Central Office Switching Systems
Telecommunications Switch - A mechanical or electrical device, which opens or closes circuits, completes or breaks an electrical path, or selects paths or circuits. Looks at incoming -voice- or -data- signals to determine the destination address. Based on that address, a transmission path is set up through the switching matrix between the incoming and outgoing physical communications ports and links.
Can be a circuit switch (voice) or packet switch (data - router).
Hardware (H/W) and Software (S/W) components, becoming more and more software intensive (i.e. softswitches), and thus more intelligent.
Central Office (CO) - Usually a telephone company building where subscriber lines are joined to switching equipment for connecting other subscribers to each other locally or long distance. This chapter focuses on switching systems in the "local" CO, which might contain one or more switches. A local switch is typically referred to as "Class 5 Switch," representing the lowest level in the AT&T hierarchy established prior to its break up.
Depending
on H/W and S/W variations, a switch can function as either:
·
CO
Switch
·
Tandem
Switch
·
PBX
·
Mobile
Telephone Switching Office (MTSO)
The primary differences in these switches are the line circuits, trunk circuits, the maintenance features and the switch capacity, all of which vary according to the application.
DCOs can
be categorized based on the line capacity:
·
Community
Dial Offices (CDOs): capacity of approximately 10K lines.
·
Central
Offices (COs): designed for urban applications of 60K, or more.
CDOs and COs use similar technology, but architecture used by CDO limits its line capacity and it is not scaleable. When capacity is reached, CDOs must be replaced.
COs, on the other hand, have line capacity based on the call rate (CR), the number of calls within a span of time. If call rate is low, COs can support around 100K lines, and if the call rate is high fewer lines are supported.
Distributed Processing
Distributed Processing - encompasses a central high-speed processor, linked to several remote processors over a data bus. Processing is initiated locally, with resulting information sent to and combined in central processing.
Central processor controls primary call processing functions, such as routing a call's path through the network.
Secondary processors handle line scanning, reception quality, supervision etc. It is to be noted that these functions do not need the access of the system database
Central Office Memory Units
DCO contains three types of memory units:
· Generic Program: Provided by equipment manufacturer, common to all switching systems and resides in the program store memory and directs call processing. Developed when office is engineered, makes use of parameters to determine the quantities and terminal addresses of equipment.
· Data store: This memory unit contains the so called translations, which are unique to the office and are the inputs by the system administrator to enable the generic program to determine the working lines and trunks and associated features and provide trunk-routing information for inter-office calls. The line translation includes information about the class of service (residence, business, ISDN, PBX, Centrex, etc.), associated line number, optional features (call waiting and forwarding, 3-way, caller ID, DMTF, etc.) and status of line (working, temporary, disconnected, out of service, etc.). The trunk translations identify signaling and terminating characteristics of the trunk: method of pulsing such as dial pulse or multi-frequency, terminating office identity, type of signaling on trunk such as loop, E & M, use of trunk such as local, toll service circuit).
· Call store: This memory unit is used to store the recent changes in the information and the progress of calls. Changes include information change in line and trunk translations, which are not merged with the systems database.
Redundancy
Having one or more "backup" systems available in case of failure of the main system.
Service reliability in PSTN is critical, lives depend on it, regulations require it.
Switches have redundancy of critical circuit elements. There might be local loop failures that can interrupt individual lines but the total central office failure is very rare, 1 hour in 20 years. (Hinsdale Incident, May 8, 1988 - Outside Chicago, C/O burns to ground
Redundancy
is provided in one of the three bases:
·
Shared Load - Identical elements divide the
total load, When one fails ... others can support services to all users without
loss of service.
·
Synchronous - Regular/Duplicate elements perform
same functions synchronously (at the same time), but only one element is online
and the other is offline on standby. When online element fails, offline standby
unit accepts load without loss of service.
·
Hot Standby - One element is online, the other
waiting with power applied, but in idle condition. When regular element fails,
standby unit switches online with momentary interruption.
Maintenance and Administrative Features
DCOs include many features to monitor system's health from a local maintenance control center (MCC) or from a remote location. These features may be classified as fault detection and correction, essential service and overload control, trunk and line maintenance features, configuration management, and database integrity checks.
