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Communication Systems - 3rd
LectureISDN
Chair in Communication Systems
Department of Applied Sciences
University of Freiburg
2010
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Communication SystemsPlan for this lecture
!oday "e "ill tal# about
$SD% layers from &hysical to %et"or# layer
'eference points and device groups
(ard"are specification
)*+21 protocol of the data lin# layer
)*+,1 protocol of the net"or# layer
$SD% call setup
-.ercises on net"or# analysis and pac#et inspection
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Communication SystemsLast lecture
'ecapitulate from last lecture
$SD% is a circuit s"itched net"or#
Circuit path bet"een t"o parties reserved for theduration of the call
'eleased upon disconnect
'emember the S$/$S stac#
$SD% implements several layers
&hysical layer implemented eg in '$ and &'$ Data lin# layer in )*+21
%et"or# layer in )*+,1
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Communication SystemsISDN and the OSI protocol stack (mostly D channel)
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Communication Systemsunction !roups " #eference Points
'eference points defining elements and servicedefinitions
LTfunction and the -! function communicatevia the interface
Devices connecting to the S/T3or S4 interfaceinclude $SD% capable telephones and FA5machines6 video teleconferencinge7uipment6 bridge/routers6 and terminaladapters
Uinterface is a t"o8"ire 3single pair4 interfacefrom the phone s"itch6 the same physicalinterface provided for &!S lines
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Communication Systemsunction !roups " #eference Points
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Communication Systemsunction !roups " #eference Points
;!9 ;ine !ermination
(andles termination of 2 "ire pair at the e.change6operating 21) or ,! line coding
!A9 !erminal Adaptor
-7uipment that supports $SD% call set up andprovides an interface for connecting to non $SD%e7uipment
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Communication Systemsunction !roups " #eference Points
!-19 !erminal -7uipment 1
!-1 is a device that is $SD%8ready*
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Communication SystemsISDN $uildin% &lock
earer channels 3 channels4 and Data channels 3Dchannels4
channels transmit customer bitstreams of any typeli#e video6 voice6 and data
D channel for call8control6 signaling
Channels multiple.ed into a single phyiscal bitstream using time8division multiple.ing 3!D
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Communication SystemsISDN Physical Layer
asic 'ate $nterface
'$ provides a total data rate of 1=0#bit/s
Standard end user connection
2 channels 3>bearer? 8 for data6 digiti@edvoice6 ***4 of =#bit/s each
1 D channel 3data channel for out8of8bandsignaling4 of 1=#bit/s
Synchroni@ation/Stuffing of 1=#bit/s
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Communication Systems'cho cancellation
-cho cancellation is used on '$ circuits becausethere is only a single pair for both transmitting andreceiving information
&'$ circuits do not have this problem because they
run off of either coa.ial cable or t"o coppert"isted pairs6 one pair for transmit and one forreceive
-cho cancellation is designed to remove the signalecho from a circuit so that your e7uipment #no"s
"hat information is being sent to it
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Communication Systems'cho cancellation
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Communication Systems'cho cancellation
!he echo canceller on the right sends a signale7uivalent to a ,v charge6 and the echo cancelleron the left sends a signal e7uivalent to a B1vcharge*
!o figure out "hat is being sent to it6 the echocanceller on the right ta#es the total voltage ofthe circuit 3B1v B ,v 2v4*
ith the total line voltage #no"n6 the echo cancellerthen applies the voltage of "hat it transmitted6 but
in the opposite polarity9 ,v becomes B,v*
!he final e7uation9 2v B B,v B1v* $t is no" #no"nthat a signal of B1v "as meant for its receiver*
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Communication SystemsISDN $asic #ate Interface
&hysical layer specifications of the U#0
perates over t"o8"ire cable up to #m3depending on cable diameter and 7uality4
S"itching center/central office provides a +0
current to po"er the %!A and one device3emergency function to be independent onlocal po"er supply for at least one telephone4
ther physical layer specifications for alternate Uinterfaces
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Communication SystemsISDN $asic #ate Interface
'$ net"or# termination is defined by the U#0interface
A special encoding 3,!4 is used9 bit digital to ,baud ternary
,! is a Ebloc# codeE that uses 'eturn8to8ero
states
Allo"s reduction of symbol rate to 120 #aud 3Gth4and thus distances up to : #m
'eduction of lo" fre7uencies in the signal spectrum
etter detection of code errors
