Download Multiplexing

Network Technology
CSE3020
Week 5
Network Technology CSE3020 - 2006
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Multiplexing & Switching
 Multiplexing (FDM & TDM)
 Switching (Circuit switching & packet switching)
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Multiplexing
 Multiplexing:
- Combining multiple data (voice) channels for transmission on
a common medium.
- Allows several transmission sources to share a larger
transmission capacity.
 Demultiplexing:
- Recovering the original channels from a multiplexed signal.
 Frequency division multiplexing (FDM) and time division
multiplexing (TDM).
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Frequency Division Multiplexing (FDM)

Each signal is modulated to a different carrier frequency.
 Carrier frequencies are separated so signals do not overlap
(guard bands).
- eg broadcast radio, television, cable TV, etc.
 Channel allocated even if no data.
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FDM System
• Analog or digital inputs (mi(t)) are modulated onto a subcarrier fi.
• Resulting analog modulated signals are then summed to produce
a composite baseband signal (mb(t)).
• fi are chosen so that bandwidths of various signals do not
significantly overlap.
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FDM System
• At the receiving end, mb(t) is passed through n bandpass filters
with response centered on fi, to separate mi(t)s.
• Then each separated component is demodulated to recover the
original analog/digitalNetwork
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FDM of Three Voiceband Signals
• Bandwidth of voice is generally 4
kHz (effective spectrum: 300 – 3400 Hz).
• Amplitude modulation at 64 kHz
carrier produces an 8 kHz bandwidth
(60 – 68 kHz).
• For efficient use of bandwidth,
transmit only lower sideband.
• Three carriers at 64, 68 and 72 kHz.
• Crosstalk (overlap): 4kHz allows
for a guard band.
• Intermodulation noise: one channel
could produce frequency components
in other channels.
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Analog Carrier Systems
 Long distance voiceband signals over high-capacity transmission
links (coaxial and microwave).
 Hierarchy of FDM schemes by AT&T (USA) and ITU-T
(international).
No. of voice Bandwidth
channels
Spectrum
AT&T
ITU-T
12
48 kHz
60-108 kHz
Group
Group
60
240 kHz
312-552
kHz
Supergroup
Supergroup
300
1.232 MHz
812-2044
kHz
600
2.52 MHz
564-3084
kHz
Mastergroup
Mastergroup
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Synchronous Time Division Multiplexing
 Multiple digital signals interleaved in time. May be at bit level or
in blocks of bytes.
 Time slots preassigned to the data sources and fixed synchronous
 Time slots allocated even if no data to be transmitted (like FDM).
 Time slots do not have to be evenly distributed amongst sources.
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TDM System
• Digital inputs (mi(t)) are briefly buffered.
• Buffers are scanned sequentially to produce a composite signal mc(t).
• Scanning is rapid so that each buffer is emptied before more data can arrive.
•Data are organized into frames of one cycle.
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TDM System
• At the receiving end, the interleaved data is demultiplexed and
forwarded to the appropriate destination buffer.
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TDM Link Control
 No headers and trailers
 Data link control protocols not needed at this level
 Flow control:
– not needed, data rate of multiplexed line is fixed
–
if one channel receiver cannot receive data, the others will
carry on.
–
the corresponding source must cease transmission. This
leaves empty slots for the corresponding channel.
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TDM Link Control
• Error control:
– if errors occur on one channel, the entire TDM frame is not retransmitted .
• Flow and Error Control can be provided on a per channel basis
by using a data link protocol such as HDLC
• The multiplexing operation is transparent to the attached stations,
to each it appears they have a dedicated link
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Data Link Control on TDM
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Framing
 Must provide Frame synchronization mechanism.
 Most common mechanism: Added digit framing.
– One control bit added to each TDM frame.
- Looks like another channel - “control channel”.
– Identifiable bit pattern used on control channel.
- e.g. alternating 01010101…unlikely on a data channel.
– Can compare incoming bit patterns on each channel with
sync pattern.
– Once synchronized, the receiver monitors the control channel.
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Pulse Stuffing
 Problem:
• Synchronizing the various data sources.
• Separate clocks in different sources drifting.
• Different data rates from different sources.
 Solution: Pulse Stuffing.
• Outgoing data rate (excluding framing bits) higher than sum of
incoming rates.
• Extra capacity is used to stuff extra dummy bits or pulses into
each incoming signal until it matches local clock.
• Stuffed pulses inserted at fixed locations in frame and removed
at demultiplexer.
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TDM of Analog and Digital Sources
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Digital Carrier Systems
 Hierarchy of TDM.
 USA/Canada/Japan use one system and ITU-T use a similar
(but different) system.
 US system:
• Based on DS-1 format.
• Multiplexes 24 channels.
• Each frame has 8 bits per channel plus one framing bit.
• 193 (24*8+1) bits per frame.
 For voice each channel contains one word of digitized data
(PCM, 8000 samples per sec)
• Data rate 8000x193 = 1.544Mbps
 DS-1 can carry mixed voice and data signals:
• 24 channels used with no sync byte.
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Synchronous Optical Network (SONET)/
Synchronous Digital Hierarchy (SDH)
 SONET is an optical transmission interface proposed by BellCore
and standardized by ANSI.
 SDH is defined by ITU-T for the use of high-speed transmission
capability of optical fiber.
 SONET and SDH are compatible.
 Signal Hierarchy:
– The lowest level is Synchronous Transport Signal level 1
(STS-1) or Optical Carrier level 1 (OC-1) and carries
51.84Mbps.
– STS-1 carries DS-3 or group of lower rate signals (DS1,
DS1C, DS2) plus ITU-T rates (e.g. 2.048Mbps).
– Multiple STS-1 combined into STS-N signal
– ITU-T lowest rate is 155.52Mbps (STM-1) and correspond to
STS-3.
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SONET Frame Format
• STS-1 frame contains 810 octets once every 125 s -> 51.84 Mbps.
• Logically viewed as a matrix of 9 rows of 90 octets.
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Statistical TDM




