The example earlier in the chapter described the operation of a common telephone which uses the loop start technique for indicating off-hook conditions. Key systems and PBX systems supporting large user communities over shared public phone lines created a need for improved signaling techniques. Specifically, these systems required quicker, more accurate methods for seizing the phone line (going off-hook) and more reliable methods for communicating dialed digits. The use of these systems also created a need for additional interface types to handle their increased signaling and trunking requirements. The interface types and signaling methods which overcome some of the limitations of loop start in larger-scale configurations are described below.
FXS FXS stands for Foreign eXchange Station. It is the standard analog telephone interface with which most people are familiar. FXS interfaces are used to connect to basic telephone devices such as phones, modems, faxes, key systems, and analog PBXs. FXS interfaces use a standard RJ-11 2-wire jack and, in effect, mimic the PSTN.
FXO FXO stands for Foreign eXchange Office. The port on a standard phone is an FXO port since it communicates with central office switching. Voice-enabled routers use FXO ports to interface with the PSTN. An FXO port acts as if it is a standard telephone requiring dialtone to place calls. The standard RJ-11 modular jack is used.
E&M E&M signaling is commonly called ''ear and mouth." However, its origin actually comes from the terms earth and magnet. Earth represented electrical ground and magnet represented the electromagnet used to generate tone. E&M signaling provides on-hook/off-hook signaling and minimizes glare. It is commonly used for PBX trunks or tie-lines. Several types of E&M exist, each specifying different ways of signaling the off-hook condition between PBX and CO switch. E&M signaling is supported over 2- and 4-wire implementations. Cisco supports E&M types I, II, III, and V in its VoIP products.
Page 43 E&M uses 6 to 8 of the pins on an RJ-48 modular jack, depending on the type of E&M used. Off-hook and incoming calls are signaled using one of the five types of E&M signaling. Since E&M interfaces don't provide dialtone, they use one of three signaling techniques to initiate dialing. They are immediate start, wink start, and delay start.
FXO/FXS Signaling Techniques
LOOP START Loop start is described in detail earlier in the chapter, but is presented here for completeness. Upon lifting the phone set and taking the phone off hook, loop-start phones complete the local loop circuit between the phone and the central office switch by connecting tip to ring through the phone set. The central office switch recognizes this, determines that phone is off hook, and responds by providing dialtone on the line. By placing the phone set back on hook, the circuit is broken and current no longer flows through the local loop. The central office switch then recognizes this and disconnects the call.
GROUND START PBXs often use ground start for signaling off-hook conditions. Ground start minimizes the risk of both sides of the link (PBX and phone switch) going off hook at the same time. Loop-start circuits run this risk because the 4-s wait in-between ring cycles provides up to4 s where an incoming call can be on the line; however, the end station does not know it because a ring tone has not yet been generated. During that time the user may pick up to the phone to dial, not knowing that an incoming call was waiting. The collision of outbound and inbound calls is called glare. Ground start avoids this by allowing either side of the link to provide instantaneous off-hook signaling. Using ground start, the PBX momentarily grounds the ring lead indicating to the phone switch that it would like service. The phone switch interprets the signal and begins listening for dialed digits. The phone switch indicates incoming calls by grounding the tip lead
toward the PBX. This virtually eliminates the possibility of glare.
Signaling Techniques
E&M signaling is not restricted to the 2-wire implementations discussed with FXO and FXS configurations. The 2- and 4-wire implementations describe the number of wires used to transmit the audio signals to and from the telephony devices. The 2-wire implementation is similar to a standard phone in that full-duplex audio signals are transmit-
Page 44
TABLE 3-2
E&M Signaling Pinout and Descriptions
Name Pin Description Types Used
E = ear or earth 7 Signal from trunking side to signaling side (router or CO to PBX)
I, II, III, IV, V
M = mouth or magnet 2 Signal from signaling side to trunking side (PBX to router or CO)
I, II, III, IV, V
SB = signal battery 1 Connects to 48-V dc battery (sometimes
ground)
II, III, IV
SG = signal ground 8 Connects to electrical ground II, III, IV T = tip 6 Audio path from signaling side to
trunking side on 4-wire circuits
I, II, III, IV, V
R = ring 3 Not used on 2-wire implementations I, II, III, IV, V
T1 = tip-1 5 Audio path from signaling side to trunking side on 4-wire circuits
I, II, III, IV, V
R1 = ring-1 4 Full-duplex audio path on 2-wire implementations
I, II, III, IV, V
ted over a single pair. The 4-wire implementations provide separate paths for receiving and sending audio signals. Table 3-2 describes the different leads, their functions, along with their associated pins on the trunking side.
