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Arhitecturi si Protocoale
de Comunicatii (APC)
Data transmission, multiplexing andswitching (overview)
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Data transmission
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Data is transmitted encoded in the parameters of an
electromagnetic wave(signal) that propagates from the
transmitter to the receiver on the transmission medium.
10110
Rx Data
10110
Tx Data Tx Signal Rx Signal
Data encoding and signal transmission:bit stringsignal.
Signal propagation through the transmission medium.
Signal reception and data decoding:signal bit string.
NICNIC
NIC = Network
Interface Card
Transmission
medium
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Data and signals
TxC
0101100
t
0 1 0 1 1 0 0 1
Tb
RxC
0101100
t
Tb
RxCTxC 0 1 0 1 1 0 0 1
Tx
Rx
RxC must be synchronized with TxC
Tx: TransmitterTxC: Transmitter clock
Rx: ReceiverRxC: Receiver Clock
Encoding: data signal Decoding: signal data
A simple data encoding (baseband transmission)
A bit string can be encoded as a digital signal with 2 levels.
Try to imagine a data encoding with more levels, based on the same
principle. Example: 4 signal levels, 2 bits/level.
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Modulation techniques
Try to imagine a data encoding with more levels, based on these techniques.
E.g., 4 phase values, or combined amplitude and phase modulation.
Basebandtransmission
1 0 1 1 0 0
t
(truncated signalbandwidth)
f
f
Carrier
signal
Amplitude
modulation
(ASK = AmplitudeShift Keying)
f
f
2ndCarrier
signal
Frequency
modulation(FSK = Frequency
Shift Keying) f
Phase
modulation(PSK = Phase
Shift Keying)f
t
t
t
t
t
Data
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Propagation delay
Time to travel from transmitter to receiver Signals travel with finite speed.
Speed depends on medium (and slightly of signal frequency):
vacuum: c = 3108m/s; conductor cable: c = 2.3108m/s, etc.
Distance d, speed c Td= d/c seconds.
Example: d = 100 Km, c = 210
8
m/sTd= 50 s
01011 01011
Tx RxTx: transmitterRx: receiver
Propagation delay Td= d/c
tt0 t0+Td
Distance d
Total packet transfer duration on a link: T = Tp+ Td = N/Rb + d/c
(the packet transmission duration plus the propagation delay)
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Packet transfer - Example 1
Send 1
Receive 1
DATA 1
ACK 1
Send 2
Receive 2
DATA 2
ACK 2
TransmissionTp = L/R
Propagation
Td = D/V
Distance D (cable length)BA
Transfer
T = Tp+Td
R = Data rate (bits/sec)
L = Packet length (bits)
D = Distance
V = Signal propagation
speed
Packet transfer, point-to-point link. Data and acknowledgement.
Stop and go error control: single unacknowledged data packet.
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Packet transfer - Example 2
Packet transfer, point-to-point link. Data and acknowledgements.
Efficient error control: multiple unacknowledged data packets.
Receive1
Send1-4 DATA 1
Distance D (cable length)BA
DATA 2
DATA 3
Receive2ACK 1
DATA 4
ACK 2
ACK 3
ACK 3
Receive3
Receive4
How much time
it takes to deliver
4 data packets?
Compare with
example 1.
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Synchronization
Bit synchronization When does a bit (cell) start/end in the received signal?
Frame synchronization When does a frame start/end in the received bit string?
Physical layer function: Maintain synchronization of
transmitter and receiver clocks.
Short distance: Share a common clock generator (e.g., within
a computer or between a computer and nearby peripherals). Large distance (networking): Include timing information in the
transmitted signal, using appropriate encoding (next slide).
Typically a Data link layer function: Define a frame format that
allows the receiver to detect start/end of frame.
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Bit synchronization
Principle
Receiver electronics use the transitions in the data signal to
adjust the local clock such that it remains synchronized with
the transmitter clock.
How to make sure there are enough transitions, even for long
sequences of 1s or 0s? Use a suitable data encoding:
Example: Manchester encoding
bit 0 = low-to-high signal transition.
bit 1 = high-to-low signal transition.
bit 0 = code bits 01.
bit 1 = code bits 10.
