QSIG is a peer-to-peer signaling system used in corporate voice networking. Internationally, QSIG is known as Private Signaling System No. 1 (PSS1). This open standard is based on the ITU-T Q.9XX series of recommendations for basic service and supplementary services. Therefore, as well as providing inter-PBX communications, QSIG is compatible with public and private ISDN.
QSIG also has one important mechanism known as Generic Functional Procedures (QSIG GF). This mechanism provides a standard method for transporting features transparently across a network.
QSIG has the following functionality:
• It enables the interconnection of multivendor equipment (standards-based protocol).
• It enables inter-PBX basic, feature transparency, and supplementary services.
• It is Interoperable with public and private ISDNs.
• It operates in any network configuration (Star, Mesh, Token Ring, and so on) and is compatible with many PBX-type interfaces.
• It does not impose restrictions on private numbering plans.
QSIG and ISDN
QSIG end-to-end signaling is maintained from PBX-to-PBX, and ISDN and ISDN User Part (ISUP) interworking is critical for end-to-end signaling in the ISDN network. The ISDN reference model for PBX-to-PBX signaling for corporate networks has two new reference points, Q and C. The reference points are as follows:
• T—Defines access to the NT2 device for ISDN PRI.
• C—Is the physical interconnection point to the PBX. It is compatible with many interfaces, including two- and four-wire analog, BRI, PRI, and radio and satellite links.
• Q—Specifies the logical signaling point between two PBXs. This reference point is used to specify signaling-system and related protocols.
A corporate network (see Figure 10) can have dedicated analog or digital channels, or VPN switched connections. Typically, a T1 or E1 digital interface is used to connect to the network.
Figure 10 Reference Model for Corporate Networks
Setup Call Proceeding
Alerting Connect Connect Ack
Voice Disconnect
Release Release Complete
Setup Call Proceeding
Alerting Connect Connect Ack
or Data Disconnect
Release Release Complete
PBX PBX PBX
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QSIG Protocols
QSIG protocols specify a signaling system and have an identical structure to that of ISDN, except for layer 3 (shown in Table 5). Layer 3 splits QSIG into the following three sublayers:
• QSIG BC—Interfaces and messages for the user and network sides are identical.
• QSIG GF—Generic layer that enables supplementary services and ANFs and provides a connection-oriented mechanism between the application entities of different PBXs.
• QSIG supplementary service and ANF protocols—Procedures for services and ANFs that are defined by the European Computer Manufacturers Association (ECMA) and the European Telecommunication Standards Institute (ETSI).
ISDN
ISDN is a network that can consist of T1, T3, E1, and E3 and has two types of subscriber access: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Each access is comprised of B and D channels.
B channels are 64-kbps channels that carry user information streams. No signaling information is carried in the B channel. The user streams include speech encoded at 64 kbps according to ITU G.711, data at or less than 64 kbps, and voice encoded at lower bit rates.
D channels are used primarily to carry signaling for circuit switching by ISDN networks. D-channel bit rates are different depending on the access method. The D channel also is capable of transmitting user packet data up to 9.6 kbps.
Table 5 QSIG Protocols
OSI Reference QSIG Protocol QSIG Standard
L1 None Based on used interface.
L2 None Identical to ISDN L2 (LAPD1).
1. Link Access Procedure on the D channel.
L3 QSIG BC ECMA 142/143; ETS3002 171/172.
2. ETSI-based standard.
QSIG GF ECMA 165; ETS300 239.
QSIG
(supplementary services)
Separate specifications, such as call forward (ECMA 173/174, ETS300 256/257) and call transfer (ECMA 177/178, ETS300 260/261).
L4
Application-based service
elements. Transparent to the network.
L5 L6 L7
Cisco Voice Telephony
ISDN
An in-depth description of each type is as follows:
• BRI—Useful when Digital Subscriber Lines (DSLs) or cable modems were unavailable and provided a fast connection to the Internet. Delivers two bi-directional 64-kbps B channels and one bi-directional 16-kbps D channel over standard two-wire telephone lines. Basic rate ISDN service typically is used for residential and small office, home office (SOHO) applications. Each B channel can transmit speech or data; the D channel transmits the signaling or call control messages. The reference configuration for ISDN is defined in the ITU specification I.411. The reference points specify the transmission medium, interface, and connectors (if used).
