Tip and Ring Circuits
The analog connection that is provided to you is referred to as a tip and ring circuit. The name comes from the tip and ring portions of the old connectors in manual switchboards. Today, the tip and ring are the two wires that compose the analog connection. Typically deployed over a two-wire circuit, the center two pins are the most commonly used in RJ-11 modular plugs.
Basically, the way the phone circuit functions is that while your phone is on hook, or hung up, the physical circuit between your phone and the CO is open. After the phone goes off hook, or is picked up to dial, the circuit is closed and -48 VDC of power is supplied to your line from batteries at the CO. This point can be confusing because the circuit is not completed, and yet you can receive a ringing tone from the CO. That ring tone is accomplished with AC voltage. There is a bridge in the phone that blocks the DC voltage from passing unless the phone is off hook. When the phone goes off hook, the switch flags the circuit as busy, and it doesn't pass any further ring tones to your receiver.
NOTE This basic service description does not take into account the advanced network services such as call waiting.
The tone that you are accustomed to hearing in your ear (your dial tone) is actually a tone generated from the switch to let you know that your circuit is ready to place a call. After you hear that tone, you begin dialing the proper digits to reach your desired party, as shown in Figure 1-8. If you are using a tone dial phone (non-rotary), you hear specific tones as you press the individual numbers. These are called dual tone multifrequency (DTMF) tones. Each character on the phone handset has its own pitch, which the switch interprets as a dialed digit. Not all technologies supply a tone for you prior to placing a call. This is covered in later chapters in this book.
Figure 1-8 Analog Telephone Circuit States
On Hook
Central Office
Central Office
555-1212
Central Office
Off Hook
-49vDC
Power Supplied from the CO Batteries, and Dial Tone Supplied from the Switch
Central Office
Copper Pair Wiring
Expert White Board: Analog Signaling Methods
Several different types of basic analog signaling provide an analog circuit with basic on-hook and off-hook functionality:
• Loop start signaling
• Reverse battery signaling
• Ground start signaling
The first type of analog signaling is loop start signaling. Loop start was actually just discussed when describing a tip and ring circuit. Loop start signaling is common in analog installations. It functions by closing a two-wire loop (circuit) to seize a circuit. When the phone is on hook the loop is open, or not completed. When the receiver is picked up, the loop is closed, seizing the circuit to the CO. The CO listens for the seizure by placing battery on the ring wire and ground on the tip wire.
Reverse battery signaling for analog lines is simple when compared to the loop start signaling method. Loop start applies battery to the ring lead and ground to the tip lead. Reverse battery signaling reverses this approach by providing battery on the tip lead and ground on the ring lead for circuit seizure.
Ground start signaling is another type of analog signaling. Ground start signaling is most commonly found in switch to switch or private branch exchange (PBX) to switch applications. This form of signaling is accomplished by applying ground to the tip wire at the local switch in the connection. Because the remote switch is monitoring the tip wire for ground, after it is detected, it closes the circuit loop to provide for an active call. E&M signaling goes by several definitions. You might hear it called Ear and Mouth, Earth and Magnet, or Receive and Transmit. Regardless of how it is defined, they all function in the same way. E&M signaling is typically only found on connections between switches, between PBXs, or between a combination of a switch and PBX. Most people use the Ear and Mouth terminology because it is easier to remember the function of each wire. At either side of the E&M connection, the switch or PBX listens for ground on the E wire and transmit ground locally on the M wire.
E&M signaling can operate in two different ways, standard mode and wink mode. In standard mode, the originating switch goes off hook and waits 210 milliseconds (ms) before sending digits. The originating switch waits between 140 ms and 200 ms because the terminating switch briefly goes off hook (between 140 ms and 200 ms) for a period of time that is called the wink.
In wink mode, the originating switch sends digits after waiting only 150 ms. The decreased wait period allows for a faster call setup process.
COs, also known as end offices (EOs), are centralized locations that include device switches, multiplexers (MUXs) and demultiplexers (DEMUXs), Digital Access and Crossconnect Systems (DACSs), and a wide variety of other equipment that is discussed in detail later in the book.
On a basic scale, look at an image of your analog phone service to your home. In Figure 1-9, the solid line represents the house connected to the CO through a pair of copper wiring. Life would be simple if this was all that was required to provide you with analog service.
Figure 1-9 Basic Representation of Analog Phone Service
Analog Phone Servic
Copper Pair Wiring
IDLC
In fact, many things have been added to improve your service, as well as save the telco money. In days of old, if the phone company wanted to provide service to 24 houses in a neighborhood, they had to run 24 pairs of copper wiring. This quickly became a great expense for the provider. Figure 1-10 shows the use of an individual line to each house. IDLCs, also known as SLCs, and time-division multiplexing (TDM) circuits were created to help alleviate this issue.
IDLCs deploy service to densely populated residential or business areas as well as saving on the amount of copper wiring. The process of saving or gaining the copper wiring back is known as pair gain. Figure 1-11 shows the deployment of an IDLC in a neighborhood.