0 1 0 1 1 0 0 1 1 1 1 0 0 0 0 1
Clock
t
tManchester
encoding
NRZ encoding
(Non-Return
to Zero)
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Attenuation and distortion
Medium changes the signal during propagation Attenuation: reduction of signal strength. Distortion, dispersion: change of signal shape.
Attenuation depends on medium properties, distance and
signal frequency. Signal shape is changed by the different
attenuation and propagation delay of the signal's components.
0101
???Tx Rx
Tx: transmitterRx: receiver
tt0 t0+TdDistance d
Effects of attenuation,distortion, dispersion
Medium bandwidth (analog) Range of signal frequencies that the medium can transmit.
(Signal components with frequency outside medium bandwidth are
practically completely attenuated.)
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Noise and other harmful signals
Our signal is not alone in the medium ... Various "noise" signals overlap with the transmitted signal.
EMI/RFI
01011 ???Tx Rx
TxRx
Crosstalk
EMI/RFI noise Electromagnetic Interference, Radio Frequency Interference.
Electromagnetic waves emitted by power lines, engines, radio
transmitters, etc.
Crosstalk signal Induced by radiation of nearby transmission media.
Reflection signal Caused by discontinuities of the medium.
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Data transmission errors
Decoding errors The receiver can fail
to decode the datacorrectly if the signalshape is too muchaltered or the clocksare not synchronizedwell enough.
TxC
0101100
t
0 1 0 1 1 0 0 1
Tb
RxC
0101100
t
Tb
RxCTxC 0 1 0 1 1 0 0 1
Tx Rx
Rx clock synchronized with Tx clock
Encoding: datasignal Decoding: signaldata
Signal attenuation, distortion,noise
t
0 1 0 1 1 0 0 1
Rx
0 1 1 1 0 0 0 1
t
Tx
Errors !
TxC
RxC
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Data networks
Challenges Scalability: Large number of computers, any distance.
Efficiency: Cost effective interconnection.
Solutions Efficient resource sharing techniques: multiplexing and
switching. Wide variety of technologies.
Interconnection devicesforward data on the linkstowards the destination:Switching and routing
Many data streams shareeach data link:Multiplexing/demultiplexing
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...
Multiplexing
TDM: Time DivisionMultiplexing
Each source is given certain time
intervalsduring which it can useall the bandwidth.
Multiple sources send at different
points in timeon the same
frequency bandwidth.
Time
Bandwidth (Hz)
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
FDM: Frequency DivisionMultiplexing
Each source is given its own
frequency bandand can use itpermanently.
Multiple sources send on
different frequency bandsat the
same time.
Time1
6
45
3
2
Bandwidth (Hz)
Using the same transmission medium for
multiple simultaneous communications
Other techniques (wireless networks): CDM (Code Division
Multiplexing). SDM (Space Division Multiplexing). Etc.
Multiplexed
link...
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FDMmux
frequency (medium bandwidth)
frequency
(signal bandwidth)
Shift signal from each
source in frequency
domain by modulation.
R bits/sR/3 bits/s
R/3 bits/s
R/3 bits/s
FDMdemux
FDM enables wireless (radio-wave) communications (analog/digital) and
the multiplexing of analog signals (e.g. TV broadcast, wireless/wired).
FDM is also used for digital transmissions, often in conjunction with TDM
or CDM, e.g., for wireless digital communications.
WDM (Wavelength Division Multiplexing) uses different light wavelengths
(light colors) to create multiple channels on optical fiber. WDM is FDM
applied to light waves.
Frequency Division Multiplexing
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Asynchronous (statistical) TDM
On demand bandwidth allocation. Much more efficient for variable
bit-rate, bursty streams. Various solutions.
Synchronous TDM
Fixed bandwidth allocation. Good for constant bit-rate streams.Inefficient for variable bit-rate, bursty streams. Simple, cost effective.
