Making Connections
© Prof. Aiman Hanna
Department of Computer Science
Concordia University
Montreal, Canada
C ommunication Carriers & Devices
The Telephone Network
 Connects 100s of millions of users
 Calls are routed first to the local office (local exchange or central office)
 Calls within the same area code can be made through direct connections
 Other calls are routed depending on the destination
 Private Branch Exchange (PBX) computer is used to route telephone calls
within a company or organization
Figure 4.2 – Telephone Network
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C ommunication Carriers & Devices
Cellular Phones

A geographic region is divided into cells each with a
base station. A cellular phone is a two-way radio
capable of communicating with the base station

The cell phone may be within more than one boundary,
however it communicates with the base station from
where the signal is stronger

Base stations communicate with a MTSO (Mobile
Telephone Switching Office), which connects to the
regular telephone network

Receiving a cell call is more complex
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C ommunication Carriers & Devices
Cellular Phones
Figure 4.3 – Cellular Grid
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C ommunication Carriers & Devices
Cellular Phones
Figure 4.4 – Cellular Phone Communication
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C ommunication Carriers & Devices
Facsimile (Fax) Machines

A paper sheet is divided into a dot matrix; each dot (Pixel) is so
tiny (200 dots per inch, 40,000 dots per square inch)

Each dot is a bit: 1 if dot is white, 0 if black

8.5x11 inches paper would produce 3,740,000 dots, and it takes
2 minutes (approx.) at the rate of 33.6 bps

Fax machines use Data Compression schemes; instead of
sending dot by dot, the fax groups the dots and defines binary
representation of them using fewer bits
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T ransmission Modes

Defines the way in which a bit group travels from
device to another

Also defines whether bits travel in both directions
simultaneously or must take turns

Different transmission modes exist:
• Serial & Parallel
• Asynchronous, Synchronous & Isochronous
• Simplex, Half-Duplex & Full-Duplex
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T ransmission Modes
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Serial & Parallel Transmission
 Parallel transmission sends bits of a byte
simultaneously on separate wires; used between PC
and printer
 Only recommended for short distances due to sync
problems
 Serial transmission uses one wire, and can be used for
long distance communication; cheaper, more reliable
but slower
Figure 4.7 –
Parallel & Serial transmission
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T ransmission Modes
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Asynchronous, Synchronous & Isochronous Transmission
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These are ways to provide serial communication
Asynchronous transmission:
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•
•
•
Bits are divided into small groups, usually bytes, and sent independently
The receiver never knows when the bits will arrive
For example, typing keyboard characters
Typical byte-oriented input-output; that is data is transmitted one byte at a
time
• A start bit is needed to alert the receive that some data is coming;
otherwise the first few bit may get lost by the time the receiver detect and
reacts to data reception
• Similarly, a stop bit is needed
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T ransmission Modes
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Asynchronous, Synchronous & Isochronous Transmission
Figure 4.8 – Asynchronous Communication
Overhead is 2/8 = 25%
Figure 4.9 – Asynchronous Transmission of
ASCII Digits 321 using NRZ Coding
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T ransmission Modes
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Asynchronous, Synchronous & Isochronous Transmission
 Synchronous transmission:
• Allows transmission of larger bit groups
• Characters are grouped into a Data Frame (simply Frame) them be transmitted
as a whole
• A generic data frame has the following pieces:

SYN: unique bit pattern that alert the receiver of frame arrival
• Also used to ensure the receiver’s sampling rate and the consistency of the arrival rate
• The receiver can then synchronize itself to the rate at which bits arrive

Control: these bits may include the following elements
• Source address
• Destination address: Needed if frame needs to go through different nodes before reaching
the destination
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Data: Actual number of data bytes
Sequence Number: Used to assemble frames at the destination in case they arrive out
of order
Frame Type: Distinguished by some protocols
Error: Error checking bits
End: End-of-frame bits
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T ransmission Modes
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Asynchronous, Synchronous & Isochronous Transmission
 Synchronous transmission:
• Much faster and has small overhead, however
• Larger frames require higher buffering; they may also occupy the link for
longer time
Figure 4.10 – Synchronous Transmission Frame
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T ransmission Modes
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Asynchronous, Synchronous & Isochronous Transmission

