Voice Over IP

Local-Loop Signaling A subscriber and telephone company notify each other of the call status through audible tones and an exchange of electrical current. This exchange of information is called local-loop signaling. Local-loop signaling consists of supervisory signaling, address signaling, and informational ... [full story]

E&M Type IV [full story]

&M Wink-Start Signaling Tie trunks have bidirectional supervisory signaling that allows either end to initiate a trunk seizure. In this way, one PBX seizes the trunk, which then waits for an acknowledgment reply from the remote end. The local end must ... [full story]

Trunk Supervisory Signaling: Wink-Start [View full size image] The following scenario summarizes the wink-start protocol event sequence: The calling office seizes the line by activating its M-lead. Instead of returning an off-hook acknowledgment immediately, the called switch allocates memory for use as a ... [full story]

E&M Immediate-Start Signaling Immediate-start signaling occurs occasionally if a PBX vendor implements wink-start, shown in Figure 2-18, but does not conform to the standards. Figure 2-18. Trunk Supervisory Signaling: Immediate-Start [View full size image] The following scenario summarizes the sequence of events for ... [full story]

E&M Delay-Start Signaling Delay-start signaling, as depicted in Figure 2-19, is the original start protocol for E&M. Figure 2-19. Trunk Supervisory Signaling: Delay-Start [View full size image] Delay-start is used when all of the equipment is mechanical and requires time to process requests. ... [full story]

Two-Wire to Four-Wire Conversion and Echo [View full size image] Telephone networks can experience two types of echo: Acoustic echo Acoustic echo frequently occurs with speakerphones, when the received voice on the speaker excites the microphone and travels back to the speaker. Electrical ... [full story]

Echo Is Always Present The magnitude or loudness of the echo is high. The delay time between when you speak and when you hear your voice reflected is significant. The listener hears the speaker twice. The two components of echo are loudness and ... [full story]

Echo Suppression [View full size image] Echo suppression has shortcomings in addressing certain echo conflict situations. Echo cancellation, a schematic of which is shown in Figure 2-23, is a more sophisticated method of eliminating echo. [full story]

Echo Cancellation Rather than breaking or attenuating the return path (as in echo suppression), echo cancellation uses a special circuit to build a mathematical model of the transmitted speech pattern and subtracts it from the return path. Note Echo cancellation applies the ... [full story]

E&M Type III Type IV, shown in Figure 2-16, is a variation of Type II. In this arrangement, the battery source and ground are reversed on the SB and M wires (as compared to Type II). This means that both ... [full story]

E&M Type II Type III, as demonstrated in Figure 2-15, is useful for environments where the M-lead is likely to experience electrical interference and falsely signal its attached equipment. When idle, Type III latches the M-lead via an electrical relay ... [full story]

E&M Type V Type V is a modified version of the Type I interface. In the Type V interface, the Cisco voice equipment (tie-line equipment) supplies battery for the M-lead while the PBX supplies battery for the E-lead. As in ... [full story]

Informational Signaling DTMF tones are used not just for address signaling but also for informational signaling. Specifically, call-progress indicators in the form of tone combinations are used to notify subscribers of call status. Each combination of tones represents a different event ... [full story]

Trunk Connections Before a telephone call terminates at its final destination, the call is routed through multiple switches. When a switch receives a call, it determines whether the destination telephone number is within a local switch or if the call needs ... [full story]

Trunk Signaling Lines and trunks must adhere to signaling standards just as telephony networks and telephone companies do. Trunk signaling serves to initiate the connection between the switch and the network. There are five different types of trunk signaling, and each ... [full story]

Loop-Start Signaling Loop-start signaling allows a user or the telephone company to seize a line or trunk when a subscriber is initiating a call. It is primarily used on local loops connecting to residences rather than on trunks interconnecting telephone switches. A ... [full story]

Ground-Start Signaling Ground-start signaling, illustrated in Figure 2-11, is a modification of loop-start signaling that corrects for the probability of glare. It solves the problem by providing current detection at both ends. Figure 2-11. Ground-Start Signaling Although loop-start signaling works when you ... [full story]

E&M Signaling E&M signaling supports tie-line type facilities or signals between voice switches. Instead of superimposing both voice and signaling on the same wire, E&M uses separate paths, or leads, for each. To call a remote office, your PBX must route a ... [full story]

PBX to Intermediate Device Signaling Type Lead On Hook Off Hook I M Ground Battery(-48 VDC) II M Open Battery(-48 VDC) III M Ground Battery(-48 VDC) IV M Open Ground V M Open Ground [full story]

Intermediate Device to PBX Signaling Type Lead On Hook Off Hook I E Open Ground II E Open Ground III E Open Ground IV E Open Ground V E Open Ground Four-wire E&M Type I signaling, shown in Figure 2-12, is actually a six-wire E&M signaling interface common in North America. One wire is the E-lead; the second wire is the M-lead, and the ... [full story]

E&M Type I [View full size image] With the Type I interface, the Cisco voice equipment (tie-line equipment) generates the E signal to the PBX by grounding the E-lead. The PBX detects the E signal by sensing the increase in current through ... [full story]

Analog-to-Digital and Digital-to-Analog Voice Encoding This section covers the fundamentals of digitally encoding voice, specifically, the basics of voice digitization and the various compression schemes that are used to transport voice while using less bandwidth. Digitizing speech was a project first undertaken ... [full story]

Analog-to-Digital Signal Conversion Step Procedure Description 1 Sample the analog signal regularly. The sampling rate must be at least twice the highest frequency, to accurately represent the original signal. 2 Quantize the sample. Quantization consists of a scale made up of 8 major divisions or chords. Each chord is ... [full story]

E1 Channel Associated Signaling In E1 framing and signaling, 30 of the 32 available channels, or time slots, are used for voice and data. Framing information uses time slot 1, while time slot 17 (E0 16) is used for signaling by ... [full story]

E1 Framing and Signaling In the E1 frame format, 32 time slots make up a frame. A multiframe consists of 16 E1 frames, as depicted in Figure 2-35. [full story]

Channel Associated Signaling - E1 [View full size image] The time slots are numbered 1 though 32, although the channels are numbered 0 through 31, as shown in Figure 2-35. Multiframe time slots are configured as follows: Time slot 1 carries only ... [full story]

Common Channel Signaling Systems Common channel signaling (CCS) differs from CAS in that all channels use a common channel and protocol for call setup. Using E1 as an example, a signaling protocol, such as ISDN Q.931, would be deployed in time ... [full story]

ISDN ISDN (Integrated Services Digital Network) is an access specification to a network. You may have studied ISDN as an access method for dial-up data systems. Because it is a digital system, ISDN makes connections rapidly. ISDN can be implemented in two ... [full story]

QSIG The QSIG (Q Signaling) protocol is based on the ISDN Q.931 standard and provides signaling for private integrated services network exchange (PINX) devices. Figure 2-39 shows how different QSIG operations map to the OSI model. Figure 2-39. QSIG Protocol [View full size ... [full story]

DPNSS British Telecom and selected PBX manufacturers originally developed the Digital Private Network Signaling System (DPNSS) in the early 1980s. It was developed and put into use before the ISDN standards were completed because customers wanted to make use of digital ... [full story]

SIGTRAN SIGTRAN, as illustrated in Figure 2-40, is a signaling protocol defined in RFC 2719 and RFC 2960. It describes the way the IP protocol carries SS7 messages in a VoIP network. SIGTRAN relies on the Stream Control Transport Protocol at ... [full story]