Time Division Multiplexing
Variable slot (or fixed). Variable cycle.R1bits/s
R2bits/s
R3bits/s
R1+R2+R3 R
R bits/s
E.g.: First In
First Served
Header Data
Fixed size slot. Fixed cycle: N slots (3)
One slot for each sourceper cycle: blue, red, green;
empty slot if no data.
R bits/s
R1 R/3bits/s
R2 R/3 bits/s
R3 R/3bits/s
Multiplexer Demultiplexer
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TDM example (historical)
This is a simple example. Actually, we need flexible techniquesable to multiplex a much larger number of data streams or/and
data streams with much higher bit-rates.
We need a digital hierarchy.
E.g., multiplex 4 E1 streams in a 8 Mbps stream, and so on.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 3131 0
E1frame: 125 s; 32 time slots; 8 bits/slot
32 channels 64 Kbps(8bit/125s). Total bit rate: 2.048 Mbps(3264Kbps).
Frame synchronization Signaling channel
E1 multiplex (ITU-T standard)
Originally designed to multiplex 64 Kbpsdigital voice channels.
Also used for WAN data links (2 Mbps
digital channel or a fraction of it).
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TDM: Digital hierarchies (I)
Plesiochronous Digital Hierarchy (PDH) Developed in 1960s-1970s. The digital signals are generated from
independent reference clocks, with (inherent) slight differences. So
they are "almost" synchronous, i.e., plesyochronous. The differences
accumulate and must be compensated by the multiplexing technique.
Disadvantage: Needs complex multi-stage multiplexers/demultiplexers.
A low level stream cannot be easily extracted from a higher level
stream (complete demultiplexing followed by re-multiplexing!).
2Mbps
E3
E0 E1 E2
32Mbps8Mbps64Kbps 2Mbps
E3
E0E1E2
32Mbps 8Mbps 64Kbps
ANSI: North America, etc.
Signal Bit-rate Channels
DS0 64 Kbps 1 DS0
DS1 (T1) 1.54 Mbps 24 DS0
DS2 (T2) 6.3 Mbps 4 DS1 (96 DS0)
DS3 (T3) 44.8 Mbps 7 DS2 (28 DS1)
- - -
ITU-T: Europe, etc.
Signal Bit-rate Channels
E0 64 Kbps 1 E0
E1 2.048 Mbps 32 E0
E2 8.45 Mbps 4 E1 (128 E0)
E3 34 Mbps 4 E2 (16 E1)
E4 140 Mbps 4 E3 (64 E1)
Obsolete
E1/E3, T1/T3 still used.
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TDM: Digital hierarchies (II)
Synchronous Digital Hierarchy Developed at the end of 1980s. Widely deployed in 1990s. The digital
signals are generated from a common and extremely accurate reference
clock (e.g., cesium atomic clock).
SONET: Synchronous Optical Network (ANSI standard).
SDH: Synchronous Digital Hierarchy (ITU-T standard, similar).
Features Lower level streams can be easily added to or dropped from a higher
level stream with a single stage multiplexer/demultiplexer (ADM).
High reliability (automatic path reconfiguration in case of faults) and
comprehensive means to control the network (enable/disable circuits),
and monitor network operation and performance, etc.
SONET (ANSI) Bit-rate SDH (ITU-T)
STS-1, OC-1 51.84 Mbps (50 Mbps) -
STS-3, OC-3 155.52 Mbps (150 Mbps) STM-1
STS-12, OC-12 622.08 Mbps (600 Mbps) STM-4
STS-24, OC-24 1244.16 Mbps (1.25 Gbps) -
STS-48, OC-48 2488.32 Mbps (2.5 Gbps) STM-16
STS-192, OC-1929953.28 Mbps (
10 Gbps)STM-64
ADMSTS-n link
Add/Drop lower level streams
(DS-m, STS-k, k
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Circuit switching
Synchronous TDM
multiplexors
Synchronous TDM
demultiplexors
S1S2
S3
A
B
C
D
E
F
G
H
Circuits
Fixed capacity communication channels. Phases: circuit setup, communication, circuit release.
Circuit switchesSwitch tiny, fixed-size data units between time slots on
synchronous TDM links, using mapping stored at circuit setup.
