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1© 2002, Cisco Systems, Inc. All rights reserved.Voice Quality
Introduction to TelephonyPart II
Digital Signaling
By:
Michael Whitley (mwhitley)
Gonzalo Salgueiro (gsalguei)
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2© 2002, Cisco Systems, Inc. All rights reserved.Voice Quality
Digital Telephony
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333© 2002, Cisco Systems, Inc. All rights reserved.
• No signal attenuation on digital circuits.
• No echo generated on digital circuits.
• Discrete values (i.e. binary 0/1) do not suffer signal degradation andthus can operate in low SNR environments.
• Repeaters, on digital circuits, regenerate perfect pulses whereasamplifiers, on analog circuits, boost signal strength and noise levels.
• Easy to switch and manipulate signal with digital components.
• Built in error checking routines into signal/frame (i.e. CRC).
• Higher speeds
• Greater call density (TDM based)
• Statistics collection (FDL)
Benefits of Digital
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444© 2002, Cisco Systems, Inc. All rights reserved.
•Assumption is that human speech information is
contained in the range of 300-3400 Hz
•Filter & use signal below 4 kHz to prevent aliasing
Digitizing Voice
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555© 2002, Cisco Systems, Inc. All rights reserved.
= Sample
8 kHz (8,000 Samples/Sec)
Codec Technique
Sampling StageAnalog Audio Source
Pulse Code Modulation - Nyquist Theorem
Voice Bandwidth =
300 Hz to 3400 Hz
The sampling frequency must be at least 2 times the largest
frequency being sampled for signal replication.
Digital Telephony
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666© 2002, Cisco Systems, Inc. All rights reserved.
Stage 1
Quantizing Stage
Quantizing Noise
100100111011001
A—Law (Europe)
µ—Law (USA –Japan)
Pulse Code ModulationAnalog to Digital Conversion
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Non- Linear Encoding
Closely Follows HumanVoice Characteristics
High Amplitude Signals HaveMore Quantization Distortion
(Both a- & - Law)
Input
Output
Linear Encoding
Relatively Easy to Analyze,Synthesize, and Regenerate
All Amplitudes Have Roughly
Equal Quantization Distortion
Input
Output
Non-Linear vs. Linear Encoding
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Companding (a-law vs. µ-law)
•ITU defines two types of PCM (G.711) algorithms:
µ-law
a-law
•Both operate at 8-bits per sample and use values between 0 and +/- 127 to
represent signal amplitude
•Both operate on a logarithmic scale
•Maximum level for µ-law is +3.17 dBM0
•Maximum level for a-law is +3.14 dBM0
•For louder input both algorithms operate similarly
•For softer input the added granularity of µ-law results in slightly improved
signal to quantizing distortion ratio
•µ-law has a slightly higher overall quantizing noise since it does not have a
digital representation for silence (i.e. all zeroes)
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• Bit Rate Of Digital Voice
= 2 x 4 kHz x 8 bits per sample
= 64,000 bits per second
CODEC
PCM
64 KBPS= DS-0
Digitizing Voice
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DS-1
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DS-1 Basics
TS1 TS3TS2 TS24TS23
8 bits * 24 timeslots = 192 bits + F bit=193bits/frame
193 bits/frame * 8000fps = 1544000bps
DS-1 Frame Format
F
Each Timeslot:
sampled at 8000 times per second ~ 2 times voice spectrum8000 samples/sec * 8 bits/sample = 64000 bps
24 DS0‘s make up a DS1 frame
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Time Division Multiplexing (TDM)
•The incoming DS0 is encoded and ready for
transmission now
•DS0‘s are bundled into DS1‘s (T1‘s) for delivery acrossthe network. T1 Framing used to accom pl ish this .
•Synchronous TDM is used to extract the DS0‘s from
the DS1‘s using a clock source for reference.