Isochronous transmission:
• With asynchronous & synchronous data do not necessarily
arrive at a fixed rate
• Time between different synchronous frames may vary
(asynchronous nature!)
• Errors may force the frame to be reset, which affects the
transfer rate further
• For some applications, such as file transfer, that is fine since
correct information is more important than delays
• Isochronous transmission is used to ensure a fixed
transmission rate without gaps in between
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T ransmission Modes
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Simplex, Half-Duplex & Full-Duplex Communication
Figure 4.11 – Simplex, Half-Duplex & Full-Duplex communication
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I nterface Standard
Communication may not occur even if both parties are using the same mechanisms!
For example, if both send at the same time, no information may reach any of them – if
one is not ready to listen then information is also lost
Hence, communication must be guided by protocols
Data Terminal Equipment (DTE), such as PCs, do not communicate directly; rather
they communicate to Data Communication Equipment (DCE), such as a modem,
which connect to the network
The connection between DTE & DCE is called DTE-DCE Interface
Figure 4.12 – DTE-DCE Interface
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I nterface Standard
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EIA-232 Interface (RS-25 Serial
Port)


RS232: 25-line cable with
25-pin connector (DB25).
Every line has a function;
for example:
• Pin 1: protective ground
• Pin 2: Transmit date DTE
 DCE
• Pin 22: Ring Indicator;
indicates DCE is receiving
a ringing signal (when
modem receives a call)
RS-232 Connector
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I nterface Standard
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EIA-232 Interface (RS-232 Serial Port)
Figure 4.14 – Sending & Receiving over RS-232
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I nterface Standard
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EIA-232 Subset

Driven by economics
and actual user needs,
some vendors only
implemented a part of
the interface using
only 9 circuits instead
of 25 (9-bin
connectors)
RS-232 Subset – 9-bin Connector
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I nterface Standard
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Null Modem
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Sometimes, it is needed to connect two computers directly
A first attempt to establish connection is plug in the wire to both ends
This however won’t work; Why?
Same circuit in each end is expected to perform the same functionality; for
example send/send or receive/receive
One solution in such case is to use a null modem
The null modem can be simply a cable
Figure 4.15 – Null Modem
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I nterface Standard
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X.21 Interface
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Uses 15-bin interface
Defined as a digital signaling interface
Control information are changed in a different way than
RS-25
The standard requires more logic circuits (intelligence)
in the DTE & DCE that can interpret control sequence
& reduce the number of connecting circuits
C (control) & I (indication) state info
T (transmit) & R (receive) data or control info
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I nterface Standard
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X.21 Interface
Figure 4.16 – Sending & Receiving over an X.21 Connection
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I nterface Standard
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Universal Serial Bus (USB)
 Not long ago, we had to deal with Serial ports, Parallel ports,
Special connections for Game controllers, Key-boards, Mice,
etc.

USB was the proper replacements to those many connectors

Very flexible in connecting many different devices

Has 7-bit addressing schemes to reference the devices, which
enables connections to 127 (excluding the DTE host itself)
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I nterface Standard
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Universal Serial Bus (USB)
Figure 4.17 – Connecting USB Devices
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I nterface Standard
Universal Serial Bus (USB)
 USB cable contains 4 wires:
2 wires for data carrying
signal in modified NRZ (0
changing, 1 same)
 The other two wires provide
low-amplitude power source
to USB devices
 USB 1.1 at 12 Mbps, USB
2.0 at 480 Mbps.
 Limited to 4.5 meters; if
longer, there is no guarantee
of electrical signal integrity
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Figure 4.18 – USB Wires
Figure 4.19 – USB Cable & Plugs
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I nterface Standard
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Universal Serial Bus (USB)
 Operates on Master/Slave mode, where the host is the master

USB Frame: 1-milisecond slice of time.