E&M signaling defines a signaling side and a trunking side for each connection. The PBX is the signaling side, while the trunking side is the telco, channel bank, or voice-enabled router. The signaling side sends its on-hook/off-hook indicators over the M lead and the trunking side sends its on-hook/off-hook indicators over the E lead. This provides each side of the link with a dedicated signaling path.
Five different types of E&M signaling exist, each with a slightly different method for signaling. All five types are presented below.
1. E&M type I signaling is popular in the United States. During inactivity, the E lead should be open and the M lead should be connected to ground. A PBX indicates an off-hook condition by connecting the M lead to the battery. The router or CO side indicates an off-hook condition by connecting the E lead to ground.
2. E&M type II signaling is also used in the United States. During inactivity, both the E and M leads should be open. A PBX indicates an off-hook condition by connecting the M lead to the signal battery (SB) lead which is connected to the battery at the CO side. The router or CO side indicates an off-hook condition by connecting the E lead to signal ground (SG), which is connected to ground at the PBX side. Type II signaling is
Page 45 symmetrical and allows for signaling nodes to be connected back to back using a crossover cable.
3. E&M type III signaling is not commonly used in modern systems. During inactivity, the E lead is open and the M lead is set to ground by connecting it to the SG lead from the CO. A PBX indicates an off-hook condition by disconnecting the M lead from the SG lead and connecting it to the SB lead from the CO. The router or CO side indicates an off-hook condition by connecting the E lead to ground.
4. E&M type IV signaling is not currently supported by Cisco's VoIP router interfaces.
5. E&M type V signaling is used in the United States and is common in Europe. During inactivity, both the E and M leads should be open. A PBX indicates an off-hook condition by connecting the M lead to ground. The router or CO side indicates an off-hook condition by connecting the E lead to ground. Type V signaling is symmetrical and allows for back-to-back connections using a crossover cable.
Table 3-3 summarizes the E&M types and the conditions they use to indicate on- and off-hook conditions.
Techniques for Seizing the Line The various E&M signaling types identify on-hook/off-hook conditions, but they do not include a method of identifying when it is safe to send dial strings. Several techniques which perform this task been developed to work with E&M ports. They are immediate start, wink start, and delay start.
1. Immediate start is the most basic technique of the three. It operates under the assumption that the device on the other side of the linkwill listen for dialed digits immediately after recognizing the off-hook
TABLE 3-3
Lead States by E&M Type and Hook Condition
Off-Hook On-Hook
E&M Type E Lead M Lead E Lead M Lead Symmetric
TABLE 3-3
Lead States by E&M Type and Hook Condition
Off-Hook On-Hook
E&M Type E Lead M Lead E Lead M Lead Symmetric
II Ground Battery Open Open Yes
III Ground Loop
current
Open Ground No
IV Ground Ground Open Open No
V Ground Ground Open Open Yes
Page 46 condition. Immediate start simply indicates the off-hook condition by using one of the E&M techniques above, waits a small time period, and then sends the desired digits.
2. Wink start is the most commonly used protocol for identifying when it is safe to send digits. The idea is for the originating station to wait for an acknowledgment from the receiving station before transmitting the digits. After the originating station goes off hook, it waits to hear a momentary off-hook signal (wink) from the receiving station. Upon hearing the wink, the originating station sends its dialed digits over the line. If the originating station detects a continuous off-hook message from the receiving station, it assumes that both stations tried to grab the link at the same time and then performs error recovery.
3. Delay dial provides a means for the CO switch to delay the originating PBX's transmission of dial digits until the switch is ready to process them. After initiating an off-hook signal, the PBX waits 200 ms. In that same time interval the CO switch senses the off-hook condition and returns a constant off-hook signal to the PBX. After waiting the allotted 200 ms, the PBX checks the E lead (from CO to PBX). If it is on hook, then the PBX dials. If it is off hook, then the PBX waits until it goes on hook before dialing. This allows the CO switch to delay the sending of digits merely by holding the line off hook until it is ready to receive the digits.