DS-0
1st Cycle2nd CycleDS-1
DS-2
DS-3
DS-4
DS-5
DS-6
6 5 4 3 2 1 06 5 4 3 2 1 0
TimeSlots ―n‖
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DS-1 Linecoding
AMI - Alternate Mark Inversion
B8ZS - Bipolar 8-Zero Substitution
8 successive 0‘s are replaced by: 000VB0VB where,
V is a bipolar violation and
B is a mark adhering to the AMI rule
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ESF Frame
TS1 TS2 … TS6 … TS12 … TS18 … TS24 F Bit
Frame1 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame2 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame3 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame4 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 0
Frame5 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame6 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame7 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame8 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 0
Frame9 12345678 12345678 …
12345678 …
12345678 …
12345678 …
12345678 MFrame10 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame11 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame12 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 1
Frame13 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame14 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame15 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame16 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 0
Frame17 12345678 12345678 …
12345678 …
12345678 …
12345678 …
12345678 MFrame18 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame19 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame20 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 1
Frame21 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame22 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame23 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame24 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 1
M - FDL C - CRC 001011 - Terminal Pattern
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DS-1 SF/ESF Frame Format
Frame
Number Term in al S ig n al li ng 7 6 5 4 3 2 1 0
1 1 d d d d d d d d
2 0 d d d d d d d d
3 0 d d d d d d d d
4 0 d d d d d d d d
5 1 d d d d d d d d
6 1 d d d d d d d A
7 0 d d d d d d d d
8 1 d d d d d d d d
9 1 d d d d d d d d
10 1 d d d d d d d d
11 0 d d d d d d d d
12 0 d d d d d d d B
Fram ing b i t Bi t in Tim eslo t Frame
Num ber Term CRC FDL 7 6 5 4 3 2 1 0
1 M d d d d d d d d
2 C1 d d d d d d d d
3 M d d d d d d d d
4 0 d d d d d d d d
5 M d d d d d d d d
6 C2 d d d d d d d A7 M d d d d d d d d
8 0 d d d d d d d d
9 M d d d d d d d d
10 C3 d d d d d d d d
11 M d d d d d d d d
12 1 d d d d d d d B
13 M d d d d d d d d
14 C4 d d d d d d d d
15 M d d d d d d d d
16 0 d d d d d d d d
17 M d d d d d d d d
18 C5 d d d d d d d C
19 M d d d d d d d d
20 1 d d d d d d d d
21 M d d d d d d d d
22 C6 d d d d d d d d
23 M d d d d d d d d24 1 d d d d d d d D
Bi t in Tim eslot Fram ing b i t
SF ESF
• SF: terminal pattern of alternating 0/1‘s in every
other frame
• ESF: terminal pattern of 001011 pattern over
every 4th frame
• SF: signaling done in LSB of frames 6 & 12
• ESF signaling done in LSB of frames 6,12,18,24
• ESF: contains 2k CRC and 4k FDL
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Debugging T1
3660a#show controller t1 2/0
T1 2/0 is down.
Applique type is Channelized T1
Cablelength is long gain36 0db
Transmitter is sending remote alarm.
Receiver has loss of signal.
Version info Firmware: 19990702, FPGA: 6
Framing is ESF, Line Code is B8ZS, Clock Source is Line.
Data in current interval (774 seconds elapsed):0 Line Code Violations, 0 Path Code Violations
0 Slip Secs, 774 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 774 Unavail Secs
Data in Interval 1:
0 Line Code Violations, 0 Path Code Violations
0 Slip Secs, 900 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 900 Unavail Secs
Receive states are:
Loss of signal - controller doesn‘t see marks on the T1.
Loss of frame - controller sees T1 marks, but can‘t frame up with the F-bit.
Yellow alarm - controller sees a Yellow Alarm on its Rx path due to remote transmitter losing its Rx.
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Debugging T1
• For Framing SF & Linecode AMI use:
channel-group 1 timeslots 1-24 speed 56
•When debugging layer 1 T1 problems, use a loopback test to verify routerhardware
•When running a loopback to the telco use pings with varying data patterns
to ensure proper linecoding (sample patterns: all 1‘s, all 0‘s, etc.)
controller T1 1/0/1
framing esf
linecode b8zs
loop local
channel-group 1 timeslots 1-24
!
interface Serial1/0/1:1
ip address 1.1.1.1 255.255.255.0
• AMI doesn’t support Clear Channel 64K DS0.
DSUCSUTelco
DTEDCE
DTEDCE
DSUCSU
Local
Loopback
Remote
Loopback
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• In-band/Out-of-band
In-band refers to signaling which uses the same frequencyrange as a voice call - ie DTMF tones, busy signal, ringback
Out-of-band uses a different frequency range that voice call - ieloop current signaling, ringing voltage
• In-channel/Out-of-channel
In-channel refers to signaling information using the samechannel as the voice/data – i.e. RBS
Out-of-channel uses a different channel than the voice/data –
i.e. D-channel in ISDN Channel Associated Signaling (CAS) is a form of in-channel,
in-band signaling
Common Channel Signaling (CCS) is a form of out-of-channel,in-band signaling
DS-1 Signaling Types
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202020© 2002, Cisco Systems, Inc. All rights reserved.