Figure 1-10 Deployment of Multiple Analog Circuits Without the Use of IDLCs u
Analog Phone Service
Copper Pair Wiring -
SLCs include various port densities, including SLC-48, SLC-96, SLC-500, and SLC-2000. You can find the larger SLCs in medium to large business areas because they allow for the deployment of a host of analog and digital services. The SLC naming conventions are Lucent coined names for the IDLC product line. Referring to the section on attenuation earlier in this chapter, IDLCs can help to alleviate the amplification issues that are associated with analog circuits. By placing the analog loop closer to the subscriber, multiple amplifications are not necessary in most cases.
Figure 1-10 Deployment of Multiple Analog Circuits Without the Use of IDLCs
- Figure 1-11 Deployment of an IDLC in a Neighborhood
- Analog Phone Service
High Bandwidth, TDM Circuits ^^^^^^^^ Copper Pair Wiring -
IDLCs use a high-bandwidth digital trunk or set of trunks to communicate back to the CO to provide service from the PSTN to the deployed area. When the loop carriers were first deployed, they typically used a T1 or E1 trunk back to the provider's CO. However, the recent developments in the area of digital technology, most notably synchronous optical network (SONET) and synchronous digital hierarchy (SDH), have enabled service providers to deploy fiber-optic trunks directly to IDLCs from the CO. These fiber links allow the service providers to deploy larger numbers of trunks, and provide more bandwidth-intensive services to their customer base. For more information on these technologies, refer to the following chapters:
• Chapter 5, "T1 Technology"
• Chapter 6, "E1, R2, and Japanese Carrier Technology"
IDLCs are deployed by using a set of standards from Bellcore that is called GR-303-CORE.
GR-303-CORE
The set of standards in GR-303-CORE was developed to specify an outline for future infrastructure deployments in reference to the use of IDLCs. The basis for these standards was to improve upon the limitations of standard IDLC deployments and to facilitate the integration of many digital mediums at the same time. GR-303-CORE is intended for use with digital mediums, but it does provide for the integration of both analog and digital mediums. GR-303-CORE provides for a way to bridge or translate analog circuits into digital trunks for transmission back to the CO.
Some of the new services designed into the framework are Hybrid Fiber Coax (HFC), xDSL, Fiber to the Curb, and the integration of ISDN equipment directly into the SLC. This design set the pace for what is now called Next Generation-IDLC (NG-IDLC). GR-303-CORE also specifies a set of standard deployment methodologies for bridging the gap among vendors and technologies.
Within the GR-303-CORE standard, there is a specification of a Timeslot Management Channel (TMC), which is for the transmission of call messaging for multiple TDM circuits. This signaling channel specifies on-hook and off-hook states with robbed-bit signaling (RBS). Although this streamlines the call setup process, it limits the channels to 56 kbps each. This loss of 8 kbps is because of the RBS stealing the least significant bit for said call states.
The new standard allows for a full digital signal Level 3 (DS-3) worth of T1 traffic (28 T1s), and the ability to provide redundancy on links that carry data traffic. In all, GR-303-CORE adds to the current deployment of the IDLC infrastructure by providing an open framework for technology integration, seamless call management over multiple TDM circuits, and the capacity for many more circuits than its predecessors did.
Internationally, the European Telecommunications Standards Institute (ETSI) has created a set of standards that provides for many of the same features by using slightly different terminology. The ETSI V5 standard set has several specifications, including V5.1 and V5.2. The V5.2 specification describes the communication between the access node (AN) and the local exchange (LE). V5.2 links can use either a single E1 trunk for communication or up to 16 E1 trunks in what is called a V5.2 bundle. One of the main differences between V5.1 and V5.2 is that V5.2 adds the capability to use primary rate access (PRA) circuits. In the case of the V5 standards, the AN is equivalent to the IDLC that GR-303-CORE uses, and the LE is the service provider's switch interface.
Each E1 carrier circuit within the V5.2 bundle is identified by using a link ID during normal operation. A provision is also made for the blocking of a specific link ID, when the AN identifies a need for disallowing traffic to a specific link. Two types of blocking are typically associated with the V5 standards, deferred and non-deferred. In the case of deferred blocking, the AN requests that a specific link ID be blocked from traffic. The LE flags all unassigned DS0s in the link as blocked, and blocks individually assigned DS0s as they become unassigned (because of call disconnect).
Non-deferred blocking operates in a slightly different way. The AN requests that a specific link be blocked to the LE. The LE switches the assigned digital service 0 (DS0s) (active calls) to a standby set of DS0s, and flags the requested link as blocked. The LE might reject the request by sending an unblock message in response to the request from the AN.
The link ID is assigned at both ends of the connection. The link ID identifies the individual links in a bundle so that both ends can agree on the location of calls between them.
The V5 standards support a host of different services between the AN and LE, which include the following
• Analog service support
• ISDN basic access (BA) and PRA Service (V5.2)
• Permanent leased line
• As with GR-303-CORE, the V5 specifications are designed to provide explicit communication instructions for analog communication through an AN, digital ISDN communication, permanent leased line (E1 facility), and the conversion between analog and digital services.
Continue reading here: SS7 Node Types
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