Transcript VoIP_1

VoIP - Implementing Trunk for
PSTN Switch and VoIP Gateway
An Examination
Ricardo Estevez
CS 522 / Computer Communication
Fall 2003
Big Picture
Gateway and Gatekeeper Signaling
Agenda
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What is a trunk?
Discuss signaling systems.
Implementing Trunk for PSTN Switch
and VoIP Gateway
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PSTN - Public Switched Telephone
Network
VoIP – Voice over IP
A trunk is a “talk path”
Trunks carry signals
Trunks are logical separations of physical
circuits.
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
T1, T3, OC-3, OC-12, E1
These circuits can carry n trunks, where n
is dependent on capacity of circuits
Implementing Trunk for PSTN Switch
and VoIP Gateway

T1 Circuit – 1.544 Mbps
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Four wire circuit with 8 pin connector –
RJ48C interface
Can carry a digital stream of 24
channels, 64 kbps each, totaling 1.536
Mbps or better known approximation:
1.544 Mbps – DS1 Signal
The 24 channels are identified with
timeslots 0 – 23
Each timeslot is known as DS0 Signal
Implementing Trunk for PSTN Switch
and VoIP Gateway

E1 Circuit – 1.92 Mbps
 Four wire circuit with 8 pin connector – RJ48C
interface
 Can carry a digital stream of 32 channels, 64
kbps each, totaling 2.048 Mbps
 The 31 channels are identified with timeslots 0
– 31
 Timeslot 0 is dedicated for framing and
synchronization
 Timeslot 16 is dedicated for signaling
 Adjusted total line rate 1.92 Mbps
 (30 channels x 64 kbps)
Implementing Trunk for PSTN Switch
and VoIP Gateway

DS3 Signal – 45 Mbps
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High capacity line equivalent to 28 T1 lines
DS3 has the most technical framing
techniques
 M13, M23 processes
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28 DS1 signals multiplex into 7 DS2
Each DS2 contains 4 DS1 signals
Combined view of 7 DS2 signals create DS3
SONET Ring commonly used to deliver DS3
Implementing Trunk for PSTN Switch
and VoIP Gateway
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Three common signaling systems
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T1 CAS (Carrier Associated Signaling)
PRI (Primary Rate Interface)
SS7 (Signaling System 7)
In-band signals travel on same
channel with voice traffic (T1 CAS)
Out-of-band signals travel on
separate channel from voice traffic
(PRI, SS7)
T1 CAS
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How a call takes place:
1.
2.
3.
4.
5.
6.
7.
Receiving gateway seizes a trunk to PSTN
switch
Signals are exchanged
Switch determines route and establishes a
connection
Address Complete Message is sent back to
caller and caller hears ring tone
Callee answers
Call takes place
Signals are exchanged to release call
T1 CAS
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CAS uses in-band signaling
CRC Error Detection – 6 bit
PRI
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PRI uses out-of-band signaling
Using a T1 circuit, one 64 kbps
channel is dedicated for signaling
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Called D channel – Data Channel
Leaving 23 64 kbps channels for
voice
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Called B channel – Bearer Channel
PRI
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NFAS - Non-facility Associated
Signaling
NFAS groups T1 PRI trunks so that
only one PRI’s signaling channel is
used, leaving other PRIs’ 24
channels for voice traffic
The signaling for PRI conforms to
standards ITU-T Q.921 and ITU-T
Q.931
PRI
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Q.921 provides full-duplex signaling between
PSTN Switch and VoIP Gateway
Layer 2 Protocol, so end-to-end
Message Sequence:
1.
2.
3.
4.
5.
Sender - Set Asynchronous Balanced Mode
Extended (SABME) establishes data-link
connection
Receiver - Connection confirmed with unnumbered
acknowledgment (UA) message
Sender/Receiver - Exchange Q.931 Messages (RR)
Sender – Disconnect message (DISC) if no more
RR messages
Receiver – Disconnect mode (DM)
Q.921
PRI
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Q.931 provides full-duplex
signaling between PSTN Switch
and VoIP Gateway
Layer 3 Protocol
Message Sequence:
1.
2.
3.
Gateway sends SETUP message
Switch replies with various cause
values
Exchange messages
Q.931
PRI
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Cause Values
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CALL PROCEEDING – call is now in progress
ALERTING – after called party has been
alerted
CONNECT – after called party has answered
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CONNECT ACK – gateway acknowledges
DISCONNECT – one party initiates
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RELEASE
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RELEASE COMP
References
Durkin, James F. Voice-Enabling the
Data Network. Cisco Press:
Indianapolis, IN, 2003
ISBN: 1-58705-014-5