During this 1-ms time frame, packets are sent (packet is a group
of bits)

All devices are clock synchronized in respect to a frame

The synchronization is not done by a common clock; rather by
the host sending a special packet at the beginning of each frame

This special packet indicates that a new frame is beginning
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I nterface Standard
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Universal Serial Bus (USB)
 USB defines 4 different transmission types:
•
•
•
•
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Control Transfer
Bulk Transfer
Interrupt Transfer
Isochronous Transfer
Control Transfer:
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•
•
•
USB devices are hot pluggable
Once plugged, the host queries the device to determine its type & bit rate
The devices responds  the host assigns an address to that device
Once this is done, the device is connected and can receive commands
from the host such as requesting their status or initiating data exchange
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I nterface Standard
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Universal Serial Bus (USB)
 Bulk Transfer:
• Some USB devices, such as scanners & digital cameras,
transfer large amount of data (bulk transfer)
• Error detection is performed and the packet may have to be
resent
• Reliable transfer, but no guarantee of timely transfer
• Many devices might be doing bulk transfer at the same time,
which may result in errors/retransmission  hence, no
guarantee on delivery time
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I nterface Standard
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Universal Serial Bus (USB)
 Interrupt Transfer:
• The world interrupt here is not that proper!
• USB devices hold the information until the host asks for them, which is
literally Polling
• The major advantage here is avoiding the complexity involved with the
interrupt system/protocol
• For example, if the host sets its polling time to the keyboard at 50 frames
(each 50 ms), then it can get up to 20 characters each second
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I nterface Standard
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Universal Serial Bus (USB)
 Isochronous Transfer:
• For some real-time devices, such as microphones and
speakers, steady transfer rate is significant
• The host can guarantee data rate for those devices by
reserving a part of each frame for them
• As with most real-time systems, error detections do not occur
here; it is simply not needed
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I nterface Standard
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Universal Serial Bus (USB)
 USB Packets
• Several exchange of packets could take place during a single
frame
• Packet types: Token, Data, Handshake
• All packets have SYN and PID (packet ID)
• SYN is a bit pattern that forces the receiving device to
synchronize its clock with the sender and adjust their
receiving bit rate
• PID identifies the packet type
• SOF indicates the Start o Frame
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I nterface Standard
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Universal Serial Bus (USB)
 USB Packets
• IN & OUT packets represent a request from the host to initiate data
transfer
• Address is a 7-bit address that identifies the device to be used
• CRC (Cyclic Redundancy Check) is used for error detection
• If errors occur, a NAK is sent to the host
• Some devices may have more than one address; for example a game
controller with multiple buttons would have multiple addresses associated
with them. The endpoint is needed to identify the exact source or
destination of the data within the device.
• For example, a game controller may have many buttons sending or
receiving different information. Each of these buttons will be indicated by
an endpoint
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I nterface Standard
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Universal Serial Bus (USB)
 USB Packets
Figure 4.20 –
USB Frames & Packets
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I nterface Standard
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FireWire
 FireWire (Apple), i.Link (Sony)
 Share common characteristics with USB
 Provide a speed of 400Mbps (USB provides 12Mbps, USB 2.0
provides 480 Mbps
 Can be used with many devices, but the main focus is on
multimedia devices, especially with digital video application
 Connects multiple devices using Daisy Chain, which means
many devices can be connected in sequence and there is no need
for a hub
 Devices have one or more FireWire port, so they can also act as
regenerators/repeaters
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I nterface Standard
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FireWire
Figure 4.21 – Connecting FireWire Devices
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I nterface Standard
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FireWire
 Uses 6 wires (2 twisted pairs TPA & TPB + 2 wires for power
source)
 Uses Data Strobe Encoding
• TPA uses some form of NRZ, where 1 is high, 0 is low
• This is however error-prone due to mis-synchronization with the sender
clock
• The sender sends a strobe signal over TPB, which stays constant
whenever the data change from 1 to 0 and vise versa
• The receiver gets both TPA & TPB signals and by XORing them, it can
create the exact sender clock
• This is a bit like Manchester Encoding, with one great difference; the
baud rate is the same as the bit rate, so there is no double BW utilization
• The only cost here is one additional twisted pair
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I nterface Standard
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FireWire
Figure 4.22 – Data Strobe Encoding
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I nterface Standard
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Multiple FireWire Buses
 USB uses Master/Slave protocol whereas FireWire uses peer-topeer protocol
 Devices may be daisy chained together to form a bus group
Figure 4.23 – Multiple FireWire Buses
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I nterface Standard
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Multiple FireWire Buses
 FireWire supports two communication modes:
Asynchronous, Isochronous

Asynchronous Communication:
• Involves exchange & acknowledgment
• Send a packet  Wait for a ACK or NACK

