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VoIP - Implementing Trunk for PSTN Switch and VoIP Gateway

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.

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VoIP - Implementing Trunk for PSTN Switch and VoIP Gateway

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  1. VoIP - Implementing Trunk for PSTN Switch and VoIP Gateway An Examination Ricardo Estevez CS 522 / Computer Communication Fall 2003

  2. Big Picture

  3. Gateway and Gatekeeper Signaling

  4. Agenda • What is a trunk? • Discuss signaling systems.

  5. 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

  6. 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

  7. 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)

  8. 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

  9. 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)

  10. T1 CAS • How a call takes place: • 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

  11. T1 CAS • CAS uses in-band signaling • CRC Error Detection – 6 bit

  12. 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

  13. 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

  14. PRI • Q.921 provides full-duplex signaling between PSTN Switch and VoIP Gateway • Layer 2 Protocol, so end-to-end • Message Sequence: • 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)

  15. Q.921

  16. PRI • Q.931 provides full-duplex signaling between PSTN Switch and VoIP Gateway • Layer 3 Protocol • Message Sequence: • Gateway sends SETUP message • Switch replies with various cause values • Exchange messages

  17. Q.931

  18. 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

  19. References Durkin, James F. Voice-Enabling the Data Network. Cisco Press: Indianapolis, IN, 2003 ISBN: 1-58705-014-5

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