Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book)
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The Central Office
The Central Office (CO) houses the digital switching equipment that terminates subscribers' lines and trunks and switch calls. The term switch is a vestige of the switchboard era, when call connections were manually created using cords to connect lines on a plugboard. Electro-mechanical switches replaced manual switchboards, and those eventually evolved into the computer-driven digital switches of today's network. Now switching between calls is done electronically, under software control.
The following section focuses on these areas of the CO:
Main Distribution Frame
Incoming lines and trunks are terminated on the Main Distribution Frame (MDF). The MDF provides a junction point where the external facilities connect to the equipment within the CO. Jumpers make the connections between the external facilities and the CO equipment, thereby allowing connections to be changed easily. Line connections from the MDF to the digital switching equipment terminate on line cards that are designed to interface with the particular type of line being connected—such as POTS, ISDN BRI, and Electronic Key Telephone Set (EKTS) phone lines. For analog lines, this is normally the point at which voice encoding takes place. Trunk connections from the MDF are terminated on trunk interface cards, providing the necessary functions for message framing, transmission, and reception.
The Digital Switch
The digital switch provides a software-controlled matrix of interconnections between phone subscribers. A handful of telecommunications vendors produce the digital switches that comprise the majority of the modern PSTN; Nortel, Lucent, Siemens, Alcatel, and Ericsson hold the leading market share. While the digital switch's basic functionality is common across vendors, the actual implementation is vendor dependent. This section provides a general perspective on the functions of the digital switch that are common across different implementations.
All digital switches are designed with some degree of distributed processing. A typical architecture includes a central processing unit that controls peripheral processors interfacing with the voice channels. Redundancy is always employed in the design to provide the high reliability that is expected in the telephony network. For example, the failure of one central processing unit results in the activation of an alternate processing unit.
The line and trunk interface cards, mentioned previously, represent the point of entry into the digital switch. These cards typically reside in peripheral equipment that is ultimately controlled by the central processor. Within the digital switch, all voice streams are digitized data. Some voice streams, such as those from ISDN facilities and digital trunks, enter the switch as digital data. Other voice streams, such as the analog phone, enter as analog data but undergo digital conversion at their point of entry. Analog lines interface with line cards that contain codecs, which perform the PCM processing to provide digital data to the switch and analog data to the line. Using the distributed processing architecture, many functions related to the individual voice channels are delegated to the peripheral interface equipment. This relieves the central processor of CPU intensive, low-level processing functions, such as scanning for on/off hooks on each individual line to determine when a subscriber wants to place a call.
The central processing unit monitors information from peripheral processors on call events—such as origination, digit collection, answer, and termination—and orchestrates the actual call setup and release. Information from these events is also used to perform call accounting, billing, and statistical information such as Operational Measurements (OM).
Although the main purpose of the digital switch is to perform call processing, much of its functionality is dedicated to maintenance, diagnostics, and fault recovery to ensure reliability.
An OM is a counter that records an event of particular interest, such as the number of call attempts or the number of a particular type of message received, to service providers. OMs can also be used to record usage in terms of how long a resource is used. Modern digital switches usually record hundreds, or even thousands of different types of OMs for various events taking place in the switch.
A modern digital switch can process many voice channels. The actual number of channels it processes varies with the switch vendor and particular model of switch, but they often process tens of thousands of voice channels in a single switch. A number of switches have capacities of over 100,000 connections.
The switch is responsible for many tasks, but one of its primary functions is connecting voice channels to create a bi-directional conversation path between two phone subscribers. All digital switches incorporate some form of switching matrix to allow the connection of voice channels to other voice channels. Once a circuit is set up between the two subscribers, the connection remains for the duration of the call. This method of setting up call connections is commonly known as circuit switching.
Figure 5-10 illustrates how a switching matrix demultiplexes individual timeslots from a multiplexed stream of voice channels and inserts them into the appropriate time slot for a connection on another facility, to connect voice channels. For example, in the figure, time slot 4 from the digital stream on the left connects to timeslot 30 of the digital stream on the right. The figure shows thirty channels, but the number of channels depends on the individual implementation of the switching matrix.
Figure 5-10. TDM Switching Matrix
Each timeslot represents a voice connection path. The matrix connects the two paths to provide a conversation path between two parties. For long-distance calls that traverse a number of switches, an individual call goes through multiple switching matrices and is mapped to a new timeslot at each switching point. When the call is set up, it occupies the voice channel that was set up through the network for the duration of the call.
Call processing is associated with the setup, maintenance, and release of calls within the digital switch. The process is driven by software, in response to stimulus from the facilities coming into the switch. Signaling indications, such as on/off-hook, dialing digits, and answer, are all part of the stimuli that drive the processing of calls.
Each call process can be represented as an originating call half and a terminating call half. When combined, the two halves are completely representative of the call. The originating half is created when the switch determines that the originator is attempting a call. The terminating call half is created when the destination has been identified, typically at the translations or routing phase. The Intelligent Network standards have established a standardized call model, which incorporates the half-call concept. A complete discussion of the call model is presented in Chapter 11, "Intelligent Networks (IN)."
