Softswitch
Encyclopedia
A softswitch is a central device in a telecommunications network
which connects telephone call
s from one phone line to another, typically via the internet
, entirely by means of software running on a general-purpose computer system. Most landline
calls are routed by purpose-built hardware, formerly using physical switchboard
s, but softswitches are the dominant 21st century trend.
Although the term softswitch technically refers to any such device, it is more conventionally applied to a device that handles IP-to-IP phone
calls, while the phrase "access server" or "media gateway" is used to refer to devices that either originate or terminate traditional "land line" (hard wired) phone calls. In practice, such devices can often do both. As a practical distinction, a Skype
-to-Skype phone call is entirely IP (internet) based, and so uses a softswitch somewhere in the middle connecting the calling party
with the called party
. In contrast, access servers might take a mobile call or a call originating from a traditional phone line, convert it to IP traffic, then send it over the internet to another such device, which terminates the call
by reversing the process and converting the IP call back to ISDN digital or analog/PSTN format, and connecting it to a destination phone number.
A softswitch is typically used to control connections at the junction point between circuit-switched
and packet-switched networks. A single device containing both the switching logic and the switching fabric can be used for this purpose; however, modern technology has led to a preference for decomposing this device into a Call Agent and a Media Gateway.
The Call Agent
takes care of functions such as billing, call routing, signaling, call services and the like, supplying the functional logic to accomplish these telephony meta-tasks. A call agent may control several different media gateways in geographically dispersed areas via a TCP/IP link.
The Media Gateway
connects different types of digital media stream together to create an end-to-end path for the media (voice and data) in the call. It may have interfaces to connect to traditional PSTN
networks, such as DS1
or DS3 ports (E1
or STM1
in the case of non-US networks). It may also have interfaces to connect to ATM
and IP
networks, and the most modern systems will have Ethernet
interfaces to connect VoIP calls. The call agent will instruct the media gateway to connect media streams between these interfaces to connect the call - all transparently to the end-users.
The softswitch generally resides in a building owned by the telephone company
called a central office. The central office will have telephone trunks to carry calls to other offices owned by the telecommunication company and to other telecommunication companies via the PSTN.
Looking towards the end users from the switch, the Media Gateway may be connected to several access devices. These access devices can range from small Analog Telephone Adaptors (ATA) which provide just one RJ11 telephone jack to an Integrated Access Device
(IAD) or PBX which may provide several hundred telephone connections.
Typically the larger access devices will be located in a building owned by the telecommunication company near to the customers they serve. Each end user can be connected to the IAD by a simple pair of copper wires.
The medium-sized devices and PBXs are most commonly used by business that locate them on their own premises, and single-line devices are mostly found at private residences.
At the turn of the 21st century with IP Multimedia Subsystem
(or IMS), the Softswitch element is represented by the Media Gateway Controller
(MGC) element, and the term "Softswitch" is rarely used in the IMS context. Rather, it is called and AGCF (Access Gateway Control Function).
, call waiting
, last call return
and three way videoconferencing
, if implemented in the network, are implemented in the feature server. The feature server works closely with the call agent, and may call upon the media server to provide these services. These features do not require the subscriber to explicitly request them but tend to be triggered within the call handling logic.
Softswitches used for transit VoIP traffic between carriers are usually called class 4 softswitches. Analogous with other Class 4 telephone switch
es, the main function of the class 4 softswitch is the routing of large volumes of long distance VoIP calls. The most important characteristics of class 4 softswitch are protocol support and conversion, transcoding, calls per second
rate, average time of one call routing, number of concurrent calls
.
Class 5 softswitches are intended for work with end-users. These softswitches are both for local and long distance telephony services. Class 5 softswitches are characterized by additional services for end-users and corporate clients such as IP PBX
features, call center services, calling card
platform, types of authorization, and other features similar to other Class 5 telephone switches.
has come to dominate the media gateway market with their AS5X line of access servers, the softswitch market is highly fragmented, with many companies producing a variety of overlapping products. Because SIP
calling has come to dominate the telecommunications world, most companies compete in that arena.
model. Users rely on softswitch infrastructure managed by a third party to deliver services to their end users without owning or operating their own softswitch.