Fault Detection and Correction
The central processor monitors the entire system for any faults and also checks itself by built in diagnostic software’s and if any fault is detected it calls for an alarm condition to the MCC and switches to a duplicate element, if available.
Essential Service and Overload Control
Switching systems are designed for traffic loads that occur on the busiest normal business days of the year. Occasionally, switches overload (WTC Bombing 9/11/01 - confluence of events, equipment loss and high rate of calls in concentrated area)
Line load control circuitry makes it possible to deny service to non-essential users, so those essential users, like public safety and government employees, can continue to place calls.
Trunk Maintenance Features
Two sides to class 5 switch: trunk side faces toward network, line side faces toward subscriber. System maintains database for the trunk performance and faults. If a faulty trunk is detected an entry into the fault report is made, which is then used by the technicians.
Line Maintenance Features
System tests the line (equipment and local plant) for any fault regularly on routine or per call basis. Line Insulation Tests (LIT) are conducted during off peak time to find out and detect incipient trouble.
Configuration Management
When a CO is originally configured, portions remain static until the C/O is later upgraded. Then the Configuration Management System (CMS) modifies the parameters to inform the switch of the presence of new equipment. The CMS also enables technicians to enter order activity through terminal or through a file transfer (i.e. add/delete subscribers and change features).
Database Integrity
Any changes made to the lines; trunks or parameter translations are recorded in the database by authorized persons. An off line copy of the database is kept so that it can be reloaded if the primary copy is corrupted.
Line Equipment Features
DCOs constructed
modularly with line cards, permitting line concentration. Concentration ratio
is the ratio between lines and trunks in a concentrated carrier system or line
concentrator. Two architectures for line interface:
·
Coder/Decoder
(Codec) - per line architecture, each line contains a separate analog-to-digital
converter, more expensive.
·
Shared
Codec - output of the analog line circuits is switched to a group of shared
codecs, increased risk of service disruption.
Transmission Performance
Decreased variability of outside plant performance.
Distributed Switching
As subscriber bases increase, current computing technology makes it more cost effective to bring switching function closer to subscriber than to keep switching function centralized and feed additional or enhanced copper wire infrastructure closer to the subscriber.
Remote switches are deployed in remote terminals miles from the C/O, normally connected by fiber optics and performing function of feeder relief.
Two types
of remote switching:
·
Remote
line switching - contains switching matrix, calls are switched through intracalling
link.
·
Remote
line module - requires two C/O links, umbilical cord
Trunk Equipment Features
Trunk interface
circuits must be capable of supporting multiple interoffice protocols, including:
·
Feature
Group A (FGA): Line side interface to IXCs and Subscribers
·
Feature
Group B (FGB): Trunk side interface to IXCs and Subscribers
·
Feature
Group D (FGD): Equal access protocol, permits Subscribers to prescubsribe to
a particular IXC for all long distance calls by dialing 1.
·
Signaling
System 7 (SS7): The SS7 network sets up the Circuit that will carry the voice
from the originating telephone to the destination telephone. It does this by
receiving and forwarding messages to all the network elements or switches involved
in the call. The SS7 network overlays the existing voice network. It offers
service providers a reliable infrastructure to deliver enhanced services, by
transporting call setup and tear down information over links that are apart
from the actual circuit used for speaking. This capability, part of the Intelligent
Network architecture, has no counterpart on IP networks. For service providers
to offer services while there are transitioning from circuit switched networks
to Packet switched networks, there is a need for a device to interpret the signaling
messages of the SS7 network to a form that the IP or packet network can understand.
Industry is rapidly developing devices to accomplish this task. The SSP collects
the digits from the calling party (originating phone) and formulates an Initial
Address Message (IAM) to the STP. The STP passes this message to the next switch
in the network. The Next switch responds to the STP with an Address Complete
Message (ACM). The STP sends this message to the originating switch. The voice
path (circuit) between the originating switch and the destination switch is
completed. Ringing is applied. When the Telephone at the destination switch
goes off hook, the destination switch sends an Answer Message to the STP. This
message is forwarded to the originating switch. When the switch receives the
message conversation between the two parties can take place.