Signaling in DS-1
• Common Channel Signaling (CCS)Instead of robbed bit, robbed ds0
Example: ISDN
• Channel Associated Signaling (CAS)Example: Robbed bit Signaling (RBS)
Frame #1
DS-0 #24 always provides signaling
information for DS-0 #1 through #23.
F DS-0 #1 DS-0 #2 DS-0 #3 DS-0 #4 DS-0 #24.......
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21© 2002, Cisco Systems, Inc. All rights reserved.Voice Quality
ISDN
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222222© 2002, Cisco Systems, Inc. All rights reserved.
ISDN Protocol Architecture
LAP-D
(Q921)
Physical
Q.931
Physical
Voice
D- Channel
Layer-1
Layer-2
Layer-3
B- Channel
For Signaling For Data Flow
WANB1-
B2-
D-
D-channel - Signaling
q921, q931
D-channel - Signaling
q921, q931
B1-
B2-
D-
B-channels - Voice
• Separate channels for signaling
and data (referred to as out-of-
band)
• ISDN allows integration ofvoice, video and data over a
single media
B-channels - VoicePacketized Voice
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232323© 2002, Cisco Systems, Inc. All rights reserved.
BRI
• 192 kbps total speed• 2 B-channels at 64 kbps
• 1 D-channel at 16 kbps
• 48 kbps framing and synchronization
• Requires an NT1 device
• Point-to-multipoint
64 Kbps
64 Kbps
16 Kbps
64K*2 + 16k
+ 48K (framing bits) =
192 Kbps
2B
D }{
BRI
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242424© 2002, Cisco Systems, Inc. All rights reserved.
PRI
• 1.544 kbps total speed
• 23 B-channels at 64 kbps
• 1 D-channel at 64 kbps (channel 24)
• 8 kbps framing and synchronization
• ESF/B8ZS
• Point to point
64K*24 + 8K (framing bits)
=1.544 Mbps (T1)23B
D
64 Kbps
64 Kbps}
PRI
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252525© 2002, Cisco Systems, Inc. All rights reserved.
ISDN Standards
• Physical Layer (1)BRI - ITU-T I.430
PRI - ITU-T 1.431• Data Link Layer (2)
ITU-T Q.921
• Network Layer (3)ITU-T Q.931
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262626© 2002, Cisco Systems, Inc. All rights reserved.
Q.921
• Q.921 protocol is used between the TE and the local ISDN switch
(not end-to-end)
• The ISDN network does not impose the use of any data link layer protocol
for the B channels
• Full duplex protocol
• Responsible for sending/receiving error-free data
• Q.921 LAPD (Link Access Procedure on the D channel) is modeled after
HDLC and LAPB (X.25).
Switch SwitchISDN PRIISDN BRI
WAN
Q.921 Q.921
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272727© 2002, Cisco Systems, Inc. All rights reserved.
Q.931
• Q.931 protocol is used between the TE and the local ISDN switch
(not end-to-end)
• The ISDN network does not impose the use of any network layer protocol
for the B channels
• Full duplex protocol
• Message based protocol responsible for call setup and call teardown
• Q.931 signaling is encapsulated in Q.921 frames
Switch SwitchISDN PRIISDN BRI
WAN
Q.931 Q.931
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28© 2002, Cisco Systems, Inc. All rights reserved.Voice Quality
Debugging ISDN
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292929© 2002, Cisco Systems, Inc. All rights reserved.
Troubleshooting ISDN
vnt-3660-33a#show isdn status
Global ISDN Switchtype = primary-ni
ISDN Serial2/0:23 interface
dsl 0, interface ISDN Switchtype = primary-ni
Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 0, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
Layer 3 Status:
1 Active Layer 3 Call(s)
CCB:callid=1F, sapi=0, ces=0, B-chan=23, calltype=VOICE
Active dsl 0 CCBs = 1
The Free Channel Mask: 0x803FFFFF
Number of L2 Discards = 0, L2 Session ID = 3
Total Allocated ISDN CCBs = 1
P h y s
i c a l P
h y si c al
D a t aL i nk
N e t w or k
I.430 (BRI) –I.431 (PRI)
LAPD –Q.921
Q.931
D-ChannelB-Channel
Voice
Samples
Indicates the ISDN interface and Telco switch are have
successfully negotiated Q.921 (ISDN L2) parameters
―ACTIVE‖ indicates that thephysical wiring looks good
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303030© 2002, Cisco Systems, Inc. All rights reserved.
