voip - implementing trunk for pstn switch and voip gateway
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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. What is a trunk? Discuss signaling systems. Implementing Trunk for PSTN Switch and VoIP Gateway. - PowerPoint PPT PresentationTRANSCRIPT
VoIP - Implementing Trunk for PSTN Switch and VoIP Gateway
An Examination
Ricardo EstevezCS 522 / Computer CommunicationFall 2003
Big Picture
Gateway and Gatekeeper Signaling
Agenda
What is a trunk? Discuss signaling systems.
Implementing Trunk for PSTN Switch and VoIP Gateway
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. 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 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 High capacity line equivalent to 28 T1 lines DS3 has the most technical framing
techniques M13, M23 processes
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
Three common signaling systems 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
How a call takes place:1. Receiving gateway seizes a trunk to PSTN
switch2. Signals are exchanged3. Switch determines route and establishes a
connection4. Address Complete Message is sent back to
caller and caller hears ring tone5. Callee answers6. Call takes place7. Signals are exchanged to release call
T1 CAS
CAS uses in-band signaling CRC Error Detection – 6 bit
PRI
PRI uses out-of-band signaling Using a T1 circuit, one 64 kbps
channel is dedicated for signaling Called D channel – Data Channel
Leaving 23 64 kbps channels for voice
Called B channel – Bearer Channel
PRI
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
Q.921 provides full-duplex signaling between PSTN Switch and VoIP Gateway
Layer 2 Protocol, so end-to-end Message Sequence:
1. Sender - Set Asynchronous Balanced Mode Extended (SABME) establishes data-link connection
2. Receiver - Connection confirmed with unnumbered acknowledgment (UA) message
3. Sender/Receiver - Exchange Q.931 Messages (RR)4. Sender – Disconnect message (DISC) if no more RR
messages5. Receiver – Disconnect mode (DM)
Q.921
PRI
Q.931 provides full-duplex signaling between PSTN Switch and VoIP Gateway
Layer 3 Protocol Message Sequence:
1. Gateway sends SETUP message2. Switch replies with various cause
values3. Exchange messages
Q.931
PRI
Cause Values CALL PROCEEDING – call is now in progress ALERTING – after called party has been
alerted CONNECT – after called party has answered
CONNECT ACK – gateway acknowledges DISCONNECT – one party initiates
RELEASE RELEASE COMP
References
Durkin, James F. Voice-Enabling the Data Network. Cisco Press: Indianapolis, IN, 2003ISBN: 1-58705-014-5