Call processing can be broken down in various ways; the following list provides a succinct view of the major stages of establishing and disconnecting a call.
Additional functions, such as billing and service interactions, also take place, but are excluded in our simple view of processing.
For a line, this initial phase of call processing occurs when a subscriber goes off-hook to initiate a call. The actual event provided to the digital switch to indicate a line origination can be a change in loop current for analog lines, or a setup message from an ISDN BRI facility. In-band A/B bit off-hook signaling, an ISDN PRI setup message, or an Initial Address Message from an SS7 signaled trunk can signal a digital trunk's origination. All of these events indicate the origination of a new call. The origination event creates the originating half of the call.
For analog line originations, the switch collects digits as the caller dials them. Inter-digit timing monitors the amount of time the caller takes to dial each digit so that the line cannot be left in the dialing state for an infinite amount of time. If the caller does not supply the required number of digits for calling within a specified time, the caller is usually connected to a digital announcement to indicate that there is a problem with dialing, a Receiver Off-Hook (ROH) tone, or both. The dialing plan used for the incoming facility usually specifies the number of digits that are required for calling.
For ISDN lines, the dialed digits are sent in an ISDN Setup message.
Translation, commonly referred to as digit analysis, is the process of analyzing the collected digits and mapping them to a result. The translation process directs calls to their network destination. The dial plan associated with the incoming line, or trunk, is consulted to determine how the digits should be translated. Different dial plans can be associated with different incoming facilities to allow flexibility and customization in the translation of incoming calls. The dial plan specifies such information as the minimum and maximum number of digits to be collected, acceptable number ranges, call type, special billing indicators, and so forth. The translation process can be somewhat complex for calls that involve advanced services like Centrex, which is often associated with business phones.
Centrex is a set of services provided by the local exchange switch to business subscribers, including features like ring again, call parking, and conferencing. Centrex allows businesses to have many of the services provided by a PBX without the overhead of PBX cost, administration, and maintenance.
The process of digit translation can produce several different results. The most common result is a route selection for the call to proceed. Other results include connection to a recorded announcement or tone generator, or the sending of an Intelligent Network Query message for calls involving Intelligent Network services. Network administrators provision dial plan, routing information, and other translation-related information on the switch. However, information returned from IN queries can be used to modify or override statically provisioned information, such as routing.
The call proceeds to the routing stage after translation processing. Routing is the process of selecting a voice channel (on a facility) over which to send the outbound call toward its intended destination, which the dialed digits identify during translation. Routing typically uses route lists, which contain multiple routes from which to choose. For calls that are destined outside of the switching node, a trunk group is selected for the outbound call. A trunk group is a collection of trunk members that are connected to a single destination. After a trunk group is selected, an individual trunk member is selected from the group. A trunk member occupies an individual time slot on a digital trunk.
Routing algorithms are generally used for selecting the individual trunk circuit. For example, members of an outgoing trunk group are commonly selected using algorithms such as least idle, most idle, and ascending or descending order (based on the numerical trunk member number).
Call connection must take place on both the transmit and receive paths for a bi-directional conversation to take place. Each involved switch creates a connection between the originating half of the call and the terminating half of the call. This connection must be made through the switching matrix, and the speech path must be cut through between the incoming and outgoing voice channels. Supervision messages or signals sent from the central processor to the peripheral interfaces typically cut through the connection for the speech path. The central processor uses supervision signals to indicate how the peripheral processors should handle lower-level functions. It is typical to cut through the backward speech path (from terminator to originator) before cutting through the forward speech path. This approach allows the terminating switch to send the audible ringback over the voice channel, to the originating switch. When the originating switch receives an answer indication, the call path should be connected in both directions.
A call may be disconnected when it is active, meaning that it has been set up and is in the talking state. Disconnection can be indicated in a several ways. For analog lines, the originating or terminating side of the call can go on-hook, causing a disconnection.
ISDN sets send a Disconnect message to disconnect the call. For trunks using in-band signaling, on-hook is signaled using the signaling bits within the voice channel. For SS7 trunks, a Release message is the signal to disconnect a call.
Figure 5-11 shows a typical call setup sequence for a line-to-trunk call. For these calls, the originator dials a number and the digits are collected and processed according to the originating line's dial plan. The dial plan yields a result and points to a list of routes to another switching node. The route list contains a list of trunk groups, from which one group will be selected, usually based on primary and alternate route choices. After the group is selected, an actual trunk member (digital timeslot) is chosen for the outgoing path. The selection of the individual trunk member is typically based on standardized trunk selection algorithms, such as:
Figure 5-11. Basic Origination Call Processing
Both a call origination endpoint and a call termination endpoint have been established in respect to the digital switch processing the call. The connection can now be made through the switching matrix between the two endpoints. The timing of the actual speech path cut-through between the external interfaces varies based on many factors, but the switch now has the information it needs to complete the full connection path at the appropriate time, as determined by software.
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