In Synchronous TDM many slots are wasted (empty slots).
Statistical TDM allocates time slots dynamically based on demand.
Multiplexer scans input lines and collects data until frame full
Data rate on line lower than aggregate rates of input lines.
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Statistical TDM Frame Formats
• Address information is required for proper delivery of data in the
frame. Thus, greater overhead.
• Generally use a synchronous protocol such as HDLC.
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Performance
 Output data rate less than aggregate input rates.
 Anticipate the average rate is less than the multiplexed line
capacity.
 May cause problems during peak periods, input exceeds capacity
Use buffer to hold excess input.
Minimum buffer size to reduce delay.
 As utilization rises, he buffer requirements and delay also
increases.
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Performance
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Required Reading
• W. Stallings, “Data and Computer Communications”
Prentice-Hall
Ch 8, Sections 8.1, 8.2, 8.3 7E/6E
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Switching Networks
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Switching Networks
• Long distance transmission is typically done over a network of
switched nodes.
• Nodes not concerned with content of data.
• Data forwarded by being switched from node to node.
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Switching Nodes
• Nodes may connect to other nodes only, or to stations and other
nodes.
• Node to node links usually multiplexed.
• Network is usually partially connected.
– Some redundant connections are desirable for reliability.
• Two different switching technologies:
– Circuit switching.
– Packet switching.
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Circuit Switching
• Circuit switching designed for voice:
- Resources dedicated to a particular call.
- Much of the time a connection is idle.
- Data rate is fixed. Both ends must operate at the same rate.
• Dedicated communication path between two stations.
• Three phases:
- Circuit establishment.
- Data transfer.
- Circuit disconnect.
• Must have switching capacity and channel capacity to establish
connection.
• Must have intelligence to work out path to destination.
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Public Circuit Switched Network
• Four generic architectural components:
– Subscribers
• Devices that attach to network, eg telephones
– Subscriber line
•
•
•
•
Link between subscriber and network
Also called the Local Loop (up to 10kms)
Uses UTP cable
Electronic switching, no delay through switch once circuit
established
– Exchanges
• Switching centres in the network
– Trunks
• Links between exchanges
• Carry multiple voice frequency circuits via FDM or synchronous
TDM
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Public Circuit Switched Network
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Circuit Switching Concepts
• Digital Switch:
– Provides a signal path between any pair of attached devices
– The path is transparent in that it appears to the attached devices that there
is a direct connection between them
• Network Interface:
– Represents functions and hardware needed to connect devices.
• Control Unit:
– Establish connections:
• Generally on demand.
• Handle and acknowledge requests.
• Determine if destination is free.
• Construct path.
– Maintain connection.
– Disconnect.
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Circuit Switching Concepts
• Control signals are the means by which the network is managed
and by which calls are established, maintained and terminated.
• Control Signaling Functions:
– Call establishment & Termination.
– Billing information.
– Control of specialist equipment.
– Two types of signaling: Common channel signaling &
Inchannel signaling.
– Common channel signaling is more flexible and powerful.
– Most widely used scheme is Signaling System Number 7
(SS7).
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Circuit Switching
• Channel capacity dedicated for duration of connection.
• If no data, capacity wasted.
• Set up (connection) takes time.
• Once connected, transfer is transparent.
• Developed for voice traffic (phone).
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Blocking
• Blocking:
– If the network is unable to connect stations because all paths are in use,
the call is blocked
– Engaged tone on voice call
• Non-blocking:
– Permits all stations to connect (in pairs) at once, to transmit data
– Possible in packet switched networks.
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Packet Switching
• Data transmitted in small packets.
– Typically 1000 octets.
– Longer messages split into series of packets.
– Each packet contains a portion of user data plus some control
information: Routing (addressing).
• Packets are received, stored briefly (buffered) and passed on to
the next node.
– Store and forward.
• Transmission time depends on the packet size.
– Optimum packet size to reduce delay & overhead.
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Packet Switching
• Station breaks long messages into packets.
• Packets sent one at a time to the network.
• Packets handled in two ways.
– Datagram.
– Virtual circuit.
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Packet Switching: Advantages
• Line efficiency:
– Single node to node link can be shared by many packets over
time.
– Packets queued and transmitted as fast as possible.
• Data rate conversion:
– Each station connects to the local node at its own speed.
– Nodes buffer data if required to equalize rates.
• Packets are accepted even when network is busy.
– Delivery may slow down.
• Priorities can be used.
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Packet Switching: Datagram
•
•
•
•
•
•
•
Each packet treated independently.
Packets can take any practical route.
Packets may arrive out of order.
Packets may go missing.
Receiver to re-order packets and recover from missing packets.
No call setup phase: Better if few packets.
More flexible: Routing can be used to avoid congested paths of
the network.
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Packet Switching: Virtual Circuit
• Preplanned route established before any packets sent.
• Call request and call accept packets establish connection
(handshake).
• Each packet contains a virtual circuit identifier instead of
destination address.
• No routing decisions required for each packet
• Network can provide sequencing and error control.
• Clear request to drop circuit.
• Loss of a node losses all circuits through that node
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Circuit & Packet Switching
• Performance
– Propagation delay
– Transmission time.
– Node delay.
.
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Required Reading
• W. Stallings, Data and Computer Communications
Prentice-Hall.
Ch 9/10, Sections 9.1-3, 10.1
6E
Ch 10,
7E
Sections 10.1-6
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