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Telecoms Systems (Week 1)
Prof. Laurie Cuthbert
Dr. Michael Chai
Dr Frank Gao
Staff
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Prof Laurie Cuthbert – weeks 1 and [email protected]
Dr Michael Chai – week [email protected]
Dr Frank Gao – week [email protected]
Changes since last year
Content has not changed Exam format different – now 4 compulsory
questions in 2 hours:– One on each week’s material
Remember:– QM rules on extenuating circumstances apply
3
Assessment
Exam: 88% Class tests: 12%
– Class test every week of teaching on Friday– Each group split into 2– You must be in the right group– Test is a question on anything taught that week– Roughly half an exam question– Each test counts 3%– Open book
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5
Emphasis on
Why How When you come to the lecture, bring:
– Pen– Paper– Lecture notes– Calculator
You will have to do problems in the class!!
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Learning Outcomes
Explain the principles of operation and architectures of circuit-switched and packet/cell-switched network; wired and mobile.
Describe the operation of transmission and switching systems.
Calculate simple numerical problems on aspects of source coding, error-control coding, Queuing Theory and Information Theory.
Extenuating circumstances
Must be for 'unplanned circumstances that outside of your control
These include medical and personal circumstances such as close family being ill, but not events such as:– planned holidays,– job interviews or internships– GRE or IELTS preparation or test– misreading timetables,– computer problems,– not being aware of rules or procedures.
Medical conditions must be sufficiently serious that they would have a major affect on your examination.
QM rules
ECs for all QM modules will be treated under QM rules
If you want to claim EC for an exam or class test you must:– Complete a form in English (from Jing Liu)– Add supporting evidence (e.g. medical certificate)– Give everything back to Jing Liu at least 1 week before
the examination board Your BUPT tutor does NOT have the authority to
approve ECs for QM modules
MODERN TELECOMMUNICATIONS
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Telecommunication system – a system for conveying content For example, the UK Telecommunications Act 1984, s.4(1)
defines it as:
“a system for the conveyance through the agency of electric, magnetic, electromagnetic, electro-chemical or electro-mechanical energy, of
(a) speech, music or other sounds;(b) visual images;(c) signals serving the impartation (whether as between persons and
persons, things and things or persons and things) of any matter otherwise than in the form of sounds or visual images; or
(d) signals serving for the actuation or control of machinery or apparatus”.
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“More communications than we know how to use” Many different technologies Developed in parallel Lead time to introduce new services
decreasing “Reliability” of software decreasing
– Online patches for mobile phones Remote working is now normal
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Legacy - wired communications using telephony
exchange
analogue digitalfax
Dial-up modemObsolete !!!!
Analogue
Digital
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Mobile communications
often a radio link to network
Cable / radio (Bluetooth)
Tablet
2G/3G/HSDPA/LTE
WLAN3G/HSDPA
WLAN
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IP allows competition with telephonyWebcam
Headset
SIP phoneDual cordless phone that
connects to a normal phone line and the computer
All telephony is going IP
Now telephony is SIP based
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Transport modes
Traditionally telephony was circuit switched:– Call set up, conversation and clear-down phases– 64 kbit/s (in digital era) allocated in both directions– Much of the capacity wasted– Analogue to digital conversion in local exchange– Control very much centralised
Now IP-based– SIP sets up and clears down connections– Transport RTP– A-D conversion in the telephone– More distributed
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Traditional network hierarchy
Access networkAnalogue
Core networkDigital
Local Exchange Local Exchange
Trunk Exchange Trunk Exchange
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Transmission
Connections are carrying little traffic are served with low capacity links – Up to 120Mbps in UK for
broadband Very high speed optical
fibre links between major cites– In excess of 500 Mbps and
support over 7000 voice calls
Switching used to be manual
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Then relays, then electronic – but specialised
Electromechanical exchangepicture courtesy of Nortel
1960s
Private electronic exchange1983
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Now just boxes of electronics – high volume
WLAN AP
IP router
IP switch
Servers
IP phone
All of these are just “boxes” ofElectronics
Wireless (GSM) network architecture
PSTN
BTS
BS
Mobile switching centre
Base station controller
Gateway MSC
BTS=Base transceiver station
History of wireless communications
1865 James Clerk Maxwell published his equations 1887 Heinrich Hertz demonstrated EM wave propagation 1893 Nicola Tesla demonstrated communication by radio 1895 Aleksandr Popov demonstrated a wireless system 1896 Guglielmo Marconi demonstrated wireless
telegraphy 1901 First wireless signal sent across the Atlantic Ocean
from Cornwall to St. John’s, Newfoundland (Canada) Marconi was not the ‘inventor’, but appreciated the
commercial opportunities offered by the new medium.