Troubleshooting ISDN Layer 2
ISDN Layer 2 establishment:vnt-3745-32a#debug isdn q921
debug isdn q921 is ON.
vnt-3745-32a#
Feb 6 22:03:42.049: ISDN Se1/0:23 Q921: User TX -> SABMEp sapi=0 tei=0
Feb 6 22:03:43.048: ISDN Se1/0:23 Q921: User TX -> SABMEp sapi=0 tei=0
Feb 6 22:03:43.056: ISDN Se1/0:23 Q921: User RX <- UAf sapi=0 tei=0
Feb 6 22:03:53.042: ISDN Se1/0:23 Q921: User RX <- RRp sapi=0 tei=0 nr=0
Feb 6 22:03:53.042: ISDN Se1/0:23 Q921: User TX -> RRf sapi=0 tei=0 nr=0
Feb 6 22:04:03.035: ISDN Se1/0:23 Q921: User RX <- RRp sapi=0 tei=0 nr=0Feb 6 22:04:03.039: ISDN Se1/0:23 Q921: User TX -> RRf sapi=0 tei=0 nr=0
TE NET
Q.921
TEI and SAPI belong to the Address field of the datalink Q.921 frame.
TEI: Terminal Endpoint Identifier : Identifies a terminal
TEI = 0-63 are used for fixed TEIs
TEI = 64 to 126 are reserved for assignment at activation
TEI = 127 is reserved for bradcasting
SAPI: Service Access Point Identifier : Defines the message type
SAPI = 0 is used for Q931 signaling information
SAPI = 16 is used for X.25 on the D-channel
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Debugging Q931 – Call Setup
SwitchSwitc
h
Outgoing
Call Made
Call Accepted
ISDN PRIISDN PRI WAN
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Debugging Q931 Call Setup
Layer 3 call establishment for a DID call:
Feb 7 15:10:01.544: ISDN Se2/0:23 Q931: RX <- SETUP pd = 8 callref = 0x000D
Bearer Capability i = 0x8090A2
Standard = CCITT
Transfer Capability = Speech
Transfer Mode = Circuit
Transfer Rate = 64 kbit/s
Channel ID i = 0xA18397
Preferred, Channel 23Calling Party Number i = 0x21, 0x81, '9199915622'
Plan:ISDN, Type:National
Called Party Number i = 0xC1, '5552104'
Plan:ISDN, Type:Subscriber(local)
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333333© 2002, Cisco Systems, Inc. All rights reserved.
Debugging Q931 Call Setup
Feb 7 15:10:01.548: ISDN Se2/0:23 Q931: TX -> CALL_PROC pd = 8 callref =0x800D
Channel ID i = 0xA98397
Exclusive, Channel 23
Feb 7 15:10:02.152: ISDN Se2/0:23 Q931: TX -> ALERTING pd = 8 callref = 0x800D
Progress Ind i = 0x8288 - In-band info or appropriate now available
Feb 7 15:10:16.205: ISDN Se2/0:23 Q931: TX -> CONNECT pd = 8 callref = 0x800D
Feb 7 15:10:16.221: ISDN Se2/0:23 Q931: RX <- CONNECT_ACK pd = 8 callref = 0x000D
TE NET
Q.931
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343434© 2002, Cisco Systems, Inc. All rights reserved.
Debugging Q931 Call Disconnect
ISDN PRIISDN PRIWAN
Switc
h
Switc
h
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353535© 2002, Cisco Systems, Inc. All rights reserved.
Debugging Q931 Call Tear Down
Layer 3 call tear down :
Feb 7 15:11:15.339: ISDN Se2/0:23 Q931: RX <- DISCONNECT pd = 8 callref =0x000D
Cause i = 0x8290 - Normal call clearing
Feb 7 15:11:15.339: ISDN Se2/0:23 Q931: TX -> RELEASE pd = 8 callref = 0x800D
Feb 7 15:11:15.359: ISDN Se2/0:23 Q931: RX <- RELEASE_COMP pd = 8 callref =0x000D
Troubleshooting Tips:
Always be aware of message direction to know who placed/cleared a call
Most disconnect cause codes are self explanatory (so don‘t try to make them difficult!)
Use ―show isdn service‖ to determine available B-channels
Find out which part of the call establishment is failing, and determine if the problem
seems to be in the ISDN negotiation or in the VoIP negotiation.
