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  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 – Mbps Four wire circuit with 8 pin connector – RJ48C interface Can carry a digital stream of 24 channels, 64 kbps each, totaling Mbps or better known approximation: 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 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 switch 2. Signals are exchanged 3. Switch determines route and establishes a connection 4. Address Complete Message is sent back to caller and caller hears ring tone 5. Callee answers 6. Call takes place 7. 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 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 messages 5. 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 message 2. Switch replies with various cause values 3. 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, 2003 ISBN: