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COMP210
Physical Layer
Dr Ahmad Al-Zubi
The Physical Layer
•
The Physical Layer performs bit by bit
transmission of the frames given to it
by the Data Link Layer.
•
The specifications of the Physical
Layer include:
•
Mechanical and electrical interfaces
•
Sockets and wires used to connect the
host to the network
•
Voltage levels uses (e.g. -5V and +5V)
•
Encoding techniques (e.g. Manchester
encoding)
•
Modulation techniques used (e.g. square
wave)
•
The bit rate and the baud rate.
Signal Transmission
• Electronic energy to send
signals that communicate
from one node to another
• Two methods of
transmitting data
¯ Digital signaling
¯ Analog signaling
Comparison of Digital
and Analog
Bandwidth-Limited
Signals
Bandwidth-Limited
Signals
Digital Signaling
• Digital signal represents
discrete state (on or off)
• Practically instantaneous
change
Digital Signaling
Current State
Encoding
• Data is encoding by the
presence or absence of a
signal
• A positive voltage might
represent a binary zero or
binary one or visa versa
• The current state indicates
the value of the data
Digital Signaling
Current State
Digital Signaling State
Transition
Analog Signaling
• Signals represented by an
electromagnetic wave
• Signal is continuos and
represents values in a
range
• Uses one or more of the
characteristics of an
analog wave to represent
ones and zeros
Characteristics of an
Analog Signal
Parts of a Wave
The maximum intensity of a wave is
called the amplitude.
The distance between two crests is the
wavelength.
Wavelen

gth ()
or period
Phase:
Amplitud
e (a)
Relative
state of one
wave to
another in
regards to
timing
Phase
differenc
e ()


The number of complete wave cycles
every second is the frequency.
The phase difference measures (as an
angle) how far ahead one wave is when
compared to another wave.
ELECTROMAGNETIC SPECTRUM
Lambda = c / f = (3 * 10**8)[m / sec] / f
f = 10 MHz
-> lambda = 30 m
f = 10 GHz
-> lambda = 3 cm
f = 10**15 Hz -> lambda = 3 * 10**-4 mm
Three equations describing
data transmissions
1. lambda = c / frequency
lambda is the wave length
c is the speed of light (3 * 10**8 m/sec)
2. Max_data_rate = 2 * freq * log(2) #of_levels
3. delta(freq) = c * delta(lambda) / lambda ** 2
Wire Propagation
Effects
•Propagation Effects
¯ Signal changes as it travels
¯ Receiver may not be able to recognise it
Original
Signal
Final
Signal
Distance
Propagation Effects:
Attenuation
• Attenuation: signal gets
weaker as it propagates
¯ Attenuation becomes greater with
distance
¯ May become too weak to recognise
Signal
Strength
Distance
Propagation Effects:
Distortion
• Distortion: signal changes
shape as it propagates
¯ Adjacent bits may overlap
¯ May make recognition impossible
for receiver
Distance
Propagation Effects:
Noise
• Noise: thermal energy in wire
adds to signal
¯ Noise floor is average noise energy
¯ Random signal, so spikes
sometimes occur
Signal
Strength
Signal
Noise
Spike
Noise Floor
Time
Propagation Effects:
SNR
• Want a high Signal-to-Noise
Ratio (SNR)
¯ Signal strength divided by average
noise strength
¯ As SNR falls, errors increase
Signal
Strength
Signal
SNR
Noise Floor
Distance
Propagation Effects:
Interference
• Interference: energy from
outside the wire
¯ Adds to signal, like noise
¯ Often intermittent, so hard to
diagnose
Signal
Strength
Signal
Interference
Time
Propagation Effects:
Termination
• Interference can occur at cable
terminator (connector, plug)
¯
¯
¯
¯
Often, multiple wires in a bundle
Each radiates some of its signal
Causes interference in nearby wires
Especially bad at termination, where
wires are unwound and parallel
Termination
Bandwidth
• Capacity of a media to
carry information
• Total capacity may be
divided into channels
• A channel is a portion of
the total bandwidth used
for a specific purpose
Bandwidth
• Baseband
¯ The total capacity of the media
is used for one channel
¯ Most LANs use baseband
• Broadband
¯ Divides the total bandwidth into
many channels
¯ Each channel can carry a
different signal
¯ Broadband carries many
simultaneous transmissions
Analog versus Digital
• Digital
¯ Is less error prone
¯ Distortion of the signal between
the source and destination is
eliminated
• Analog
¯ Little control over the signal
distortion
¯ Old technology
Analog versus Digital
In digital communication, it is often possible
to reconstruct the original signal even after it
has been effected by noise
voltag
e
voltag
e
tim
e
voltag
e
analogue
signal
tim
e
voltag
e
digital
signal
tim
e
analogue signal +
noise
When noise
effects an
analogue
signal, it is hard
to deduce the
original signal.
tim
e
digital signal +
noise
The original
digital signal
can be
deduced
despite the
noise.
Benefits of Digital
Transmission
• Reliability
¯ Can regenerate slightly
damaged signals
¯ There are only two states.
Change to closest
¯ E.g., if two states are voltages
+10v (1) and -10v (0) and the
signal is +8v, the signal is a 1
Original
ReceivedRegenerated
Benefits of Digital
Transmission
• rror detection and
E
correction
¯ Can correct errors in
transmission
- Add a few bytes of errorchecking information
- Can ask for retransmission
if an error is detected
Benefits of Digital
Transmission
• Encryption
¯ Encrypt (scramble) messages so
that someone intercepting them
cannot read them
Benefits of Digital
Transmission
• Compression
¯ Compress message before
transmission
¯ Decompress at other end
¯ Compressed message places
lighter load on transmission
line, so less expensive to send
¯ Not always used
10101001
1010
Original
Signal
Compressed
Signal
Modulation
• Because attenuation is frequency dependent,
modems use a sine wave carrier of a particular
frequency, and then modulate that frequency.
Various modulations include:

Amplitude
modulation:
Two
different amplitudes of
sine wave are used to
represent 1's and 0's.

Frequency
modulation: Two (or
more)
different
frequencies, close to
the carrier frequency,
are used.

Phase
modulation:
The phase of the sine
wave is changed by
some fixed amount.
Binary Signal
Nyquist’s Limit

Suppose we know the bandwidth (H) of a
channel and the number of signal levels
(V) being used. What is the maximum
number of bit we could transmit?

Nyquist’s Limit says:

Max_bps = 2*H*log(base 2)(V)
bits/sec

For example, if the bandwidth is 3100Hz
and we are using 16 level modulation
then the maximum number of bits per
second is:
max_bps = 2  3100  log2(16) =
24800 bps
Analog Signaling
Amplitude Modulation
(ASK)
Analog Signaling
Frequency Modulation
(FSK)
Analog Signaling
Phase Shift Keying
(FSK)
Constellation Diagrams

We can represent the different
combinations of phase shifts and
amplitudes using a constellation diagram.
101
0110
001
45
0000
15
110 010
000 100
1000
011
111

Each dot represents a combination of
phase shift (the angle from the positive x
axis) and amplitude (the distance from the
origin).
Baud Rate / Bit Rate




The maximum number of times a signal
can change in a second is called the baud
rate.
The number of bits (1s and 0s)
transmitted in a second is called the bit
rate.
In the examples we have seen so far, the
bit rate and the baud rate are the same.
This is not always the case.
The bit rate can be higher than the baud
rate if we use more than two signal levels
(more later on…).
Modems
• Modulation demodulation
• Used to connect a digital
computer to an analog
phone system
• Can be installed internally
a card inserted into the
motherboard
• Can be connected to the
serial port (external
modem)
Modems
• Transfer speeds
¯ Bit rate BPS
¯ Baud
¯ Bandwidth
• Compression
¯ appears to increases speed by
decreasing the number of bits
sent (usually some data does not
compress well)
¯ sending and receiving modem
must use same compression
standard
Modems
• Error detection and
correction
¯ asynchronous modems use
parity check
¯ checksum counting the number
of data words sent
Digital Modem
• Miss named
• Used to connect to a digital
telephone
• ISDN (integrated Services
Digital Network) is an
example
• Again do not connect
digital modems to an
analog phone
• Higher quality lower errors
Transmission Modes
Parallel Mode
Serial Mode
Channel Types