Isochronous Transfer:
• With this mode, FireWire guarantee that data is sent at a
steady rate; there is no waiting for ACKs or resending of
packets
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I nterface Standard
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FireWire Arbitration
 Since there is no master host, what happens if two devices
attempt to send at the same time
 Devices are configured in a tree hierarchy, with one device at the
root; each device selects an ID based on its location in the tree
 The root device acts an arbiter; when devices under it wish to
transfer, the root decides which one gets the bus based on some
form of priority
 This process is only part of the arbitration, and it works with
some arbitration methods:
• Fairness Arbitration, and
• Urgent Arbitration
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I nterface Standard
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FireWire Arbitration
 Fairness arbitration: Fairness interval allows all competing
devices to access the bus once. No device monopolizes the bus;
the fairness interval starts again after all devices that wish to
send use the bus once
 Urgent arbitration allows the devices to be prioritized within a
fairness interval (asynchronous packets interval)
 Root device has the highest priority among all in the group
 To guarantee Isochronous transmission, the root device acts as a
Cycle master. Each cycle starts with a cycle-start-packet, which
marks the start of an Isochronous cycle
 Starting the Isochronous cycle regularly guarantees Isochronous
transmission
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I nterface Standard
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FireWire Arbitration
Figure 4.23 – FireWire Arbitration
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M ultiplexing
It is possible to connect each device of a network
directly to that network, however each of these
connection carries its cost
Alternatively, multiplexing can be used
A multiplexer, or mux, routes transmission from
multiple sources to a single destination
Multiplexer
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M ultiplexing
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Frequency-Division Multiplexing (FDM)
 Used with analog signals; a common uses are TV & radio
 The available BW is divided into separate ranges or channels
 Each device shares a part of the available BW, a channel, and
keeps that portion at all times
Figure 4.27 – Amplitude Modulation
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M ultiplexing
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Frequency-Division Multiplexing
 The modulated signals from all inputs are combined
into as a single, more complex analog signal
 The channels themselves are separated by a guard
band
Figure 4.29 – FDM
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M ultiplexing
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Time-Division Multiplexing (TDM)
 Used with digital signals
 TDM keeps the signals physically distinct but logically packages them
together
 The optimal performance is achieved when the combined input rate is equal to
the output rate
 A faster combined input rate would result in signals being dropped and a
slower input rate would results in frames that are partially full so the channels
are underused
Figure 4.30 – TDM
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M ultiplexing
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Statistical Time-Division Multiplexing
 In practice, it may not be possible to keep input & output rates the same
 Keeping the frame size fixed would simply the protocol but underutilize the
channels
 An alternative is to use Statistical Multiplexer, sometimes called
Concentrator
 Since the order in one frame is not the guaranteed, a more complex logic is
there to resolve the frame correctly
Figure 4.31 – Statistical TDM
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M ultiplexing
Wave-Division Multiplexing
 Similar to FDM, but based on
optics – Potential bit rate is 1000
Gbps (Tera bps)
 Light consists of several
wavelengths (refer to spectrum of
frequencies)
 Prism spreads the light into
different colors (to different
wavelengths)
 Each source can operate at a
specific wavelength
 All signals are combined
before transmission, and
separated at the receiver
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Figure 4.32 – Light Reflecting through a Prism
Figure 4.33 – Wave-Division Multiplexing
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D igital Carriers
T1
 A standard used for long-distance communication
 Uses TDM to combine many voice channels into one DS1 frame
 T1 refers to the circuit, DS1 refers to the signal
 DS1 frame has 24 channels of 8 bits each, and one framing bit
for synchronization
Figure 4.34 – DS1 Frame
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D igital Carriers
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T1
 8-bit voice samples are taken from each of the 24 channels at a
rate of 8000 samples per second
 Each sample occupies one slot in the DS1 frame
 The receiving mux extract the bits from each slot and route them
to the appropriate destination (the voice is heard at the other
side)
Figure 4.34 – T1 Carrier System
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D igital Carriers
T1
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T1 rate:
• 8-bit sample * 8000 samples/second  64 Kbps
• To support this rate, T1 must transmit a DS1 frame each 1/8000 seconds
 must transmit 8000 * 193 bits each second
•  Date rate of 1.544 Mbps
This rate is considered slow compared to optical fiber capabilities
That is the reason there are other carriers with more channels and faster bit
rate
T1 is not only used for voice communication; other companies lease phone
lines to transfer digital information between computers
Carrier
Digital Signal No.
No. of Channels
Bit Rate, Mbps
T1
DS1
24
1.544
T2
DS2
96
6.312
T3
DS3
672
44.736
T4
DS4
4,032
274.176
North American Communication Carriers
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C ontention Protocols
Access to the medium from many entry points is called contention
Unless controlled, contention may lead to fatal problems
Contention protocols are used to avoid such problems
Figure 4.39 – No Contention Protocol
Figure 4.40 – Stop-and-Go Access Protocol
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C ontention Protocols
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Aloha Protocols
 Earliest contention protocol in 1970s by Univ. of Hawaii, called
Pure ALOHA
 Several stations to central station (Menehune) by radio
communication
 f1 for broadcast, f2 (different frequency than f1) for ACK
 Any station can transmit; if collision then wait random time
Figure 4.41 –
Aloha System
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C ontention Protocols
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Slotted Aloha Protocols
 Any overlap in signals, even a small one, would force
retransmission
 Hence, a minimal safe period to transmit two signals is 2T (T is
time period)
 So, to allow a device to transmit, you should reserve 2T for that
 Not to waste such time, Slotted Aloha is used
 Devices can only send at the beginning of each slot
Figure 4.42 – Transmission Using Pure Aloha & Slotted Aloha
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C ontention Protocols
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Slotted Aloha Protocols
 Slotted Aloha has a higher success rate than Pure Aloha
 However, with increased traffic, the different may not
be that significant
Figure 4.43 – Success Rate for Pure Aloha & Slotted Aloha
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C ontention Protocols
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Carrier Sense Multiple Access Protocols (CSMA)

