and printers. SCSI interfaces provide for faster data transmission rates (up to 80 Mbps) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface. Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.

SCSI Specs The following varieties of SCSI are currently implemented: • SCSI-1: Uses an 8 -bit bus, and supports data rates of 4 Mbps • SCSI-2: Same as SCSI-1, but uses a 50 -pin connector instead of a 25 -pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI. • Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16 -bit transfers. • Fast SCSI: Uses an 8 -bit bus, but doubles the clock rate to support data rates of 10 MBps. • Fast Wide SCSI: Uses a 16 -bit bus and supports data rates of 20 MBps. • Ultra SCSI: Uses an 8 -bit bus, and supports data rates of 20 MBps. • SCSI-3: Uses a 16 -bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI. • Ultra 2 SCSI: Uses an 8 -bit bus and supports data rates of 40 MBps. • Wide Ultra 2 SCSI: Uses a 16 -bit bus and supports data rates of 80 MBps

END Class 6