COMP 421 /CMPET 401

1
COMP 421 /CMPET 401

• COMMUNICATIONS and NETWORKING
• CLASS 6

2
Physical Layer

• Refers to transmission of unstructured bits over
  physical medium
• Deals with characteristics of and access to the
  physical medium

3
Data Link Layer

• Provides for reliable transfer of information
  across physical link
• Includes
• transmission of blocks of data (frames)
• synchronization
• error control
• flow control

4
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

5
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

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Asynchronous Communications
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Asynchronous Character Stream
1 to 2 Stop Bits
Odd Even None
5 to 8 data bits
1
Idle State
Stop Bits
Next Idle State
Start Bit
P Bit
0

• Parity bit is set so that the total number of 1s
  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.

8
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

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Synchronous Frame
8-bit flag
Control fields
Control fields
8-bit flag
Data Field

• 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

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Synchronization
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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

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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)

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Pleisiochronous Transmission

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

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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

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Generic Communications Interface Illustration
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DTE and DCE
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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 020 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 devices
RS-232 port through its Receive Data (RD) lead.
RS-232 is compatible with these standards ITU
V.24, V.28 ISO IS2110.
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RS-232C (EIA 232C)

• EIAs 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
DISTANCE (ft) 1200
1000 2400 500 4800
250 9600 150
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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

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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
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Limits
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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.
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The DTE Connector
24
The DCE Connector
25
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

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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-
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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
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RS-232 DB-25 Connectors
DB Connector-Data Bus Connector
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RS-232 DB-25 Pinouts
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Important Pins
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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)
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Limited Distance Modem Example (Point-to-Point)

• Only a few circuits are necessary
• Signal Ground (7)
• Transmitted Data (2)
• Received Data (3)
• Request to Send (4)
• Clear to Send (5)
• DCE Ready (6)
• Received Line Signal Detector Carrier Detect (8)

• Additional circuits necessary sometimes
• DTE Ready(20)
• Ring Indicator (22)

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RS-232 DB-9 Connectors

• Limited RS-232

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Electrical Specifications

• Specifies signaling between DTE and DCE
• Uses NRZ-L encoding
• Voltage lt -3V binary 1
• Voltage gt 3V binary 0
• Voltage could be as high as ? 25 volts
• Rated for gt20Kbps and lt15M
• greater distances and rates are theoretically
  possible, but not necessarily wise

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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)

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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

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Control Lines
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Control Lines
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Clocks
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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.
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RS-232 Signals (Asynch)
Even Parity
Odd Parity
No Parity
See ASCII Table 3.1, Page 83
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Connection Establishment
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Dial Up Operation (1)
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Dial Up Operation (2)
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Dial Up Operation (3)
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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.
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Voltage Levels
48
Asynchronous Operation
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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.
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Limited Distance Modem Example (Point-to-Point)

• Only a few circuits are necessary
• Signal Ground (7)
• Transmitted Data (2)
• Received Data (3)
• Request to Send (4)
• Clear to Send (5)
• DCE Ready (6)
• Received Line Signal Detector Carrier Detect (8)

• Additional circuits necessary sometimes
• DTE Ready(20)
• Ring Indicator (22)

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Null Modem Cable

• Allows DTE to DTE direct communication

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Balanced Interfaces
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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)
Computer or Terminal
Modem
Transmit (2)
Receive (3)
Request to Send (4)
Clear to Send (5)
Data Set Ready (6)
Common Return (7)
Carrier Detect (8)
Date Terminal Ready (20)
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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

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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.
RS-449 was originally intended to replace RS-232,
but RS-232 and RS-449 are completely incompatible
in mechanical and electrical specifications
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RS-449 Pins
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RS-530
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.
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RS-530 Speed and Distance
Terminated Circuits
10 MHz 10 Meters6 MHz 17 Meters2 MHz 40
Meters1 MHz 100 Meters100 KHz 1000 Meters10
KHz 1000 Meters Non- Terminated Circuits
1 MHz 10 Meters100 KHz 100 Meters56 KHz 110
Meters10 KHz 1000 Meters
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RS-530
    EIA-530 Interface Reference
25 pin D-SUB MALE connector  at the DTE
(Computer)
             25 pin D-SUB FEMALE connector at
the DCE (Modem)
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RS-530 PINS
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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
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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.
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A Comparison
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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
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V.35 Connector / Pins
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V.35 Cable Recommendations
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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
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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
• Thickness0.86 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 type2 row, 50 pin, shielded tab connectors
  AMP plug part number
• Receptacle type2 row, 50 pin, receptical header
  with rails and latch blocks.

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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.
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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.
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Electrical Specifications
Electrical SpecificationsA differential "1" is
defined as (D) - (D-) gt 200 mV and a "0" is
defined as (D) - (D-) lt -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.
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IEEE 1394
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SCSI
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SCSI Specs
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END Class 6