·
In-band
signaling - signaling between C/Os and between C/Os and Subscribers using T1/E1
interface.
·
D-channel
signaling - PRIs between C/O and subscribers exchange signals over the D channel.
Dialable Bandwidth
Some services such as video conferencing and imaging may require more bandwidth than the available 64 KB circuits. Some ISDN-equipped DCOs can multiplex (transmit 2 or more channels over a single channel) channels to provide a capacity of N C 64 service.
Local Central Office Equipment Features
Local central offices contain several peripheral software and hardware components that facilitate maintenance and special software features.
Trouble Indicating and Alarm Systems
Stored Program Control switches have internal diagnostic capabilities. In addition to audible alarms, trouble-indicating registers on the maintenance console can be printed, so that a backup system can be put into operation. In the case of remote equipment, alarms can be transmitted to surveillance centers.
Automatic Number Identification (ANI)
ANI automatically identifies from look-up tables, the calling party for billing purposes. It identifies the party to be billed in case of two party lines.
In case of single party lines, identification is obtained from their line circuit. In electronic offices this is a table lookup function. In electromechanical offices, separate equipment translates the billed telephone number from the line equipment.
Automatic Message Accounting (AMA)
Automatic Message Accounting interrogates the ANI to determine the identity of the calling party. Can be classified as local (LAMA), handled through the local C/O or centralized (CAMA).
Records details at each stage of connection, called/calling parties numbers initially registered, time of connection/termination, common identifying number for storage reference.
In case of four-party lines the ANI might not provide the identity, in which case the operator is bridged in on the line to receive the calling party’s number and key it into the AMA equipment.
Coin Telephone Interface
Coin telephones are assigned to a separate class of service that provides access to special coin trunks. The coin trunk supports either a Dial tone first, Coin first or a Post pay type of telephone.
Common Equipment
The common
equipment to all the types of central offices are:
·
Power
equipment
·
Local
Measured Service (LMS) equipment
·
Traffic
Measuring equipment
·
Network
Management
The Advanced Intelligent Network (AIN)
An AIN provides service based on customized logic located in the network. Central offices use SS7 Network to communicate with the central nodes. Makes otherwise costly upgrades to equipment more practical.
The SS7 links with a central database known as the Service Control Point (SCP). The SCP responds with a message and instructs the switch on how to handle the call.
Local Number Portability (LNP)
The provision of local number portability (LNP) is one of the obligations that the Telecommunications Act of 1996 ('96 Act) imposes on all carriers offering local services in order to foster a pro competitive, deregulatory national policy framework. Number portability comes in several forms (service provider LNP, service flavor LNP, geographic LNP, etc.). Congress recognized that LNP will lower barriers to entry and promote competition in the local exchange marketplace by enabling customers to switch to a new local service provider without having to change their telephone numbers. In that spirit, number portability permits consumers to select a local telephone company based on service, quality, and price, rather than on a desire to keep a particular telephone number.
No telecommunications provider is immune to the issues of deploying LNP. This mandate is faced by all, (but clearly the greatest burden is on Incumbent LECs) regardless of network technology. In its initial rule making, the FCC directed the North American Numbering Council (NANC) to provide national oversight of LNP administration. At that time, the FCC declined to mandate a particular technology to implement LNP however, saying sufficient momentum exists within the industry to deploy compatible methods nationwide. Location Routing Number (LRN) technology emerged as the dominant mechanism. LNP has been a huge undertaking, requiring significant infrastructure upgrades and a high level of cooperation between ILECs, CLECs and IXCs.
With LRN technology, all carriers' switches are assigned unique 10-digit numbers conforming to the North American Numbering Plan format. Any calls to an NPA-NXX in which number portability is available require a database check. If a dialed number has been ported, or moved to a different carrier, it is temporarily replaced with the location routing number of the new carrier's switch so that it can be routed there. At the new switch, the dialed number replaces the location routing number so that the call can be routed to the ported customer. If the number has not been ported, the call is routed to the original switch.