Why wireless?
No more cables– No cost for installing wires or rewiring – Wiring is infeasible or costly in some areas, e.g.. rural areas, old
buildings… Mobility and convenience
– Allows users to access services while moving: walking, in vehicles…
Flexibility– Roaming allows connection any where and any time
Scalability– Easier to expand network coverage compared to wired networks.
Challenges of wireless
Limited resources: finite radio spectrum – Frequency reuse, breaking cells into smaller cells, more efficient medium access
technology, e.g. CDMA… Supporting mobility - Location management, handover, … Maintaining Quality of Service (QoS) over unreliable wireless links
– Radio propagation attenuation: path loss, shadowing, multipath fading. Connectivity and coverage - roaming and internetworking Security
– Wireless channels are “open”– Certification and authentication
Integrated services (voice, data, multimedia, etc.) over a single network – service differentiation, priorities, resource sharing,...
Mobile terminal battery life You will learn more about all of this later
Emerging and existing wireless technology
Mobile Wireless: – 2G: GSM, TDMA, CDMA– 2.5G EDGE, GPRS– 3G W-CDMA, HSDPA, HSUPA– 4G - LTE
Fixed Wireless: – MMDS, LMDS, Satellite dish, Microwave
Wireless LAN: – IEEE 802.11, Ad-hoc, Bluetooth,
WiMaxWireless LAN
Mobile cellular
Point-Point/ Multipoint Wireless
Satellite wireless
Types of wireless network
WPAN (Wireless Personal Area Network)– typically operates within about 30 feet
WLAN (Wireless Local Area Network)– operates within 300 yards
WMAN (Wireless Metropolitan Area Network )– operates within tens of miles
WWAN (Wireless Wide Area Network )– operates over a large geographical area, mobile
phone, …
Features of mobile communications
Mobile phones are portable, convenient, move with people. – By their nature, they are location aware.
Limited frequency bandwidth Low power: max mobile transmit power
– 125mW for WCDMA– 2W peak for GSM900– 1W for GSM1800/1900
Point to multi-point, not broadcast
Cellular concept
Late 40s: AT&T developed cellular concept for frequency re-use Break the service area into cells Shrink the cell size; adopt intensive frequency re-use Add more cells to add more capacity Mobility management is required
Evolution of mobile networks
“It is dangerous to put limits on wireless” Guglielmo Marconi in 1932…….
1970’sProposed late 1980’s
GSM launched in 19921990’s –present
Proposed in 1998Launched in UK 2003
1G2G
2.5 G3G
Evolution of mobile networks
1G2G
3G
4G ?
2.5G
NTTTACSNMT
AMPS
GSM
IS-136
IS-95
PDC
GPRS
HSCSD
EDGE
IS-95B
W-CDMA (3GSM)
TD-SCDMA
cdma2000
Speech service Analogue
transmission
Speech & low rate data service
Digital transmission Speech, data, multimedia
services Bit rate up to 2 Mbit/s Digital transmission
Higher bit rate ? New
applications ?