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36© 2002, Cisco Systems, Inc. All rights reserved.Voice Quality
Channel Associated Signaling
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CAS Signaling on T1
• Channel Associated Signaling (CAS) robs the least significant bitfrom the voice channel to convey supervisory signaling
• These robbed bits are known as A/B/C/D bits and they are in everysixth frame of a DS-1
Super Frame (SF) - A/B
Extended Super Frame (ESF) - A/B/C/D
• Throughput is capped at 56k for RBS circuits since 1 of every 8 bitsis robbed from every DS0
• Robbing the LSB for voice has no noticeable effect, whereas all bitscarry equal weight in the case of data
• Common CAS signaling types used in digital RBS are: fxs/fxo loop start
fxs/fxo ground start
E&M (immediate, wink-start, fgd, delay)
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Digital Signaling Schemes
Bit Frame
A 6th
B 12th
C 18th
D 24thAddress Signaling
(Dial Pulse)
DS-1 Frame
Supervision
On/Off Hook
Audio
Address Signaling
(DTMF)
Framing
Bit
LSB
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393939© 2002, Cisco Systems, Inc. All rights reserved.
ESF RBS FrameTS1 TS2 … TS6 … TS12 … TS18 … TS24 F Bit
Frame1 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame2 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame3 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame4 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 0
Frame5 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame6 1234567A 1234567A … 1234567A … 1234567A … 1234567A … 1234567A C
Frame7 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame8 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 0
Frame9 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame10 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame11 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame12 1234567B 1234567B … 1234567B … 1234567B … 1234567B … 1234567B 1
Frame13 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame14 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame15 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame16 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 0
Frame17 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame18 1234567C 1234567C … 1234567C … 1234567C … 1234567C … 1234567C C
Frame19 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame20 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 1
Frame21 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame22 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 C
Frame23 12345678 12345678 … 12345678 … 12345678 … 12345678 … 12345678 M
Frame24 1234567D 1234567D … 1234567D … 1234567D … 1234567D … 1234567D 1
M - FDL C - CRC 001011 - Terminal Pattern ABCD - RBS Bits
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404040© 2002, Cisco Systems, Inc. All rights reserved.
Standard (E&M) Trunk Type A/B Bits
Direction State A B C D
Txmit On Hook 0 0 0 0
Txmit Off Hook 1 1 1 1
Receive On Hook 0 0 0 0
Receive Off Hook 1 1 1 1
E&M Signaling
• Simplest of the digital signaling protocols
• Provides both disconnect and answer supervision aswell as glare avoidance
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414141© 2002, Cisco Systems, Inc. All rights reserved.
E&M Signaling
There are 4 types of E&M Signaling that are supported onCisco routers:
wink-start (fgb) – wink is used to notify the remote side it can send theDNIS.
wink-start with wink-acknowledge (fgd) – similar to wink, except a
second wink is used to acknowledge the receipt of DNIS. Sending ANIafter the wink-ack is also an option.
immediate start – off-hook is ―immediately‖ followed by DNIS. Nowinks are involved.
delay-start – similar to wink-start. Wink is variable length and should
remain active until receiver is ready to accept DNIS.
For a more detailed explanation of each of these signaling protocols,refer to Part I - Analog Telephony.
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E&M Signaling
Note: Disconnect (Router Network)
is also symmetrical.
1
2
3
4
5 7
6A=1 B=1
A=1 B=1
A=0 B=0
A=0 B=0
Incoming Call (Network Router): 1) Network goes off-hook. A-bit = 1, B-bit =1
2) Router sends wink. A-bit =1, B-bit =1 for 200ms. This only occurs when using wink-start or wink-start with wink acknowledgement. Ignore this step for immediate start.
3) Network sends DNIS information (typically in-band dtmf tones).
4) Router sends a wink acknowledgement. A-bit =1, B-bit = 1 for 200ms. This only occurs for wink-startwith wink acknowledgement. Ignore this step for immediate start or wink-start.
5) Router goes off-hook when call is answered. A-bit =1, B-bit =1.
Outgoing Call (Router Network): Same procedure as above except the ‗Network' describedabove would be the ‗Router' and vice-versa. This is due to the fact that the signaling is symmetric.
Call Disconnect (Network Router):
6) Network goes 'on-hook'. A-bit = 0, B-bit = 0
7) CPE will go 'on-hook'. A-bit = 0, B-bit = 0
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FXS/FXO Loop Start Signaling
• Loop start signaling was originally designed as astation side signaling protocol (i.e. channel bank) butcan be used as a trunk signaling protocol.
• Loop start is not widely deployed as a digital signalingprotocol.
• In loop start signaling the FXS side only uses the B bitand the FXO side only uses the A bit to communicatecall information.
• A big disadvantage of loop start signaling is the lack ofdisconnect supervision and answer supervision due tono defined state change from FXS to FXO.
• Another limitation of loop start signaling is that itprovides no incoming call channel seizure. Thereforea glare situation could arise where both parties (FXO &FXS) try to simultaneously place calls.
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FXS/FXO Loop Start State Table
Note: State Table
is from FXO‘s
Perspective.