– U reference point—Specifies the transmission characteristics of the local loop. The two-wire interface operates at 160 kbps (2B+D + 16 kbps for overhead) over standard copper twisted wires.
– S/T reference point—Provides a four-wire connection to ISDN-compatible terminals or terminal adapters. The interface operates at 144 kbps (2B+D) between the ISDN device and the network termination device. Up to eight ISDN devices can be connected to the S/T interface.
– R reference point—Provides connection using EIA/TIA-232 and V.35 interfaces for non-ISDN devices. The devices connect to the terminal adapter. This reference configuration also specifies the set of functions required to access ISDN networks:
– Network Termination 1 (NT1)—Outside the United States, NT1 is on the network side of the defined user-network interface and is considered part of the service provider network.
NT1s terminate the two-wire local loop and provide four-wire S/T bus for ISDN terminal equipment (TE).
– Terminal Equipment 1 (TE1)—ISDN-compatible devices that connect directly to the S/T connector on the NT1.
– TE2—Non-ISDN compatible devices that require terminal adapter (TA) interconnection.
– TA—An ISDN-compliant interface to NT1s and standard interfaces for TE2s. These standard interfaces include EIA/TIA-232, V.35, EIA/TIA-449, and X.21.
• PRI—Designed for telephone switches, computer telephony, and voice processing systems. PRI can be made into as many as 24 and 32 phone calls.Corresponds to two primary rates: 1.544 Mbps (T1) and 2.048 Mbps (E1). PRIs typically are used in medium to large business applications. PRI is comprised of 23 64-kbps B channels and one D channel. The interface structure for T1 is 23B + D (North America and Japan). The interface structure for E1 is 30B + D (Europe). The configuration and reference points for PRI are similar to those for BRI, and the differences:
– U reference point—A four-wire interface that operates at T1 PRI rates.
– T reference point—Provides access to the Network Termination 2 (NT2) device.
– NT2—PBX equipment can provide such NT2 functions as Layer 2 (L2) and Layer 3 (L3) protocol handling as well as multiplexing, switching, interface termination, and maintenance.
NT2s also can provide connections to ISDN-compatible TE1s and non-ISDN compatible TE2s.
ISDN was designed to overcome the deficiencies of the PSTN by the following:
• Providing an internally accepted standard for voice, data, and signaling.
• Making all transmission circuits end-to-end digital.
• Adopting a standard out-of-band signaling system.
• Bringing significantly more bandwidth to the desktop.
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Some ISDN features are as follows:
• Call waiting—The calling party is placed in a queue until the called party accepts or rejects the call.
• Specialized numbering and dialing plans—Centralized management of all ISDN terminals, including PBXs, key systems, and so on.
• Credit card calling—Automatic billing of certain or all calls into accounts independent of the calling line.
• Calling line identification presentation—Provides the calling party the ISDN phone number and address, in some cases, of the called party. The called party can accept or reject the call.
• Calling line identification restriction—Restricts presentation of the calling party’s ISDN phone number and address to the called party.
• Closed user group—Restricts conversations to or among a select group of phone numbers, local, long distance, or international.
• Desktop videoconferencing—Enables the display of the calling party’s video image on the desktop device.
• E-mail—ISDN can carry information to and from unattended phones as long as the phones are equipped with the required hardware and software.
• Simultaneous data calls—Two users can talk and exchange information over the D packet and the B circuit.
Trunking
The terms line and trunk are often used synonymously, but there is a distinction. A trunk can connect key systems in private telephone systems, routers, and switches. A line connects to a PBX, switch, or other communication system, telephone, computer terminal, or other endpoint. There are several kinds of trunks, such as tie, loop-start, ground-start, and ISDN (T1, E1, BRI, and PRI). Some use analog signaling and others use digital signaling.