1G systems
Analogue– Speech– Some data at 1.2kbit/s
Designed for car use First handportable – Motorola “Brick” (DynaTAC 8000X )
– 1983– 800g– 30 mins talk time– USD 3995
Insecure– Eavesdropping– Cloning
Almost no roaming
Some 1G systems
No real roaming apart from NMT
System Band (MHz)
Example Locations
AMPS 800 US, Canada, Mexico, Australia, New Zealand, Hong Kong, Brazil, Argentina
TACS 900 UK, Ireland, Spain, Italy, Austria
NMT 450/900 Denmark, Finland, Norway, Sweden, Belgium, Austria, France, Hungary, Netherlands, Spain
NTT 800 Japan (First cellular system 1979)
2G Systems
Speech and low bit rate data service Digital transmission Designed to be more secure Almost exclusively handportable
2G Systems
System Band (MHz)
Example Locations
D-AMPS (IS-54, IS-136)
800 North and South America
CDMA (IS-95) 800/1900
North and South America, S Korea, China
GSM 900/18001900
World-wide (except Korea and Japan)1900 MHz US and Canada
JDC/PDC 800/1500
Japan
GSM
Officially launched in 1992 Multiple access: TDMA 8 channels (frames of 8 time
slots) on each carrier FDD (Frequency Division Duplex) – different
frequencies for uplink and downlink 200kHz carrier bandwidth 9.6kb/s net data (13kb/s encoded voice) (Almost) worldwide availability with multi-band
handset Useful link www.gsmworld.com
GSM worldwide success
Over 860 networks in 220 countries/areas Still growing: No of GSM + 3 GSM subscribers
– 11/9/2011 00.55 (CN time) 5,231,269,752– In the next 5 mins an increase of: 7,106 !!!!!– World Population at same time: 6.914 billion– Penetration 59%
System Uplink (MHz) Downlink (MHz)
GSM850 824 -849 880 -915
GSM900 890 -915 935 –960
GSM1800 (DCS1800) 1710 –1785 1805 –1880
GSM1900 (PCS1900): 1930 –1990 1850 –1910
GSM network architecture – core components
BTS: Base Transceiver Station BSC: Base Station Controller BSS: Base Station Subsystem MSC: Mobile Switching Centre HLR: Home Location Register VLR: Visitors Location Register AuC: Authentication Centre GMSC: Gateway MSC PSTN: Public Switched
Telephone Network
MSC
VLR
HLRAuC
BSC BSC
GMSC
PSTN
MT
BTS
BSS
GSM network architecture – other elements
EIR: Equipment Identity Register– Record of status of phone– White / grey /black (stolen)
SMS-C: Short Message Service Centre OMC: Operation and Maintenance Centre
BS
Locating a Mobile terminal
When a MT moves from one location area to another:– MT initiates the location updating procedure.– HLR is notified by the new MSC/VLR.– HLR removes old MSC/VLR information– HLR confirms and updates the new MSC/VLR.– location area update is confirmed with the MT.
Mobile Terminating call
MSC
VLR
HLR
BSC
BSC
GMSC
BTS
Location area
MSC
VLR
PSTN
traffic
signalling
Visited network
Home network
Roaming incoming call
MSC
VLR
HLR
BSC
BSC
GMSC
BTS
Location area
MSC
VLR
PSTN
MSC
VLR
BTS Roaming leg paid by recipient
Visited network
Home networkRoaming outgoing call
MSC
VLR
HLR
BSC
BSC
GMSC
BTS
Location area
MSC
VLR
MSC
VLR
BTS
PSTN
Billing centre
Mobile Terminal
GSM Mobility Management: Authentication
A3 algorithm A3 algorithm
Challenge: RAND
Response: SRESMT
SRESMSC
If equal, then authenticated
Key Ks in SIM Key Ki in MSC
If results match, Ks=Ki and the user is genuineOnly information transmitted over the air is RAND and SRESMT
Randomnumber
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How does communications everywhere affect the global economy? Increasingly reliant on communications
technology for business Variety of actors with competing interests Communications systems becoming the target
of cyber-terrorist attacks Communications networks now part of the
national large-scale critical infrastructure.
INFORMATION CONVERSION
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Transmission of analogue information
Multiplexing Demultiplexing
Information:‘Hello! How are you?’ You and I understand but not the telephone!
Analogue signal can be understood by electrical systems but problematic!
So all new systems digital
Information Conversion
Different sources of information are presented with different formats at the input of transmitter.
Formatting transforms the source information to a compatible digital format for digital processing.
Four basic stages of information conversion
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Overview of Digital Communication System
FormatSource encode
EncryptChannel encode
MultiplexPulse
modulateBandpassmodulate
Freq.spread
Multipleaccess
FormatSource decode
DecryptChannel Decode
DemultiplexPulseDetect
DemodulateFreq.
despreadMultipleaccess
Transmitter
Receiver
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Transmission side
FormatSource encode
EncryptChannel encode
MultiplexPulse
modulateBandpassmodulate
Freq.spread
Multipleaccess
Transmitter
Transform the source information into bits, assuring compatibility between the information and the signal processing within the DCS.