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
which connects telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
s from one phone line to another, typically via the internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, entirely by means of software running on a general-purpose computer system. Most landline
Landline
A landline was originally an overland telegraph wire, as opposed to an undersea cable. Currently, landline refers to a telephone line which travels through a solid medium, either metal wire or optical fibre, as distinguished from a mobile cellular line, where transmission is via radio waves...
calls are routed by purpose-built hardware, formerly using physical switchboard
Telephone switchboard
A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...
s, but softswitches are the dominant 21st century trend.
Although the term softswitch technically refers to any such device, it is more conventionally applied to a device that handles IP-to-IP phone
IP Phone
A VoIP phone uses voice over IP technologies allowing telephone calls to be made over an IP network such as the Internet instead of the ordinary PSTN system. Calls can traverse the Internet, or a private IP network such as that of a company...
calls, while the phrase "access server" or "media gateway" is used to refer to devices that either originate or terminate traditional "land line" (hard wired) phone calls. In practice, such devices can often do both. As a practical distinction, a Skype
Skype
Skype is a software application that allows users to make voice and video calls and chat over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system...
-to-Skype phone call is entirely IP (internet) based, and so uses a softswitch somewhere in the middle connecting the calling party
Calling party
The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....
with the called party
Called party
The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....
. In contrast, access servers might take a mobile call or a call originating from a traditional phone line, convert it to IP traffic, then send it over the internet to another such device, which terminates the call
Call termination
Call termination, also known as voice termination, refers to the routing of telephone calls from one telephone company, also known as a carrier or provider, to another.The terminating point is the called party or end point...
by reversing the process and converting the IP call back to ISDN digital or analog/PSTN format, and connecting it to a destination phone number.
A softswitch is typically used to control connections at the junction point between circuit-switched
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
and packet-switched networks. A single device containing both the switching logic and the switching fabric can be used for this purpose; however, modern technology has led to a preference for decomposing this device into a Call Agent and a Media Gateway.
The Call Agent
Call Agent
In a telecommunications system, a Call Agent is a Media Gateway Controller when used in the context of MGCP. It is concerned with the handling of specific services to users. MGCP is a server client protocol developed by Cisco, to make Industry Standard. MGCP is the only Server Client Voice...
takes care of functions such as billing, call routing, signaling, call services and the like, supplying the functional logic to accomplish these telephony meta-tasks. A call agent may control several different media gateways in geographically dispersed areas via a TCP/IP link.
The Media Gateway
Media gateway
A Media gateway is a translation device or service that converts digital media streams between disparate telecommunications networks such as PSTN, SS7, Next Generation Networks or PBX...
connects different types of digital media stream together to create an end-to-end path for the media (voice and data) in the call. It may have interfaces to connect to traditional PSTN
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
networks, such as DS1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
or DS3 ports (E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
or STM1
STM-1
The STM-1 is the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 155.52 Mbit/s. Higher levels go up by a factor of 4 at a time: the other currently supported levels are STM-4, STM-16, STM-64 and STM-256...
in the case of non-US networks). It may also have interfaces to connect to ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
and IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
networks, and the most modern systems will have Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
interfaces to connect VoIP calls. The call agent will instruct the media gateway to connect media streams between these interfaces to connect the call - all transparently to the end-users.
The softswitch generally resides in a building owned by the telephone company
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
called a central office. The central office will have telephone trunks to carry calls to other offices owned by the telecommunication company and to other telecommunication companies via the PSTN.
Looking towards the end users from the switch, the Media Gateway may be connected to several access devices. These access devices can range from small Analog Telephone Adaptors (ATA) which provide just one RJ11 telephone jack to an Integrated Access Device
Integrated access device
An Integrated Access Device is a customer premise device that provides access to wide area networks and the Internet. Specifically, it aggregates multiple channels of information including voice and data across a single shared access link to a carrier or service provider PoP...
(IAD) or PBX which may provide several hundred telephone connections.
Typically the larger access devices will be located in a building owned by the telecommunication company near to the customers they serve. Each end user can be connected to the IAD by a simple pair of copper wires.
The medium-sized devices and PBXs are most commonly used by business that locate them on their own premises, and single-line devices are mostly found at private residences.