Guaranteed bandwidth. Low, constant transfer delay.
Ideal for real-time, constant bit-rate traffic (audio, video).
Inefficient for variable bit-rate traffic.
Examples: telephone network, ISDN, SONET/SDH.
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SONET/SDH networks
OC-n
DCC
ADM
ADM
Survivable SONET ring
OC-N
OC-nOC-n
DCC
TM
TM
TM
TM
ADM DCC
ADM
TM
OC-n
OC-n
DS--n
OC-n
TM - Terminal Multiplexer.ADM - Add/Drop Multiplexer.DCS - Digital Cross-Connect.
Hub
OC-n
DS-n
STS-n
(ATM,
IP)
SONET & SDH allow the creation of high speed circuit-switched
networks, that can provide an arbitrary mesh of high-capacity
digital circuits (STS-n, DS-n).
Used to interconnect switches in the core (backbone) of PSTN
and packet switched networks.
Typical topologies: Ring (dual, survivable). With hub (star)
extensions.
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Packet switching
Packets
Encapsulated data units with routing information in header. Packet switches
Switch variable-size packets between asynchronous TDM links
based on information in the header and forwarding tables.
Dynamic bandwidth allocation. Variable transfer delay. More
efficient for variable bit-rate traffic.
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Packet switching: connection-oriented
Virtual circuit (VC)
Logical path set up between network nodes across a packet-switched network. Identified on each link by a VC identifier (VCI).
Phases: VC setup, communication, VC release.
Switching table
Indicates how to forward packetson VCs: maps VCI on each input
link to output link and next VCI.
QoS supportOrdered packet delivery.
Can guarantee QoSbyreserving resources on VC
(bandwidth, delay).
Examples: MPLS, Frame
Relay, ATM, X.25 (obsolete).
Packet switch 1Input Output
Link VCI Link VCI
2 16 3 24
... ... ... ...
Packet switch 3Input Output
Link VCI Link VCI
1 24 2 43
... ... ... ...
Packet switch 5Input Output
Link VCI Link VCI
3 43 2 19
... ... ... ...
Switching tables
VCI data
S2
S1
S3
S4
S5
S6
2 1
31 2
1 2
34
16
24 43
19
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Packet switching: connectionless
Routing table
Gives next hop on the path
to each known destination.
Best effort service
Packet delivery and ordered
delivery not guaranteed.
Can add some QoS support(traffic classes, priorities, and
resources allocated per class).
Datagrams
Standalone packets, forwarded independently of each other,based on source and destination addresses in the header.
Examples: Internet Protocol
(IP), Novell IPX (obsolete).
DA,SA data
R1R3
R4
R6
R2 R5
yxyx yxyx
yx yx
yx
Router R1DA Next hop
y R3, R4
... ...
Routing tables
Router R3DA Next hop
y R6
... ...
Router R6DA Next hop
y -
... ...
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Packet transfer - Example 3
Send
Receive
DATA
ACK
Receive
Packet transfer. Store and forward packet switching. Data and acknowledgement.
DATA
ACK
Distance D Distance D Distance D
DATA
ACK
BA
How much time
it takes to deliver a
data packet?
Compare with
example 1.
This example assumes
that the packet queues in
all the switches are empty
(no queuing delay ).
How much is the transferdelay if the packet queues
are not empty?
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Example (historical): Frame Relay (I)
Light-weight packet-switching, connection-oriented Layer 2 packet (frame) switching. Successor of X.25.
Developed after 1988 in the framework of ISDN (ITU-T).
Frame Relay virtual circuits (VC) Called Data Link Connections (DLC).
Distinguished by DLC identifiers (DLCI).
DLCI have local (link) or global (network) significance.
Frame Relay
WANPhysical
Data Link
Link bandwidth: up to 45 Mbps.
DLCI=1
DLCI=1DLCI=9
DLCI=2
DLCI=5
DLCI=4
DLCI=3
DLCI=7DLCI=4
Frame Relayswitch
Frames
Layer-2 connection-oriented packet
switching.
