Digital&AnalolgSignaling

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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)




Digital Telephony



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




•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




= 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




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



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)



• 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



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



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‖



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



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



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


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


11 M d d d d d d d d

12 1 d d d d d d d B






18 C5 d d d d d d d C





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



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.



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



• 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




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.......




ISDN




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




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




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




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




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




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




Debugging ISDN




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




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




Debugging Q931 – Call Setup

SwitchSwitc

h

Outgoing

Call Made

Call Accepted

ISDN PRIISDN PRI WAN




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)




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




Debugging Q931 Call Disconnect

ISDN PRIISDN PRIWAN

Switc

h

Switc

h




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.




Channel Associated Signaling




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)




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




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




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




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.




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




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.




FXS/FXO Loop Start State Table

Note: State Table

is from FXO‘s

Perspective.

Disconnect

Supervision is

done with A bit




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




• 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




FXS/FXO GroundStart State Table

Note: State Table

is from FXO‘s

perspective




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




Debugging CAS




Debugging Topology

5400 PBX 3745

T1 E&M

Wink Start

PBX

T1 E&M

Wink Start

IP

Setup Direction

Disconnect Direction




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.




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




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


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



EM_WAIT_WINKDOWN, E_DSP_SIG_0000]em_wink_onhook


*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*/




Debugging CAS



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: htsp_timer_stop


/* 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) */




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











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



/* User dials dtmf 1234

once call is connected.

Router received 1234 from

T1 interface.*/

/* Router disconnects call.

Duration is approx 121 sec*/




Debugging CAS


EM_WAIT_ONHOOK,

E_DSP_SIG_0000]htsp_report_onhook_sig


EM_WAIT_ONHOOK, E_HTSP_EVENT_TIMER]em_wait_timeout






EM_CLR_PENDING, E_HTSP_EVENT_TIMER]em_clr_timeout



*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



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. */




Telephony Lab




Lab Topology

Router A

FXS

FXS

Router B

T1

E&M

T1

E&M

Ext. 1234

Ext. 4321




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.

Digital&AnalolgSignaling

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