At the turn of the 21st century with IP Multimedia Subsystem
IP Multimedia Subsystem
The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem is an architectural framework for delivering Internet Protocol multimedia services. It was originally designed by the wireless standards body 3rd Generation Partnership Project , as a part of the vision for evolving mobile...
(or IMS), the Softswitch element is represented by the Media Gateway Controller
Media Gateway Controller
A Media Gateway Controller is a system used in certain Voice over IPtelephony architectures. An MGC controls a number of dumb terminals, the Media Gateways...
(MGC) element, and the term "Softswitch" is rarely used in the IMS context. Rather, it is called and AGCF (Access Gateway Control Function).
Feature server as a part of softswitch
The feature server, often built into a call agent/softswitch, is the functional component that provides call-related features. Capabilities such as call forwardingCall forwarding
Call forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...
, call waiting
Call waiting
Call waiting , in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new...
, last call return
Last Call Return
Last-call return, also known as call return, is a telephony feature offered by telephone service providers to give a called party the time and telephone number of the last received call, and may also offer the facility to place a call back to the calling party.The feature is typically listed as...
and three way videoconferencing
Videoconferencing
Videoconferencing is the conduct of a videoconference by a set of telecommunication technologies which allow two or more locations to interact via two-way video and audio transmissions simultaneously...
, if implemented in the network, are implemented in the feature server. The feature server works closely with the call agent, and may call upon the media server to provide these services. These features do not require the subscriber to explicitly request them but tend to be triggered within the call handling logic.
Class 4 and Class 5 softswitches
VoIP Softswitches are subdivided into two classes. Class 4 softswitches and Class 5 softswitches.Softswitches used for transit VoIP traffic between carriers are usually called class 4 softswitches. Analogous with other Class 4 telephone switch
Class 4 telephone switch
A Class 4, or Tandem, telephone switch is a U.S. telephone company central office telephone exchange used to interconnect local exchange carrier offices for long distance communications in the Public Switched Telephone Network....
es, the main function of the class 4 softswitch is the routing of large volumes of long distance VoIP calls. The most important characteristics of class 4 softswitch are protocol support and conversion, transcoding, calls per second
Calls per second
Calls Per Second or CPS refers to how many telephone calls can be handled in a second.CPS is one measure of the performance of Switching systems....
rate, average time of one call routing, number of concurrent calls
Erlang unit
The erlang is a dimensionless unit that is used in telephony as a statistical measure of offered load or carried load on service-providing elements such as telephone circuits or telephone switching equipment. It is named after the Danish telephone engineer A. K...
.
Class 5 softswitches are intended for work with end-users. These softswitches are both for local and long distance telephony services. Class 5 softswitches are characterized by additional services for end-users and corporate clients such as IP PBX
IP PBX
An IP PBX is a business telephone system designed to deliver voice or video over a data network and interoperate with the normal Public Switched Telephone Network ....
features, call center services, calling card
Telephone card
A telephone card, calling card or phone card for short, is a small plastic card, sized and shaped like a credit card, used to pay for telephone services. It is not necessary to have the physical card except with a stored-value system; knowledge of the access telephone number to dial and the PIN is...
platform, types of authorization, and other features similar to other Class 5 telephone switches.
Media gateways and softswitches in practice
While CiscoCisco
Cisco may refer to:Companies:*Cisco Systems, a computer networking company* Certis CISCO, corporatised entity of the former Commercial and Industrial Security Corporation in Singapore...
has come to dominate the media gateway market with their AS5X line of access servers, the softswitch market is highly fragmented, with many companies producing a variety of overlapping products. Because SIP
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
calling has come to dominate the telecommunications world, most companies compete in that arena.
Hosted switch services
Due to the high costs of infrastructure and its maintenance, some softswitch solutions are now offered through a Hosted service providerHosted service provider
A Hosted Service Provider is a business that delivers a combination of traditional IT functions such as infrastructure, applications , security, monitoring, storage, Web development, website hosting and email, over the Internet or other wide area networks...
model. Users rely on softswitch infrastructure managed by a third party to deliver services to their end users without owning or operating their own softswitch.