Disconnect
Supervision is
done with A bit
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FXS/FXO Loop Start Signaling
Incoming Call (Network Router):
1) Network toggles B-bit to indicate ringing. Standard U.S. ringing cadence (i.e. 2sec on, 4sec off)
2) Router will detect the 'ringing' and go 'off-hook'. A-bit goes from 0 to 1.
Outgoing Call (Router
Network):
3) Router will go 'off-hook'. A-bit goes from 0 to 1.
4) Network will provide dial-tone. No signaling change.
5) Router will send digits (typically in-band dtmf tones).
Disconnect (Network Router):
6) Router detects in-band that the call has dropped (i.e. supervisory tones).
7) Router will go 'on-hook'. A-bit goes from 1 to 0.Disconnect (Router
Network):only step 7 will occur
1
2 7
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• Very similar to loop start signaling
• Inherent deficiencies in loop start were resolved
• Advantages over loop start include:
Glare prevention via channel seizure instead of only ringing
Answer & disconnect supervision via state change when FXStransitions between off-hook & on-hook
Better trunk signaling method, yet still makes station provisioningsimple
• Some disadvantages are:Not widely deployed
More complex state machines
FXS/FXO Ground Start Signaling
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FXS/FXO GroundStart State Table
Note: State Table
is from FXO‘s
perspective
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FXS/FXO Ground Start Signaling
Incoming call (Network Router):1) Network goes off-hook. A-bit transitions from 1 to 0. ―Ringing‖ also occurs by B-bit
toggling between 0 and 1 (0=ringing, 1=no ringing).
2) Router detects ringing AND seizure and goes ‗off -hook‗ via A-bit = 1.
3) Network stops ―ringing‖. B-bit is now 1. A-bit still equals 0.
Outgoing call (Router Network):4) Router transmits ‗ground on ring‘ by setting B-bit to 0. A-bit already 0.
5) Network goes ‗off -hook‘. A-bit goes from 1 to 0. B-bit is set to 16) Router goes ‗off -hook‘. A-bit & B-bit transition from 0 to 1.
7) Router detects in-band dial tone and typically sends in-band DTMF digits.
Disconnect (Network
Router):8) Network goes ‗on-hook‘. A-bit transitions from 0 to 1.
9) Router goes ‗on-hook‘. A-bit transitions from 1 to 0.
Note:
Disconnect (Router Network)
Steps 8&9 above will be reversed
1
2
3
4
5
6
8
9
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49© 2002, Cisco Systems, Inc. All rights reserved.Voice Quality
Debugging CAS
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Debugging Topology
5400 PBX 3745
T1 E&M
Wink Start
PBX
T1 E&M
Wink Start
IP
Setup Direction
Disconnect Direction
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Debugging CAS
Debug output on an AS5400 for an inbound CAS wink-start call:
/* Inbound cal l comes in to the rou ter (i .e. network goes o ff -h ook) */
*May 27 23:44:21.979: from Trunk(7): (2/0): Rx LOOP_CLOSURE (ABCD=1111)
/* 200 msec w ink sen t by the rou ter */
*May 27 23:44:22.199: from Trunk(7): (2/0): Tx LOOP_CLOSURE (ABCD=1111)*May 27 23:44:22.399: from Trunk(7): (2/0): Tx LOOP_OPEN (ABCD=0000)
/* Router rec eives DNIS d ig its an d then goes o ff -h ook once cal l is answer ed */
*May 27 23:44:30.375: from Trunk(7): (2/0): Tx LOOP_CLOSURE (ABCD=1111)
/* Disconnec t received by the rou ter (i .e. call was connected fo r approx . 121 seconds) */
*May 27 23:46:31.443: from Trunk(7): (2/0): Rx LOOP_OPEN (ABCD=0000)
/* Router d isconnec ts call from its s ide */
*May 27 23:46:31.759: from Trunk(7): (2/0): Tx LOOP_OPEN (ABCD=0000)
On AS5xxx platforms use the ―debug cas‖ command to
troubleshoot CAS signaling issues.
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525252© 2002, Cisco Systems, Inc. All rights reserved.