Sense the medium at the beginning of a slot, send if the medium is free, else wait for next slot

p-persistent CSMA:
•
•
Continue to sense the active medium
If free, send with a probability p (0 < p  1)
 p=0 never transmits (wait again) ;
 p=1 always transmits (collision chances are higher)

Nonpersistent CSMA: check periodically, if free send else wait for one time slot and
check again
Figure 4.44 – Success Rate for CSMA & Aloha Protocols
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C ontention Protocols
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Collision Detection (CD)
 Instead of sending entire frame then discover that collision has
occurred when no ack is received, sense the medium for collision
and stop transmitting if occurs
 This will avoid the medium from being unusable during collision
 Commonly used with CSMA – called CSMA/CD
Figure 4.45 – Collision with and without Detection
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C ontention Protocols
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Collision Detection (CD)
 Two issues worth considering:
• Frame size
• Distance

Frame Size:
• The frame has to be of a minimum size so the device can
detect collision before it finishes
• If too large, a device can monopolize the medium
• So, how small should a frame be?

Depends on the maximum time it takes to detect collision
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C ontention Protocols
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Collision Detection (CD)

Example: Assume:
• 10 Mbps bit rate,
• Largest distance between two devices is 2 KM
• Signal propagate at a rate of 200 meter/µsec
To propagate 2 KM it takes 10 µsec
To propagate 4 KM (worst case, go & come back), we need 20 µsec
Rate of 10 Mbps is the same as 10 bits each µsec
In 20 µsec we have 200 bits or 200/8 = 25 bytes
• This is the minimum size a frame can be so CD can be made
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C ontention Protocols
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Collision Detection (CD)
 The other issue with CD is distance
 For example CD does not work well with satellite since the time
needed to travel back and forth between ground and satellite is
too big due to the large distance
Binary exponential back-off algorithm
 Varies the waiting time before sending again if collision occurred
 If first collision then wait 0, or 1 slots
 Second collision then wait randomly for 0, 1, 2, or 3 slots
 ..........................…
 …………………..
 If n successive collisions then wait for random # of slots between 0 and
2n-1, when n > 16 give-up and signal to higher layer!
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C ontention Protocols
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Token Passing
 Instead of sending whenever it wishes, a device will take turns in
sending with the other ones
 Capture token to send data frame
 If data then remove token and transmit data frame; else pass
token to neighbor
 Only sender can put the token back on ring after receiving it
back
 One frame per token
 Advantage: contention is much controlled than the previous
protocols
 Disadvantages:
• All devices must be known
• Complexity (what happen if the token is lost or if the device that has
control over it fails)
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C ontention Protocols
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Token Passing
Figure 4.46 – Token Ring Network
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