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Frame Relay (II)
7
6
9
9
9 8 12
5 6 5
Input OutputPort DLCI Port DLCI
1 9 4 8
1 5 4 6
2 7 3 11
Input OutputPort DLCI Port DLCI
2 6 3 9
Input Output
Port DLCI Port DLCI
1 9 2 9
Input OutputPort DLCI Port DLCI
1 8 4 12
1 6 4 5
Return paths not shown
in the switching tables!
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Example: ATM
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PBX
VoiceVideo
/Audio
Data
Cells:
5 octets
header,
48 octets
payload.
ATM: Asynchronous Transfer Mode
First universal digital carrier: voice, video, data.Foundation of Broadband Integrated Services
Digital Network (B-ISDN): first attempt to unify
data, telephone, and video/audio networks.
Widely deployed in the 1990s, being phased out.
Cell switching: convergence of technologies As in circuit switching, ATM is connection-oriented
and uses small, fixed-size data units (known as cells).
However, ATM uses asynchronous TDM and packet
switching on virtual circuits.
Small, fixed-size cell reduces the end-to-end delay
and jitter (required by telephony) and simplifies the
design and implementation of high-speed switches.
Asynchronous TDM improves the efficiency for a
broad range of QoS requirements (see next slide).
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ATM service categories (ATM Forum)
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Service category Description AAL
CBR
(Constant Bit Rate)
Fixed data rate, low delay and delay variationspecified in service contract and guaranteed.
Non-compressed real-time video, audio (tele-
conferencing, telephony, video-on-demand).
Type 1
VBR-RT (real-time
Variable Bit Rate)
Variable data rate, low delay and delay variation
specified in service contract and guaranteed.
Compressed real-time video, audio.
Type 2
VBR-NRT
(non real-time VBR)
Similar with VBR-RT, but delay constraints not
guaranteed. Other real-time applications.Type 2
UBR
(Unspecified Bit Rate)
Best effort service. No traffic and QoS
commitment. E-mail, ftp.Type 3/4
ABR
(Available Bit Rate)
Variable data rate specified in service contract,
minimum rate guaranteed, higher rate provided
whenever resources are available. Bursty traffic.
Type 5
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i110.4.3.0/24
i1R1
i2 i3i2 i1R2
i2
R4
i1R3 10.5.0.0/16
IP packet forwarding without MPLS
DA=10.5.1.1 DA=10.5.1.1 DA=10.5.1.1
R1: Routing table
Destination Out IF NH
10.4.3.0/24 i2 R2
10.5.0.0/16 i2 R2
R2: Routing table
Destination Out IF NH
10.5.0.0/16 i3 R3
10.4.3.0/24 i1 R4
R3: Routing table
Destination Out IF NH
10.4.3.0/24 i1 R2
10.5.0.0/16 i2 Direct
DA=10.5.1.1
A routing protocol determines the routes.
Each router on the path to the IP packet's destination makes a
longest match routing table lookup to find the route to be taken by
the packet (destination-based FEC determined at each hop).
Let's add now MPLS and destination-based label switched paths.
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i1R1 R2
i2 i3
i1R5
i1R4i3
i1 i2
R3i2i1
Packet forwarding using MPLS
Ingress router classifies the packets, determines the appropriate LSP(destination, service class), then labels and forwards them on the LSP.
Internal routers forward the packets along the path according to the label
and switching tables (instead of destination address and routing table).
LSP egress router removes the labels and then forwards the packets.
LSPs are set up based on IP routing tables using label distribution protocols.
Label switching table
In IF In L Out IF Out L
i2 L1 i3 L2
Edge LSRLSR
LSR
LSREdge LSR
MPLS domainLSR = Label Switching Router
Label switching table
In IF In L Out IF Out L
i1 L2 i2 L3
LSP
LSP = Label Switched Path
IP IP L1 IP L2 IP L3 IP
At Edge:Ingress LSRClassifies IP packets& Adds labels (MPLSheader)
At Edge:Egress LSRRemoves labels(MPLS header) &Forwards IP packets
In Core:LSRs forward packets based on labels.
Label switching (aka label swapping)