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