!hree states9 negative pulse6 no pulse6 positive pulse
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Communication SystemsISDN $asic #ate Interface
%e.t state 3S1 8 S4 to be transmitted is indicated incolumn labled Io
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Communication SystemsISDN $asic #ate Interface
Alternate encoding 3e*g* used in the US49 21) 2bit digital to 1 baud 7uaternary representation
21) transmission can be simply described as anamplitude modulation scheme for DC pulses
$nput voltage level can be one of distinct levels
rdering of data bloc#s depends on the encodingused
its )uarternary oltage
00 8, 82*001 81 80*:,
10 B, 2*0
11 B, 0*:,
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Communication SystemsISDN rame ormat
-ach frame 20 bits long For data rate of 1=0 #b/s each frame 1* ms
Frame overhead 8 1= #b/s
D channel 8 1= #b/s
2 channels at = #b/s 8 12: #b/s
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Communication SystemsISDN rame ormat
Sync field consists of + )uaternaries 32 bits each4 inthe pattern B, B, 8, 8, 8, B, 8, B, 8,*
1 B 2 B D is 1: bits of data consisting of : bitsfrom the first channel6 : bits from the second
channel6 and 2 bits of D channel data
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Communication Systemsk &it streams from s*itchin% center to N+$,
-ach frame consists of 120 ternary steps 2JB1JD ta#es 10: steps in ternary bloc#s 3!4
"ith 2H steps each
Sync channel occupies 11 steps and a
>maintenance? channel 3
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Communication Systemsk &it streams from N+$, to s*itchin% center
Connection is full8duple. over the t"o "ires -cho compensation and terminating set is needed
%!A splits the data streams to separate up anddo"n onto the S
0bus
C i i S
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Communication SystemsISDN $asic #ate Interface
$nstead of the traditional "all soc#et 3analogtelephony4 a %!A 3net"or# terminal baseadapter4 is needed at end users site
%!A provides the S0bus to "hich end user devices
are connected Unidirectional on pair of "ires for each direction
Allo"s up to 12 "all soc#ets6 : $SD% devices 3oranalogous devices via a/b converter4
&rovides device po"er up to 6
C i i S
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Communication SystemsISDN S
&rovides the same and D channels as U#0
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Communication SystemsS ,-I code
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Communication SystemsISDN Data Link Layer
'elevant for the control channel %o distinct layering for channels
&C< or data directly put into frames as sho"n onprevious slides
;A&D ;in# Access &rocedure on D channel
Derived from (igh8;evel Data ;in# Control &rotocol3(D;C4
roadcasts only for net"or# termination device
D2 frame margin octet of binary pattern9 01111110
C i ti S t
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Communication SystemsData link layer for the D channel
Keeping of frame se7uence -rror discovery
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Communication SystemsLayer . for the D channel
Flag Character is part of the (eader information6
he.adecimal H-
Address is t"o bytes 3octets4 long6 and consists of
up to five fields Service Access &oint $dentifier 3SA&$4
Command/'esponse 3C/'4 bit
Address -.tension 3-A04 bit indicates "hether final
octet of the address or not !erminal -ndpoint $dentifier 3!-$4
Address -.tension 3-A14 bit6 same as -A0
Communication Systems
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Communication SystemsLayer . for the D channel
Control one or t"o octets 3bytes4 in length6 indicatesone of three frame formats
$nformation
Supervisory
Unnumbered
$nformation carries ;ayer , Call Control 3)*+,14 data
$t may carry Unnumbered Informationdata 3!-$assignment4 or 5$D 3Connection
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Communication SystemsData link layer for the D channel
Service Access &oint $dentifier 3SA&$4 =8bit fieldidentifying the point "here ;ayer 2 provides aservice to ;ayer ,
SAPI Description
0 Call control procedures1 &ac#et
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Communication SystemsData link layer for the D channel
&rotocol handles the !-$ 3!erminal -ndpoint$dentifier4 allocation
All devices on S0using the same bus and have to
be addressable
!-$ assignment started by the connected devicesafter successful initiali@ation of physical layersynchroni@ation
%on automatic assignment uses $D0 =,6automatic = 12=
!here is a special group !-$ 12H
&rotocol elements
$nformationlo"ermost bit is set to 0
Communication Systems
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Communication SystemsData link layer for the D channel
&rotocol elements Receive Ready 8 3014
Set Asyncronous Balance Mode E.tended 83=F/HF4
Unnumbered Information 8 30,4 Disconnect 8 3,/,4
Unnumbered Ac#no"ledgement 3=,/H,4
Flo" control uses se7uence numbers for sending
and receiving
00:E1:04:00:...