Debugging CAS
T1 7/2 is up:Loopback: NONEDS0 Type Modem <-> Service Channel Rx Tx
State State A B C D A B C D---------------------------------------------------------------------------------------------------------1 cas - - insvc idle 0 0 0 0 0 0 0 02 cas-voice - - insvc connected 1 1 1 1 1 1 1 13 cas - - insvc idle 0 0 0 0 0 0 0 0
4 cas - - insvc idle 0 0 0 0 0 0 0 05 cas - - insvc idle 0 0 0 0 0 0 0 06 cas - - insvc idle 0 0 0 0 0 0 0 07 cas - - insvc idle 0 0 0 0 0 0 0 08 cas - - insvc idle 0 0 0 0 0 0 0 09 cas - - insvc idle 0 0 0 0 0 0 0 010 cas - - insvc idle 0 0 0 0 0 0 0 011 cas - - insvc idle 0 0 0 0 0 0 0 012 cas - - insvc idle 0 0 0 0 0 0 0 013 cas - - insvc idle 0 0 0 0 0 0 0 0
14 cas - - insvc idle 0 0 0 0 0 0 0 015 cas - - insvc idle 0 0 0 0 0 0 0 016 cas - - insvc idle 0 0 0 0 0 0 0 017 cas - - insvc idle 0 0 0 0 0 0 0 018 cas - - insvc idle 0 0 0 0 0 0 0 019 cas - - insvc idle 0 0 0 0 0 0 0 020 cas - - insvc idle 0 0 0 0 0 0 0 021 cas - - insvc idle 0 0 0 0 0 0 0 022 cas - - insvc idle 0 0 0 0 0 0 0 023 cas - - insvc idle 0 0 0 0 0 0 0 0
24 cas - - insvc idle 0 0 0 0 0 0 0 0
vnt-5400-37a#show controller t1 7/2 timeslots 1-24
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Debugging CAS
*Apr 23 17:04:35.807: htsp_timer_stop3 htsp_setup_req
*Apr 23 17:04:35.807: htsp_process_event: [1/1:0(1), EM_ONHOOK,
E_HTSP_SETUP_REQ]em_onhook_setup
*Apr 23 17:04:35.807: em_offhook (0)vnm_dsp_set_sig_state:
[recEive and transMit1/1:0(1)] set signal state = 0x8
*Apr 23 17:04:35.807: htsp_process_event: [1/1:0(1),
EM_BRANCH, EM_EVENT_WINK]
*Apr 23 17:04:35.807: em_start_timer: 550 ms
*Apr 23 17:04:35.811: htsp_timer - 550 msec
*Apr 23 17:04:36.059: htsp_process_event: [1/1:0(1),
EM_WAIT_WINKUP, E_DSP_SIG_1100]em_wink_offhook
*Apr 23 17:04:36.059: em_stop_timers
*Apr 23 17:04:36.059: htsp_timer_stop*Apr 23 17:04:36.059: em_start_timer: 1200 ms
*Apr 23 17:04:36.059: htsp_timer - 1200 msec
*Apr 23 17:04:36.259: htsp_process_event: [1/1:0(1),
EM_WAIT_WINKDOWN, E_DSP_SIG_0000]em_wink_onhook
*Apr 23 17:04:36.259: em_stop_timers
*Apr 23 17:04:36.259: htsp_timer_stop htsp_wink_ind
On 26xx/36xx/37xx platforms use the ―debug vpm signaling‖
command to troubleshoot CAS signaling issues.
/* Router goes off-hook */
/* Router receives off-hook part
of wink*/
/* Router receives on-hook part
of wink- duration is 200ms*/
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Debugging CAS
*Apr 23 17:04:36.259: htsp_timer - 70 msec
*Apr 23 17:04:36.331: htsp_process_event: [1/1:0(1),
EM_WAIT_DIALOUT_DELAY, E_HTSP_EVENT_TIMER]
em_imm_send_digits em_send_digits htsp_dial
*Apr 23 17:04:36.943: htsp_process_event: [1/1:0(1), EM_WAIT_DIAL_DONE,
E_DSP_DIALING_DONE]em_offhook_digit_done
*Apr 23 17:04:36.943: htsp_timer_stop2 htsp_progress
*Apr 23 17:04:36.943: htsp_timer2 - 850 msec*Apr 23 17:04:36.943: htsp_process_event: [1/1:0(1),
EM_WAIT_FOR_ANSWER,E_HTSP_VOICE_CUT_THROUGH]
*Apr 23 17:04:36.943: htsp_timer_stop2
*Apr 23 17:04:40.263: htsp_process_event: [1/1:0(1),EM_WAIT_FOR_ANSWER,
E_DSP_SIG_1100]em_wait_answer_offhook
*Apr 23 17:04:40.263: em_stop_timers
*Apr 23 17:04:40.263: htsp_timer_stop
*Apr 23 17:04:40.263: htsp_timer_stop2
/* Router transmits DNIS digits. Not seen
via ―debug vpm signal‖. View outbound
digits with ―debug vtsp session‖ */
/* Finished sending digits */
/* Router sees off-hook
(answer supervision) */
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Debugging CAS
*Apr 23 17:04:43.571: htsp_digit_ready: digit = 31*Apr 23 17:04:43.571: htsp_process_event: [1/1:0(1),