A channel is any conduit for sending
information between devices.
There are three basic types of channel:
simplex, half-duplex and full-duplex.
A simplex channel is unidirectional, which
means data can only be sent in one
direction. For example, a TV channel
only carries data from the transmitter to
your TV set. Your TV set cannot send
information back.
Channel Types


A half-duplex channel allows information
to flow in either direction (but not
simultaneously).
Devices at either end of the channel must
take it in turns to transmit information
whilst the other listens. For example, a
walky-talky either transmits or receives
but not both at the same time.
Channel Types



A full-duplex channel allows data to be
sent in both directions simultaneously.
A full-duplex channel can be formed from
two simplex channels carrying data in
opposite directions. This may make it
more expensive than a half-duplex
channel.
There is no waiting for turns or for the
devices swap roles, as is the case with a
half-duplex channel. This means fullduplex can be faster and more efficient.
Network Media Types
• Types of Media
¯ Cable (conducted media)
-
Coaxial
Twisted pair (UTP)
Shielded twisted pair (STP)
Fiber optic
¯ Radiated
-
Infrared
Microwave
Radio
Satellite
Media Selection
Criteria
• Cost
¯ For actual media and
connecting devices such as NICs
hubs etc
• Installation
¯ Difficulty to work with media
¯ Special tools, training
Media Selection
Criteria
• Capacity
¯ The amount of information that
can be transmitted in a giving
period of time
¯ Measured as
- Bits per second bps (preferred)
- Baud (discrete signals per
second)
- Bandwidth (range of
frequencies)
Media Selection
Criteria
• Node Capacity
¯ Number of network devices that
can be connected to the media
• Attenuation
¯ Weakening of the signal over
distance
Media Selection
Criteria
• Electromagnetic
Interference (EMI)
¯ Distortion of signal caused by
outside electromagnetic fields
¯ Caused by large motors,
proximity to power sources
• Other noise sources
¯ White (Gaussian) noise
¯ Impulse noise
¯ Crosstalk
¯ Echo
Cable Media
• Unshielded Twisted Pair
UTP
• Shielded Twisted Pair STP
• Coaxial
• Fiber optic
Unshielded Twisted
Pair (UTP)
• One or more pairs of
twisted copper wires
insulated and contained in
a plastic sheath
• Twisted to reduce
crosstalk
Unshielded Twisted
Pair (UTP)
• Categories
¯ categories 1 and 2
- voice grade
- low data rates up to 4 Mbps
¯ category 3
- suitable for most LANs
- up to 16 Mbps
¯ category 4
- up to 20 Mbps
Unshielded Twisted
Pair (UTP)
• Categories
¯ category 5
- supports fast ethernet
- more twists per foot
- more stringent standards on
connectors
• Data grade UTP cable
usually consists of either 4
or 8 wires, two or four pair
• Uses RJ-45 telephone
connector
Unshielded Twisted
Pair (UTP)
Shielded Twisted Pair
(STP)
• Same as UTP but with a
aluminum/polyester shield
• Connectors are more
awkward to work with
• Usually comes in pre made
lengths
• Different standards for IBM
and Apple
Shielded Twisted Pair
(STP)
Coaxial Cable
Coax
• Two conductors sharing
the same axis
• A solid center wire
surrounded insulation and
a second conductor
Coaxial Cable
Coaxial Cable
Coax
• Size of Coax
¯ RG-8, RG-11
- 50 ohm Thick Ethernet
¯ RG-58
- 50 ohm Thin Ethernet
¯ RG-59
- 75 ohm Cable T.V.
¯ RG-62
- 93 OHM ARCnet
Fiber Optic Cable
• Thin strand(s) of glass or
plastic protected by a
plastic sheath and strength
wires or gel
• Transmits laser (single
mode) or LED (multi mode)
• Single mode more
expensive but can handle
longer distances
Fiber Optics
• Three examples of a light ray from
inside a silica fiber impinging on
the air/silica boundary at different
angles.
• Light trapped by total internal
reflection.
Optical Fibers