Digital Information
Textual Information
Analogue Information
PulsemodulateEncoder
QuantiseSample
Formatting Textual Data
Textual information compromises a sequence of alphanumeric characters.
Each alphanumeric character is transformed into binary by character coding. Most popular character coding method is ASCII
Encoded into sequence of k bits called [symbols]
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[1]
You do not have to remember this
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Sampling (ideal sampling)time domain
frequency domain
original signal x(t)
t tt
Sample pulse xp(t) Sampled signal xs(t)
f
0-fm fm
f
fs 2fs-fs-2fs 0
f
fm fs-fm-fs 0
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Sampling (real sampling)time domain
frequency domain
original signal x(t)
t tt
Sample pulse xp(t) Sampled signal xs(t)
f
0-fm fm
f
fs 2fs-fs-2fs 0
f
fm fs-fm-fs 0
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Sampling frequency
fs > 2fmax fs < 2fmax
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Aliasing (ideal sampling)original signal
signal sampled with fs > 2 fm
signal sampled with fs = 2 fm
signal sampled with fs < 2 fm
aliasing occurs
f
0-fm fm
f
0-fm fm fs
f
0-fm fm fs 2fs
f
0-fm fmfs 2fs
You must remember the term aliasing
Aliasing in more detail
original signal
signal sampled with fs > 2 fmax
signal sampled with fs = 2 fmax
signal sampled with fs < 2 fmax
aliasing distortion occurs
fmax
fs
fs 2fs
low-pass filter can recover original signal
recovery not possible - spectra interfere
Oversampling
original signal
oversamplingsignal sampled with fs > 2 fmax
signal sampled with fs = 2 fmax
signal sampled with fs < 2 fmax
aliasing distortion occurs
fmax
fs
fs 2fs
low-pass filter can recover original signal
recovery not possible - spectra interfere
Easier to implement filter
Needs to be ideal filter
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Sampling Theorem
To prevent aliasing and hence to allow the original signal to be recovered the sampling frequency (fs) must be given by:
fs ≥ 2 fmax
where fmax is the highest frequency present in the original signal.
This is the SAMPLING THEOREM and is a fundamental theorem.
Notice that fmax is the highest frequency present, NOT the highest frequency of interest.
Oversampling
A process of sampling a signal at more than twice the higher frequency than the highest frequency present in the original signal.
Oversampled signal is normally expressed with the oversampled factor of .
fs = fmax ; ≥ 2 Oversampling makes it easier to design a simpler
filter to recover the original signal
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Summary
Different types of network – how they are linked and different network speeds
Analogue, textual and digital transmission Character coding – ASCII Sampling, anti-aliasing and oversampling
QUANTISATION
Scope
Linear quantisation and non-linear quantisation
Companding Delta modulation
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Quantisation and PCM
Quantisation results from mapping continuous analogue values to the discrete vales that can be represented digitally.
May be linear or non-linear Pulse-code modulation (PCM) is a method used
to digitally represent sampled analogue signals – there are many others.
PCM invented in 1948 by Sir Alec Reeve at STL Harlow, UK
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Principle of PCMtime domain
original signal x(t)
t tt
Sample pulse Sampled signal xs(t)
Sample amplitude represented by N bits
Linear quantisation
Peak-to-Peak Voltage, Vpp=Vp-(-Vp) = 2Vp
Quantisation interval, q,
(step size) uniformly distributed over the full range
Approximation will result in an error no larger than ±q/2
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q
Quantising level
Quantised signal
Quantising levels
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Vp
-Vp
0
Quantising level
N levels gives a range of (N-1)q
-Vp
0
Quantising level
Vp
N levels gives a range of Nq
Quantising distortion
quantising levelsignal changes state half-way between quantising levels
error q
error signal
+q/2
-q/2
quantised signal
Quantising error power
Error is approximately sawtooth over the quantisation region, apart from the dwell regions.
+q/2
-q/2
t
-q/2 +q/2
error e
p(e)1/q
A sawtooth waveform has a uniform pdf: all values are equally likely. The area under the pdf must be 1 so that the amplitude is 1/q. Note that p(e)=0 outside the range +q/2 to -q/2
D e p e de eq
deq
q
2 2
2
2 2112 = =
Power in quantising distortion =
Notes
this holds for reasonable well-behaved signals without frequent dwell regions.
quantising error leads to distortion, not noise, because it is causally related to and dependent on the input: the same input will always produce the same output.
the statistics of the distortion are independent of the statistics of the input.
this approximation shows that the distortion power is constant and depends only on the step size.
