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COMP 421 /CMPET 401
COMMUNICATIONS and NETWORKING
CLASS 6
Physical Layer
Refers to transmission of unstructured bits
over physical medium
 Deals with characteristics of and access to
the physical medium

Data Link Layer
Provides for reliable transfer of
information across physical link
 Includes:

–
–
–
–
transmission of blocks of data (“frames”)
synchronization
error control
flow control
Asynchronous & Synchronous Transmission


Timing problems require a mechanism to
synchronize the transmitter and receiver
Two solutions exist
– Asynchronous
– Synchronous




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




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




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 56kbps 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
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



Used in parallel
communication
Large blocks of bits
transmitted without
start/stop codes
Synchronized by a
clock signal or
clocking data




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 RS232 or V.35. The data,
being binary, is usually represented by two
physical states. For example, +5v may represent
1 and -5v 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

Low-speed terminals and PCs commonly
use asynchronous transmission
– inexpensive

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
 Standards define what signals are sent, and
how
 Some standards also define the physical
connector to be used

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 DB25 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
IS2110.
RS-232C (EIA 232C)

EIA’s “Recommended Standard” (RS)

Specifies mechanical, electrical, functional, and
procedural aspects of the interface

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

Newer version of RS-232-C adopted in
1987

Improvements in grounding shield, test and
loop-back signals

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


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
Limits
The standards for RS-232 and similar interfaces
usually restrict RS-232 to 20kbps or less and
line lengths of 15m (50 ft) or less. These
restrictions are mostly throwbacks to the days
when 20kbps 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 20kbps
over a 15m line would imply. Most 56kbps DSUs
are supplied with both V.35 and RS-232 ports
because RS-232 is perfectly adequate at speeds
up to 200kbps.
DTE / DCE
If the full EIA232 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 DB25 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 DB25
connector, and utilizes the same 22 available pins for signals and ground.
The DTE Connector
The DCE Connector
Mechanical Specifications

25-pin connector with a specific arrangement of leads

DTE devices usually have male DB25 connectors while
DCE devices have female

In practice, fewer than 25 wires are generally used in
applications
RS232 DB25 Connector
RS-232 Serial PC Port Connector DB-25
DB-25M
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



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)

Only a few circuits are
necessary:
–
–
–
–
–
–
–

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

Limited RS-232
Electrical Specifications
Specifies signaling between DTE and DCE
 Uses NRZ-L encoding

– Voltage < -3V = binary 1
– Voltage > +3V = binary 0
– Voltage could be as high as  25 volts

Rated for >20Kbps and <15M
– greater distances and rates are theoretically
possible, but not necessarily wise
Functional Specifications

Specifies the role of the individual circuits

Data circuits in both directions allow full-duplex
communication

Timing signals allow for synchronous transmission
(although asynchronous transmission is more common)
Procedural Specifications
Multiple procedures are specified
 Simple example: exchange of
asynchronous data on private line

– 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 30v/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 -3v to -25v with respect to
signal ground (pin 7) are considered logic '1' (the marking condition),
whereas voltages of +3v to +25v are considered logic '0' (the spacing
condition). The range of voltages between -3v and +3v is considered a
transition region for which a signal state is not assigned.
Voltage Levels
The truth table for RS232 is:
Signal > +3v = 0
Signal < -3v = 1 <-3v>
The output signal level usually swings
between +12v and -12v. The "dead
area" between +3v and -3v is designed
to absorb line noise. In the various RS232-like definitions this dead area may
vary. For instance, the definition for V.10
has a dead area from +0.3v to -0.3v.
Many receivers designed for RS-232 are
sensitive to differentials of 1 volt or less.
Asynchronous Operation
Signal Timing
The EIA232 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 EIA232 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 1ms.
3 - For Data and Timing signals, the transit time through the transition region should be
a - less than 1ms for bit periods greater than 25ms,
b - 4% of the bit period for bit periods between 25ms 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)

Only a few circuits are
necessary:
–
–
–
–
–
–
–

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

Allows DTE to DTE direct communication
SG
DTR
DSR
RTS
CTS
CD
TD
RD
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 < -0.2v =0
VA-VB > +0.2v=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

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
 Provides for a 37-pin connection, cable lengths
up to 200 feet, and data transmission rates up to
2 million bps
 Equates with the functional and procedural
portions of R-232-C

– the electrical and mechanical specifications are
covered by RS-422 and RS-423
RS-449
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 DB37 and DB9 connectors. RS449 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
1
2
EIA
CKT
Description
From
DCE
SI
Shield
Signaling Rate Indicator
*C
*T
4
5
22
23
SD
ST
Send Data
Send Timing
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
RL
IC
Remote Loopback
Incoming Call
*C
SR
TT
Signaling Rate Selector
Incoming Call
18
19
TM
SG
Test Mode
Signal Ground
20
28
RC
IS
Receive Common
Terminal in Service
32
33
SS
SQ
Select Standby
Signal Quality
34
36
37
NS
SB
SC
New Signal
Standby Indicator
Send Common
14
15
16
17
35
*D
To
DCE
*D
*C
*C
*C
*C
*C
*C
*T
*C
*C
*C
*C
*C
*C
RS-530
RS-530 — Supersedes RS-449 and complements RS232. 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
1
2
3
4
5
6
Description
Circuit
Paired with
TxD
RxD
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
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 TxD
CB
BA
5
2
15
Transmit Signal Element Timing
DB
12
16
17
18
19
20
21
22
23
24
25
Rtrn RxD
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
Dir
RS-422
RS-422 — Defines a balanced interface with no accompanying
physical connector. Manufacturers who adhere to this standard use
many different connectors, including screw terminals, DB9, DB25 with
nonstandard pinning, DB25 following RS-530, and DB37 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
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
differential
Drivers per Line
1
1
32
Receivers per Line
1
10
32
Maximum Cable Length
50 feet
4000 feet
4000 feet
Maximum Data Rate
20 kbps
10 Mbps
10 Mbps
±25V
-0.25 to +6V
-7 to +12V
±5V
±2V
±1.5V
±15V
±5V
±5V
3kW to 7kW
100kW
54kW
Max. Driver Output Current (Power on)
n/a
n/a
±100mA
Max. Driver Output Current (Power off)
VMAX/300W
±100mA
±100mA
30V/ms max.
n/a
n/a
±15V
-7V to +7V
-7V to +12V
Receiver Input Sensitivity
±3V
±200mV
±200mV
Receiver Input Resistance
3kW to 7kW
4kW
12kW
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 —
V.35 has been around for quite some time and was
originally designed for a 48K bps modem, that's right
officially it's top speed is 48Kbps. However, it has been
shown if implemented correctly 2.048Mhz 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/RS449 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 T3plus 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 mW
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 and an overall jacket which makes it a STP-cable. The alternative cable is used for the 1.5 Mbps
version and has a B-type 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
Electrical Specifications
A differential "1" is defined as (D+) - (D-) > 200 mV and a "0" is defined as
(D+) - (D-) < -200 mV.
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 400Mbps(in 1394a) and 800Mbps (in 1394b). Products supporting
the 1394 standard go under different names, depending on the
company. Apple, which originally developed the technology, uses the
trademarked name FireWire. Other companies use other names, such
as i.link and Lynx, to describe their 1394 products.
A single 1394port 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.
•Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.
•Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps
END Class 6