The main advantage of optical fiber is
the great bandwidth it can carry.
There are three main bands of
wavelength used.
Attenuat
dB/km
0.80
band
1.30
band
1.55
band
2.
0
1.
5
1.
0
0.
5
0 0. 0. 1. 1. 1. 1. 1. 1. 1. 1. 1.
8 9 0 1 2 3 4 5 6 7 8
Wavelen (microns)
Optical Fibers



One problem with optical fibers is that the
light pulses become distorted over
distance due to dispersion.
Dispersion occurs when photons from the
same light pulse take slight different paths
along the optical fiber. Because some
paths will be longer or shorter than other
paths the photons will arrive at different
times thus smearing the shape of the
pulse.
Over long distances, one pulse may
merge with another pulse. When this
happens, the receiving device will not be
able to distinguish between pulses.
Overcoming Dispersion

Normal fiber optic cable is called
multimode because photons can take
different paths along it. The more
expensive monomode fibre optic
overcomes dispersion by having a core so
thin that the light can only take one path
along it.
Fiber Optic Networks
A fiber optic ring with active
repeaters.
Fiber Optic Networks
Star connection
Fiber Cables
A comparison of
semiconductor diodes and
LEDs as light sources.
Characteristics of
Cable Media
Wireless Media
• Uses the earth’s
atmosphere as a
conducting media
• Main types
¯ radio wave
¯ microwave (including satellite)
¯ infrared
The Electromagnetic
Spectrum
The electromagnetic
spectrum and its uses for
communication.
Radio Transmission
• (a) In the VLF, LF, and MF
bands, radio waves follow the
curvature of the earth.
• (b) In the HF band, they bounce
off the ionosphere.
Radio Wave
• Most radio frequencies are
regulated
• Must obtain a license from
a regulatory board (CRTC,
FCC)
• A range of radio
frequencies are
unregulated
Radio Wave
• Low power single
frequency
¯ uses one frequency
¯ limited range 20 to 30 meters
¯ usually limited to short open
environments
Radio Wave
• High power single
frequency
¯ long distance may use repeaters
to increase distance
¯ line of sight or bounced of the
earth’s atmosphere
¯ uses a single frequency
Radio Wave
Spread Spectrum
Maintains security of the radio
transmission by:
1. Spreading the carrier signal
frequency
2. Modulating the carrier
frequency by a Pseudo
Random signal
Radio Wave
Spread Spectrum
Two methods:
1. Frequency hopping – carrier
frequency keeps on changing
according to the Pseudo Random
code
2. Direct Sequence Spread SpectrumPseudo Random digital signal
modulates the carrier directly
Transmitter and receiver must use the
same Pseudo Random code
Radio Wave
Spread Spectrum
Direct Sequence Spread Spectrum
(DSSS) is used as the Physical
Layer for the Mobile networking
(IEEE 802.11)
Radio Wave
Microwave
• Terrestrial
¯ line of sight
¯ use relay towers
¯ uses license frequencies
Communication
Satellites
Communication Satellites
One disadvantage
of satellite
transmission is the
delay that occurs
because the signal
has to travel out
into space and back
to Earth
(propagation delay).
One problem
associated with
some types of
satellite
transmission is
raindrop attenuation
(some waves at the
high end of the
spectrum are so
short they can be
absorbed by
raindrops).
Communication
Satellites
The principal satellite bands.
Communication
Satellites
VSATs using a hub.
Globalstar
Infrared
• Uses same technology as
remotes for T.V.
• signals can not penetrate
objects
• Can be point to point or
broadcast
• Point to point requires
precise alignment of
devices
• Point less immune to
eavesdropping
Multiplexing
• several lines (one for each
device) enter a multiplexer
(mux) at the host side
• the host side mux
combines all incoming
signals
• combined signals are
transmitted to a mux on
the receiving side
Types of
Multiplexers
• Frequency Division
Multiplexing (FDM)
• Time Division Multiplexing
(TDM)
• Statistical Time Division
Multiplexing (STDM)
Frequency Division Multiplexing
• The frequency spectrum is divided up among the
logical channels - each user hangs on to a particular
frequency. The radio spectrum (and a radio) are
examples of the media and the mechanism for
extracting information from the medium.
Frequency division
multiplexed circuit
Frequency Division Multiplexing