Quantiser SDR
Consider a quantiser with a maximum range of ±V and N bits. Then we have:
q V DV V
N q N N
22
4
2
1
12 3 2
2
2
2
2 and hence
If the signal power is S then we can define a signal to distortion ratio (SDR) :
SDRS
V
N
3 22
2
or in dB: SDRS
VN(dB)
10 477 60210 2log . .
Quantiser impairment curve
As the slope of the clipping line is very steep, the quantiser must be operated well away from that boundary
Such an impairment curve is not suitable for signals such as speech that have a very wide dynamic range (speech around 30dB). For instance, if S varies by 30dB this will not give satisfactory results. A flat impairment curve is needed.
SDR (dB)
log (S/V2)
clippingusable
increase N
These 2 curves have the same power (S) but clipping will have a different effect on each.
Dynamic Range (Dy)
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. .
) ( log 10 10 dB
SQNRacceptablegiveswhichpowerMin
overflownopowersignalpossibleMaxDy
Dy = max possible SQNR (dB) min acceptable SQNR (dB)
12/
log10
12/
log10
:onquantisati uniformFor
210210 q
powerMin
q
powersignalMaxDy
This final expression for Dy is well worth remembering, but it only works for uniform quantisation!
ITU Recommendation – what we need
A better impairment curve is obtained by non-liner quantisation known as companding (compressing and expanding). This is done in a codec (coder - decoder). here are two standardised coding laws: the (generally 7-bit) µ-law used in N. America and Japan and the 8-bit A-law used in the rest of the world (including Europe)
33.2dB
CCITT (now ITU-T)recommendation
SDR(dB)
input level (power)
approx. 30dB
Speech and Linear Quantisation
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The higher values of the quantisation are rarely used for speech.
SNR is worse for lower signals as quantisation noise is the same for all signal magnitudes.
Non-uniform quantisation can provide fine quantisation of the weak signals and coarse quantisation of the strong signals.
Non-linear quantisation
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♦ 8-bits per sample not sufficient for good speech encoding with uniform quantisation.
♦ Problem lies with setting a suitable quantisation step-size.
♦ One solution is to use non-linear quantisation.
♦ Step-size adjusted according to amplitude of sample.
♦ For larger amplitudes, larger step-sizes used as illustrated next.
♦ ‘Non-linear’ because step-size changes from sample to sample.
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Linear Quantisation and Non-linear Quantisation
0 0
15 15
Non-linear quantisation
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Non linear quantisation uses the logarithmic compression and expansion function. Compress at the transmitter and expand at
the receiver. Compression process changes the distribution of
the signal amplitude. Lower amplitude signals strength to higher
values of quantisation. As the result the compressed speech signal is
now more suitable for linear quantisation. The logarithmic compression and expansion
function is also called Companding.
Implementation of companding (in principle)
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Pass x(t) thro’ compressor to produce y(t). y(t) is quantised uniformly to give y’(t) which is
transmitted or stored digitally. At receiver, y’(t) passed thro’ expander which
reverses effect of compressor. analogue implementation uncommon but shows
concept well.
Com-pressor
Uniform quantiser
Expanderx(t) y(t)
Transmitor store
y’(t) x’(t)
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Companding
F xx A x
A
x A x
AA
x A A x
sgn
ln
sgn ln
ln1
1
11 1 1
and
There are two compading standards: A-law compression (used mainly in Europe) and µ-law compression (used in North America and Japan).