One of the larger questions was under what circumstances a particular carrier would be responsible for doing a database check for ported numbers. Many industry groups favored the "N-1" approach, which requires the second to the last carrier handling a call to do the lookup. For local calls between two carriers, the originating carrier would be responsible for checking the database. For local calls to and from customers served by the same carrier, that carrier would do the check. For long-distance calls, it would be the Interexchange carrier's responsibility. The FCC adopted this approach, finding this to be the most efficient and effective method of implementing LNP. Also, the FCC has outlined a cost recovery system for carriers performing database "dips" for other carriers that don't have their own systems but must deal with ported numbers.
LCOs provide a variety of services, which is primarily software controlled. Some of the principal features provided by most of the electronic end offices are listed below:
Call-Processing Features
These features allow the customers to recall the C/O equipment by a momentary on-hook flash. Features like calling waiting, calling blocking, call forwarding and distinctive ringing, etc are provided by the C/O equipment.
Other features like call forward remote access, which enables the user to activate or deactivate call forwarding from a remote location and gabline, a feature which allows user to dial a single number to which multiple callers can be simultaneously connected are also provided by the LECs.
Custom Local Area Signaling Services (CLASS)
Many LECs offer a package of services known as the CLASS. This brings the users most of the features that are available in the PBX. These features depend upon the SS7 (see above) for communication between central offices or through ISDN.
Some of the Principal features are Anonymous Caller Rejection, Automatic Callback, Automatic Callback, Calling Name Delivery, Calling Name and Number Blocking, Call Waiting and a load of other features.
This is not to be confused with the ANI. The difference between the two is rooted in how the signaling is handled in the C/Os.
The user cannot block ANI. On the other hand, if the call comes from a PBX equipped with ISDN trunks, the station number may show as the CLID if the PBX is programmed to send station numbers to the central office.
Most LECs offer centralized voice messaging service to their subscribers. Voice Messaging Services can include services such as the Automated Attendant, IVR, Fax messaging, Fax-on-demand, etc.
Centrex is a PBX like service, which is located at the C/O. Centrex allows Direct Inward Calling (DID) to a telephone number and Direct Outward Calling (DOC) from a number.
Emergency Reporting
When a subscriber dials 911, the local C/O switches an emergency call over a dedicated group of trunks to a public safety answering point (PSAP). Emergency operators have special features that enable them to trace calls and hold up a circuit to re-ring the calling party. Also, databases allow emergency operators to pull identifying information with an emergency call.
Routing to Service Facilities
Service and repair facilities.
Multiline Hunt
Rotary line group connects incoming calls to an idle line from a group of lines allocated to a user.
Call Processing
Calls are typically processed using scanners, signal distributors, registers, generic programming, call store, data store, time slots and network or switch fabric.
Applications
Selection of a Digital Switch is a complex process that depends on the maintenance strategies, service offerings, and cost objectives of the LECs.
Standards
Subscriber line and trunk interface standards are published by Electronics Industry Association (EIA).
DCO performance criteria are published by Telcordia Local Switching System General Requirements (LSSGR). This defines the features and technical specifications required of C/Os in North America.
DCOs operate at high frequency, hence Electro Magnetic Radiation shielded cabinets are used for the radiating components. These are governed by FCC regulations under Part 15, Subpart J.
The different criterions for evaluating a DCO are Cost, compatibility etc. Some of them are given below:
Maintenance Features and Reliability
DCOs should have a high degree of reliability. The processors must be capable of self-diagnosis and the system back should be done at a regular basis.
The manufacturer should provide technical assistance in solving the unusual maintenance problems.
Training and Documentation
The manufacturer should provide the technical documentation and training to the users in designing, engineering, maintaining the system.
Other features
Other features like the Environment, Earthquake Resistance, Multi-class Operation, Capacity and Technical Support are to be taken into account in evaluating the DCO.
Local Switching Systems, with the right kind of software can perform the function of a PBX, Tandem Switch or a MTSO (Mobile Telephone Switching Office), though the hardware difference is not so significant.
The key to the application lies in the features provided in its generic program. Some manufactures produce systems, which can actually handle all the three application by suitable variations in hardware and software.