One problem with FDM is that it cannot
utilise the full capacity of the cable.
It is important that the frequency bands do
not overlap. Indeed, there must be a
considerable gap between the frequency
bands in order to ensure that signals from
one band do not affect signals in another
band.
FDM is usually used to carry analogue
signals although modulated digital signals
can also be sent using this technique.
Wavelength Division Multiplexing
• The same as FDM, but applied to fibers. There's
great potential for fibers since the bandwidth is so
huge (25,000 GHz).
• Fibers with different energy bands are passed
through a diffraction grating prism
• Combined on the long distance link
• Split at the destination
• High reliability, very high capacity
Time Division Multiplexing (TDM)

Like FDM, time division multiplexing (TDM) saves
money by allowing more than one telephone call
to use a cable at the same time.
Multiplexer
Demultiplexer
Device#1
Terminal#1
Terminal#2
Terminal#3

#3 #2 #1
#3 #2 #1
Device#2
Device#3
Instead of dividing the cable into frequency bands,
TDM splits cable usage into time slots. Each
channel is given a regular time slot in which to
send a PCM signal.
Time Division Multiplexing
• In TDM, the users take turns, each one
having exclusive use of the medium in a
round robin fashion. TDM can be all digital.
Time Division
Multiplexing
Statistical Time
Division Multiplexing
(STDM)
• allows connection of more
nodes to the circuit than
the capacity of the circuit
• works on the premise that
not all nodes will transmit
at full capacity at all times
• must transmit a terminal
identification
• may require storage
Digital Technology in Telephone Networks