The A-law is given by the mathematical expression:
¨ However, it is not used like this, but as a segmented, piece-wise linear approximation. The segmented A-law uses 13 segments (0, ±1® 7) with A=87.6
¨ 8-bit code consist ofi) polarity bit P (range is ±V)ii) 3 segment decoding bits XYZiii) 4 bits (abcd) specifying intra segment value on a linear scale
-211 +211
+27
-27
m-law A-law
Compression law derivation
Variation of SQNR with amplitude of sample
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48
36
24
12
VV/2V/4V/16 3V/4
A-law
Uniform
Amplitudeof sample
0
SQNR dB
A-law encoding table
Segment Coder input range output code quantum interval
0 0-V/128 P 000 abcd V/20481 V/128-V/64 P 001 abcd V/20482 V/64-V/32 P 010 abcd V/10243 V/32-V/16 P 011 abcd V/5124 V/16-V/8 P 100 abcd V/2565 V/8-V/4 P 101 abcd V/1286 V/4-V/2 P 110 abcd V/647 V/2-V P 111 abcd V/32
P is a polarity bitabcd is a 4-digit intra-segment code
PCM for telephony
8kHz sampling to satisfy Sampling Theorem8 bits per sample with A-law encoding
64kbit/s digital speech signal
Note that the sampling rate of 8kHz leads to a period of 125ms between speech samples
f
v
v
t
f
v
v
t
low-pass anti-aliasing filter
3.4kHz
3.4kHz
Sampling frequency of 8kHz equivalent to a sample every 125ms
Sampling theorem applied to telephony (PCM)
By bandlimiting the incoming speech signal to 3.4kHz and sampling at 8kHz, the sampling theorem is satisfied
8-bit A-law compander
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Delta modulation (DM)
A simpler way than PCM Provides a staircase version of the message signal by
referring to the difference between the input signal and its approximation
Quantization is done using 2 levels:– Positive difference: +– Negative difference: -
Provided that the input signal does not change too rapidly from sample to sample, this approximation works well
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DM illustration
Binary sequence at modulator output
m(t)Input signal
mq(t)Staircase approximation of m(t)
111111010000000000101111110101010
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DM Discrete-time relations
Let– Ts be the sampling period
– e(nTs) be the error signal
– eq(nTs) be the quantised error signal
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DM transmitter
Sampled Inputm(nTs) ∑ Quantiser Encoder
DM data sequence
Delay Ts
∑mq(nTs-Ts)
+
-+
+
Comparator
Accumulator
e(nTs) eq(nTs)
mq(nTs)
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DM quantisation error
Slope overload distortion Granular noise
mq(t)
m(t)
Slope overload distortion
Granular noise
dt
)t(dmmax
Ts
To minimise slope overload distortion
Summary
Sampling process is restricted by Nyquist criterion – Aliasing will occur during undersampling.
The non-linear quantisation is more effective than the linear quantisation at lower quantisation values.
A-Law and µ-Law compression are used in the non-linear quantisation.
Delta Modulation is a simpler way than PCM. DM is efficient technique for signals that changes less rapid. Slope overload distortion at DM
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ATM
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ATM - Main features
Legacy now – but structure and principles appear in modern systems
Connection-oriented transfer mode The cells are much shorter than in a conventional packet
network to get reasonable delay variance. Overhead is minimised to maximise efficiency (e.g. there is
no error correction mechanism for payload) Cells are transported at regular intervals; there is no space
between cells, idle periods on the link carry unassigned cells.
ATM provides cell sequence integrity.
ATM cell stream
cell-based transfer mode this means that information from is transferred as fixed-length cells
unassigned cell
cell from source 1
cell from source 2
cell from source 3
ATM cell stream
Principle of ATM
cell-based transfer mode – this means that information from the source is
transferred as fixed-length cells – no “white space” between cells - if no information
an empty cell is sent instead.
ATM cell structure
48 octet payload
5 octet header
Details are given later
with short blocks of data it takes time to assemble one cell for transmission
Cell assembly delay
payload
ATM header
this is cell assembly delay: for 64kbit/s it is 6ms
Segmentation
Large data packets need to be segmented:
data segment
cell stream
This is done in the ATM Adaptation Layer (AAL) - considered later
ATM relative merits
Advantages of ATM– ease of handling VBR services– inherent multiplexing of the cells with ATM offers
ease of integration of sources onto one link– the network operator only has to provide one
connection (one access link) to the customer and all the services can be provided over this link.
Disadvantages– cell delay variation– cell assembly delay.