EM_CONNECT, E_VTSP_DIGIT]
*Apr 23 17:04:43.775: htsp_digit_ready: digit = 32
*Apr 23 17:04:43.775: htsp_process_event: [1/1:0(1),
EM_CONNECT, E_VTSP_DIGIT]
*Apr 23 17:04:43.987: htsp_digit_ready: digit = 33
*Apr 23 17:04:43.987: htsp_process_event: [1/1:0(1),
EM_CONNECT, E_VTSP_DIGIT]
*Apr 23 17:04:44.187: htsp_digit_ready: digit = 34
*Apr 23 17:04:44.187: htsp_process_event: [1/1:0(1),
EM_CONNECT, E_VTSP_DIGIT]
*Apr 23 17:06:41.711: htsp_timer_stop3
*Apr 23 17:06:41.711: htsp_process_event: [1/1:0(1),
EM_CONNECT, E_HTSP_RELEASE_REQ]em_conn_release
*Apr 23 17:06:41.711: htsp_timer_stop*Apr 23 17:06:41.711: htsp_timer_stop2
em_onhook(0)vnm_dsp_set_sig_state:
[recEive and transMit1/1:0(1)] set signal state = 0x0
*Apr 23 17:06:41.711: em_start_timer: 400 ms
*Apr 23 17:06:41.711: htsp_timer - 400 msec
/* User dials dtmf 1234
once call is connected.
Router received 1234 from
T1 interface.*/
/* Router disconnects call.
Duration is approx 121 sec*/
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Debugging CAS
*Apr 23 17:06:42.055: htsp_process_event: [1/1:0(1),
EM_WAIT_ONHOOK,
E_DSP_SIG_0000]htsp_report_onhook_sig
*Apr 23 17:06:42.111: htsp_process_event: [1/1:0(1),
EM_WAIT_ONHOOK, E_HTSP_EVENT_TIMER]em_wait_timeout
*Apr 23 17:06:42.111: em_stop_timers
*Apr 23 17:06:42.111: htsp_timer_stop
*Apr 23 17:06:42.111: em_start_timer: 400 ms
*Apr 23 17:06:42.111: htsp_timer - 400 msec
*Apr 23 17:06:42.511: htsp_process_event: [1/1:0(1),
EM_CLR_PENDING, E_HTSP_EVENT_TIMER]em_clr_timeout
*Apr 23 17:06:42.511: em_stop_timers
*Apr 23 17:06:42.511: htsp_timer_stop
*Apr 23 17:06:42.511: em_start_timer: 10000 ms*Apr 23 17:06:42.511: htsp_timer - 10000 msec
*Apr 23 17:06:42.511: htsp_process_event: [1/1:0(1), EM_PARK,
E_DSP_SIG_0000]em_park_onhook
*Apr 23 17:06:42.511: htsp_timer_stop
*Apr 23 17:06:42.511: htsp_timer_stop2
htsp_report_onhook_sig
/* Remote disconnects call */
/* Router rechecks theEM state of remote after
timer pops. This ensures
it was a disconnect and
not a hookflash. */
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57© 2002, Cisco Systems, Inc. All rights reserved.Voice Quality
Telephony Lab
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Lab Topology
Router A
FXS
FXS
Router B
T1
E&M
T1
E&M
Ext. 1234
Ext. 4321
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Lab Exercises
1. Configure the T1 with AMI/SF. Change 1 side to B8ZS and look at the showcontroller output. Next change 1 side to ESF and view the show controller output.
2. Configure both sides for ESF/B8ZS.
3. Make a loopback plug and plug into the T1.
4. Hook an analyzer into the T1, loopback the controller to the analyzer and bert thecontroller.
5. Configure Router A with extension 1234 and Router B with extension 4321.
6. Configure 6 timeslots all with different signaling types between the 2 Routers.
7. Configure dial-peers such that Router A can call Router B and vice versa usingany of the 6 channels between the 2 routers. (i.e. use an access code)
8. ―Debug vpm signal‖ and capture debugs to a file. Notate all signaling statechanges.
9. Debug vtsp session to see out-pulsed digits.
10. Configure the router so you can place hairpin calls to a second phone off Router A(extension 1235).
11. On the analyzer, watch the signaling bits for idle and active channels.
12. Listen to the audio for an active call with the analyzer.
13. Change the CAS T1 to a PRI.
14. Debug isdn q931 to view call setup and teardown messaging.
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