Over the past 30 years, much of the traditional
analogue telephone network has been replaced by
digital technology.
A device called a codec (coder/decoder) is used to
convert analogue voice signals into digital
information that can be handled by the digital
technology.
WAN Transmission
Media
• Public Switched Telephone
Network
• High speed, High
bandwidth dedicated
leased circuits
• High speed fiber optic
cable
• Microwave transmission
links
• Satellite links
Services provided by
PSTN
• Voice – Plain Old
Telephone Service (POTS)
Based on the Voice
Channel (3600 Hz)
• Data transmission services
¯ consist of services such as :
-
switched 56
X.25
T1, T3 circuits
Frame relay
ISDN
ATM
Switching
• Switching send data
across different routes
• Three types of switching
¯ Circuit switching
¯ Message switching
¯ Packet switching
Switching
• Circuit Switching:
A connection
(electrical, optical, radio) is established
from the caller phone to the callee phone.
This happens BEFORE any data is sent.
• Message Switching: The connection is
determined only when there is actual data
(a message) ready to be sent. The whole
message is re-collected at each switch and
then forwarded on to the next switch. This
method is called store-and-forward. This
method may tie up routers for long periods
of time - not good for interactive traffic.
• Packet Switching: Divides the message
up into blocks (packets).
Therefore
packets use the transmission lines for only
a short time period - allows for interactive
traffic.
Circuit Switching
• Connects the sender and
receiver by a single
physical path for the
duration of the session
• PSTN uses circuit
switching
• Before transmission a
dedicated circuit must be
established
Circuit Switching
• Advantages
¯ guaranteed data rate
¯ once connected no channel
access delay
• Disadvantages
¯ inefficient use of the
transmission media (idle time)
¯ long connection delays (first
time)
Message Switching
• Each message is treated
as an independent unit
¯ has its own source and
destination address
• Each is transmitted from
device to device
• Each intermediate device
stores the message until
the next device is ready
¯ store and forward
Message Switching
• Route messages along
varying paths for more
efficiency
• Switching devices are
often PCs with special
software
Message Switching
• Advantages
¯ efficient traffic management
¯ reduces network congestion
¯ efficient use of network media
¯ messages can be sent when
receiver down
• Disadvantages
¯ delay of storing and forwarding
¯ costly intermediate storage
Packet Switching
• Packet switching breaks
messages into packets
• Packets travel different
routes (independent
routing)
• Each packet has its own
header information
• Packets small enough to
be stored in RAM thus
quicker than message
switching
Packet Switching
• Advantages
¯ improves the use of bandwidth
over circuit switching
¯ can adjust routes to reflect
network conditions
¯ shorter transmission delays
than message switching (stored
in RAM)
¯ less disk space
¯ smaller packets to retransmit
Packet Switching
• Disadvantages
¯ More RAM
¯ More complex protocols
¯ more processing power for
switching device
¯ Greater number of packets
greater chance for packet loss
or damage
Packet Switching
• Two methods of packet
switching
¯ Datagram packet switching
¯ Virtual circuit packet switching
Datagram Packet
Switching
• Message divided into a
stream of packets
• Each packet has it’s own
control instructions
• Switching devices route
each packet independently
Datagram Packet
Switching
• Switching devices can
route packets around busy
network links
• Require sequence
numbers to reassemble
• Small packet size
facilitates retransmission
due to errors
Virtual Circuit Packet
Switching
• Similar to circuit switching
• Before transmission of the
sending and receiving
device agree on:
¯ maximum message size
¯ network path
¯ establish a logical connection
(virtual circuit)
• All packets travel on the
same virtual path
CIRCUIT SWITCHED versus PACKET
SWITCHED NETWORKS
(a) circuit switching
(b) message switching
(c) packet switching
NARROWBAND ISDN - WHAT IS IT?
Integrated
Services
Digital Network:
A
completely digit, circuitswitched phone system.
Integrates voice and nonvoice services.
ISDN allows integration of
computers and voice. It
means that caller ID can be
used to look up your
account on the computer
so that by the time a
human answers the phone,
a
screen
has
your
information
already
available.
Integrated Services Digital Network
ISDN SYSTEM ARCHITECTURE:
• ISDN uses TDM to handle multiple
channels. For home use, the NT1
(Network Terminator) connects the
twisted pair going to the phone company
with the house wiring. Various ISDN
devices can be connected to this NT1.
• Businesses may have more channels
active than the home configuration
internal bus can handle. So a PBX (
Private Branch eXchange ) is used to
provide the internal bus containing more
switching capacity.
This in turn is
connected to NT1.
Integrated Services Digital Network