Cell delay variation caused by queueing delays
cells from source being tracked
this cell delayed by 1 slot because of queueing delays
expected delay through the network
increased gap reduced gap
B-ISDN protocol reference model
Normal OSI 7-layer model does not apply - separate B-ISDN model
Control Plane User Plane
ATM Layer
Physical Layer
Management plane
Higher LayersHigher Layers
ATM Adaptation Layer
Layer management
Plane management
Original physical layer interfaces (ITU)
Optical or electrical SDH framed or cell-based Bitrates:
– 155.52 Mbit/s upstream and downstream– 622.08 Mbit/s in at least one direction (symmetry
of this interfaces not yet defined)
ATM layer -1
characteristics of ATM layer independent of physical medium.
ATM uses virtual connections for information transport: the virtual path and the virtual channel
All functions of ATM layer are supported by the ATM cell header.– Cell multiplexing and demultiplexing– Cell Virtual Path Identifier (VPI) and Virtual Channel
Identifier (VCI) translation– Cell Header generation/extraction– Generic Flow Control
Cell structure
octet 1
bit 8 bit 1NNI header
1st octet, UNI header
VPI
VPI VCI
VCI
VCI PT
CLP
HEC
5 octet header
48 octet payload
GFC VPI
NNI: network node interfaceUNI: user network interface
VPs and VCs
Physical layerVirtual Path (VP)
Virtual Channel (VC)
Each VP within the physical layer has its own distinct Virtual Path Identifier (VPI); each VC within a VP has its distinct Virtual Channel Identifier (VCI)
VPI & VCIs: specific to a link
input port P
VPIa, VCIb
output port Q
VPIx, VCIy
routeing information in switch
input outputport VPI VCI port VPI VCI....... ...... ...... ....... ..... ...... P a b Q x y....... ...... ...... ....... ..... ......
VC & VP Switching
VC switch
Endpoint of VPC
VP switchVP switch
representation of VC and VP switching
representation of VP switching
Virtual Paths (VPs)
VP generic name for a bundle of VC links, all VC links in the bundle having the same endpoints
Virtual path links concatenated to form Virtual Path Connection (VPC)
VPs provide logical direct routes between switching nodes via intermediate cross-connects.
VP identified by VPI - routeing translation tables in each node provide VPI translation.
VP concept may also be used in access network to provide virtual leased lines, or to allow access to competing operators.
AAL introduction
ATM Adaptation Layer (AAL) performs the necessary mapping between the ATM layer and the next higher layer at the edge of the network.
Functions of AAL depend upon higher layers (ie on services as well)
Examples of service provided by AAL:– handling of quantization effect from cell payload size – handling of transmission errors – handling of lost and mis-inserted cell conditions– flow control and timing control– segmentation and reassembly
AAL - principles of segmentation
user information
AAL
ATM layer
addition of Convergence Sublayer header and trailer protocol information to the user information
Segmentation
addition of Segmentation and Reassembly Sublayer header and trailer protocol information to every segment
payload for the cell
addition of the ATM header
cell
header cell payload
Segmentation without end segment indication
1 2 3 4 5 1 2 3 4 51 2 3 4 5
data units
data segmented into cells
received cells - with one missing because of cell loss
1 2 3 5
2 3 4 52 3 4 5 11 2 3 5 1
1 2 3 4 51 2 3 4 5
reconstructed segments - all in error because of the slip of 1 cell
Segmentation with end segment indication
1 2 3 4 5 1 2 3 4 51 2 3 4 5
data units
data segmented into cells
received cells - with one missing because of cell loss
1 2 3 5 1 2 3 4 51 2 3 4 5
reconstructed segments - only 1 in error because reconstruction starts again after end segment received
end segment indicator
1 2 3 5 1 2 3 4 51 2 3 4 5
this data unit in error these data units are correct
Connection admission control (CAC)
Network decides at call set-up whether to accept a (VP or VC) connection request. Criteria:– sufficient resources (QoS) available for connection
request across network – agreed QOS of existing calls not affected
Call can have more than one connection: CAC procedures should be performed for each
CAC needs the following information:– source traffic characteristics– required QOS class.
Connection admission control (CAC)
CAC uses this information to determine:– whether the connection can be accepted or not– the traffic parameters needed by usage parameter
control– the allocation of network resources.
UPC continued
Functions– checking validity of VCIs and VPIs– checking traffic volume per VPC and VCC to
ensure contract not violated Actions on violation
– Discarding of cells– Dropping connection– Tagging of cells– (Punitive charging)