ctets M for sending and M for receiving in theinformation frame
Communication Systems
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Communication SystemsData link layer for the D channel error detection
D channel protocol uses rather sophisticated errordetection protocol
Ienerates frame chec#sums
Ienerator polynom
g(x) = (x +1)(x15+x14+x13+x12+x4+x2+x +1)
g(x) = x16+x12+x5+1
1= bit frame chec#sum
$nverted residue of binary division
p1(x) = xk (x15+x14+...+x2+x +1)
p2(x) = x16d(x)
Communication Systems
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Communication SystemsData link layer for the D channel error detection
Chec#ing for added or lost binary @eros !hus cyclic (amming codes implemented
-rror detection for one6 t"o and three bit error
Communication Systems
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Communication SystemsLink 'sta&lishment
;ayer 2 establishment process1* !- 3!erminal -ndpoint4 and the %et"or# initially
e.change 'eceive 'eady 3''4 frames6 listeningfor someone to initiate a connection
2* !- sends an Unnumbered $nformation 3U$4 frame"ith a SA&$ of =, 3management procedure6 7uerynet"or#4 and !-$ of 12H 3broadcast4
,* %et"or# assigns an available !-$ 3in the range =812=4
Communication Systems
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Communication SystemsLink 'sta&lishment
;ayer 2 establishment process 3cont*4*!- sends a Set Asynchronous alanced
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Communication SystemsISDN Net*ork Layer
$SD% %et"or# ;ayer specified by the $!U )8seriesdocuments )*+,0 L +,+
;ayer , used for the establishment6 maintenance6 andtermination of logical net"or# connections bet"een t"odevices
Communication Systems
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Communication SystemsNet*ork layer for the D channel
DSS1 protocol handles the call setup of the callingand called site
Call destruction after finishing the session
'estarting and par#ing if re7uired
-rror handling
Communication Systems
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11/0/10 ,+
yDSS/ layer 0 protocol
&rotocol Discriminator &art of the ;ayer , header information
Single byte 3octet4 that is usually set to a value of00001000 3he.adecimal E0:E4 8 meaning )*+,1 callmaintenance
'eference alue consists of either t"o or three bytes3octets4
'$ systems have a H8bit Call 'eference value 312Hreferences4
%o particular end8to8end significance
-ither end can assign an arbitrary value
Used to associate messages "ith a particularlychannelconnection
Communication Systems
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11/0/10 0
yDSS/ layer 0 protocol
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yDSS/ layer 0 protocol messa%e types
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yDSS/ layer 0 protocol information elements
-ach type of message has
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yDSS/ layer 0 protocol information elements
%et"or# Specific Facilities 3Useful for %orthAmerican &'$ calls 8 identifies net"or# type6Carrier $D6 Carrier Service!ypeNA!S/SD%/ASDS6etc*O4
Calling &arty %umber 3caller $D4
Calling &arty %umber sub8address
Called &arty %umber 3destination number6 type ofnumberNun#no"nO6 numbering plan4
Called &arty %umber sub8address
hen $nformation -lements consist of multipleoctets6 the follo"ing octet describes ho" manybytes 3octets4 are in the $nformation -lement
Communication Systems
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Layer 0 Call Setup
Follo"ing steps setup an $SD% call1*Caller sends a S-!U& to the S"itch*
2* $f the S-!U& is K6 the s"itch sends a CA;;&'Ceeding to the Caller6 and then a S-!U& tothe 'eceiver*
,*'eceiver gets the S-!U&* $f K6 then rings thephone and sends an A;-'!$%I message to theS"itch
*S"itch for"ards the A;-'!$%I message to the
Caller
Communication Systems
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Layer 0 Call Setup
Follo"ing steps setup an $SD% call 3cont*4*hen the receiver ans"ers the call6 is sends a
C%%-C! message to the S"itch
=*S"itch for"ards the C%%-C! message to theCaller
H*Caller sends a C%%-C! ACKno"ledge messageto the S"itch
:*S"itch for"ards the C%%-C! ACK message tothe 'eceiver
+*All DoneP connection no" up
Communication Systems
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Practical '1ercise ISDN
y no" completely prepared for $SD% &hysical to%et"or# analysis
;oo# into the pac#ets
'elate actions to pac#ets on the $SD% bus
$dentify different layers ;imited hard"are again
!hus the class is split again Qust changing the t"oe.ercises 32 R ,4 from last Friday
&lease re8seat appropriately
Communication SystemsP i l ' i ISDN
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Practical '1ercise ISDN
ou see the typical users premises installations3basic rate interface '$4 "ith UK0
and S0
connectors in operation
S0
bus can connect up to eight devices
e careful "ith bus setup 3"iring4
-very device >sees? the complete traffic
As# your neighbors for advice 3they might havedealt "ith the tas# already last time4