THE ISDN INTERFACE:
• Typically a number of channels are combined
together. In the USA, Primary Rate ISDN
contains 23 channels (each 64 kbps carrying
voice or data) + 1 channel for signaling and
control (16 kbps digital channel.) In Europe,
instead of 23 channels, 30 are used.
• The primary Rate is designed to connect to a
business with a PBX. As it turns out, most
companies now need far more capacity than 64
kbps for the many uses beyond voice. So this is
less than adequate.
• N-ISDN may have a life as a connection to
homes for people wanting to download images
etc. But it's not useful for serious business
applications.
B-ISDN
(Broadband Integrated Services
Digital Network)
• Broadband transmission  A type of data
transmission in which a single medium (wire)
can carry several channels at once (ex. Cable
TV).
• Baseband transmission  one signal at a time
(most communication between computers).
• B-ISDN  will offer video on demand, live
television from many sources, full motion
multimedia email, CD-quality music, LAN
interconnection, high-speed data transport for
science and industry and many more, ALL over
the telephone line.
• A digital virtual circuit capable of 155 Mbps
• Underlying technology that makes B-ISDN
possible  ATM (Asynchronous Transfer Mode)
Comparing Virtual Circuits and Circuit
Switching
A Virtual Circuit  A kind of circuit-switched path implemented
with packet switching
The service offered is connection oriented (from the customer's
point of view) but is implemented with packet switching.
Services offered include:
• Permanent
virtual
circuits that remain in
place for long periods of
time.
• Switched virtual circuits
that are set up and torn
down with each request.
• The
method
for
establishing
these
circuits is shown in the
Figure. The circuit is
really entries in a series
of
switches,
each
mapping
a
circuit
number
onto
a
forwarding line.
Why all the interest in
ATM?
• These days it is more common
for companies to want to
interconnect networks. There is
one international standard for
ATM networks and so
interconnection is easier.
• ATM can be used to provide both
LAN and WAN networks.
• ATM can be made to behave like
other standard networks and so
you do not have to throw away all
your old equipment.
• ATM provides a high speed
network.
ATM Technology
• ATM is based on Cell Relay
technology. This uses virtual circuits
to carry small packets (just 53 bytes
long) over a predetermined path
through the network.
• Section of the path can be shared by
other virtual circuits, thus ensuring
that the network is used more
efficiently than in the case of circuit
switching.
• Little error checking is performed by
the network (this keeps overheads
down). Instead, the transmitting and
receiving hosts are responsible for
error checking.
Establishing a Connection
• When information needs to be
communicated, the sender
NEGOTIATES a "requested path" with
the network for a connection to the
destination.
• When setting up this connection, the
sender specifies the type, speed and
other attributes of the call, which
determine the end-to-end quality of
service.
• The network then determines a path
through the network, and sets up a
virtual circuit along this path.
• An analogy for this is sending mail.
One can choose to send 1st class,
overnight, 2 day delivery, etc. and can
ask for certified mail.
ATM Cells
The format of the ATM cell looks
like this:
Bits:
1
2
VP
I
16
3 1
8
VCI
T P
H
CR
C
Payload
48 bytes
VPI
Virtual Path ID
VCI
T
Virtual Channel
ID
Payload Type
P
Priority
H
Header
CRC Checksum
5 bytes (40 bits) are used to carry
header data and the remaining 48
bytes carry data.
The Virtual Channel ID is used
identify which virtual circuit the
cell is to be routed along.
ATM Cell Transmission
– Unlike in synchronous
communication, there is no
requirement that cell input rigidly
alternate among sources
– Continuous cell stream is not
required
– ATM cells can be transmitted on
many types of transmission
technologies
– ATM current common speed is
155Mbps (OC3c), 622Mbps
(OC12) and 45Mbps (T3) also getting
common.
– Fiber media preferred. Short runs
may be coax or twisted pair.
ATM Speed
• ATM can deliver data at rates of either
155.52 Mbps or 622.08 Mbps and
higher data rates are likely to follow.
• ATM can operate at these high speed
because specialised switching
mechanisms have been developed that
can switch the short 53 byte cells very
quickly through the network.
• ATM can work over a variety of media.
Coaxial cable and optical fibre are the
most commonly used.
• ATM technology is currently being
used to develop the next generation of
high-bandwidth telephone systems.
ATM Switches
Operation
– Cells arrive at 360000 cells/sec per 155Mbps
connection
• Cycle time of 2.7 ms
– Many ports per switching fabric
• Fixed size cells facilitate fast switching
– Switch cells
• Keep discard rate very low
• Never reorder cells on virtual circuit
– Problem: Two cells destined for same output
– Problem: Head-of-line blocking