Resources
From the dial tone to the first spoken words, your telephone call sets in motion
a chain of events that transfers voice and data across the global telephony network.
Signaling is a vital communications component that provides telephone companies
with the ability to connect calls and offer services like caller ID, prepaid calling
cards and mobile roaming.
Ulticom's Signaling Resources section is designed to help you understand this vital
communications element. From a basic history of signaling, to a more advanced look
at signaling protocols, this section of the website provides a variety of information
about one of the least understood aspects of the telephone system.
Signaling Background
Since the invention of the telephone by Alexander Graham Bell in the 1870's, the
public switching network that started out as manually operated switchboards connecting
individual wires has evolved into an intelligent network - a network that is today
capable of more than just switching and counting the duration of telephone calls.
It took several decades, in fact nearly a century, to improve the public switching
network's ability to implement a universal numbering plan, recognize and route on
dialed digits to interconnect two end users over a network that is scalable nationwide
and beyond. Still by 1980, all it could do was to simply connect callers and charge
based on usage. The resultant service was deservedly called Plain Old Telephony
Service (POTS).
Back then, the network routed the calls by hunting inter-machine trunks in the direction
of the terminating central office and sending multiple frequency (MF) tones, in-band,
to repeat the dialed digits - in most cases just to find out that the terminating
device was unattended or busy, thus inefficiently using network resources.
With the introduction of SS7 standards in the early 1980s and realization by mid
1980s, the public switching network not only became efficient but intelligent. The
SS7 standards allowed implementation of a separate, out-of-band, network for signaling
between individual switches on the network.
The initial and obvious use of SS7 was call control - in the form of well-defined
messages and scenarios implemented within the ISDN User Part (ISUP) layer of the
protocol. The result was significantly improved call completion ratio and increased
network efficiencies, thus improved profits for service providers.
The next big gain was possible by the introduction of the Transaction Capabilities
Application Part (TCAP) layer of SS7. TCAP enabled switches to query databases and
the service logic resident in Service Control Points (SCP), "switch-less" nodes
on the SS7 network.
Through the adoption of finite-state machine based Intelligent Network (IN) call
models, SS7 capable switches, called the Service Switching Points (SSPs), were able
to transact with SCP based applications to inject intelligence to mid-call decision
points, opening the door to implementation of sophisticated call routing, payment
policies, and voice messaging.
The 800-service led this movement beginning with 1987. This was followed by the
introduction of Enhanced Service Platforms (ESPs), Service Nodes (SNs) and Intelligent
Peripherals (IP) to provide voice messaging services, and "follow-me" or "personal
assistant" type of adjunct call-control services. Local Number Portability capability
was added later to support the deregulation act of 1996, allowing subscribers to
change their service provider without having to change their directory numbers.
The SS7 signaling architecture was designed using packet switching principles, but
with very stringent and carefully calculated requirements, to deliver a robust,
real-time, highly scalable and universal network. While, primarily based on packet
switching technology, the SS7 network achieved Quality of Service (QoS) by using
circuit-switched point-to-point links for its Layer 1, the Physical Layer. This
design compromise helped SS7 to earn itself carrier-grade status from day one.
The SS7 network became so widely deployed and powerful that it found some initially-not-thought-out
uses moving forward:
Wireless capitalizes on SS7
The wireless industry adopted SS7 to implement mobility management, thus making
the PSTN's signaling and switching infrastructure reusable for the wireless service.
Proven signaling capabilities of SS7 and its extensibility to add new transaction
capabilities were key to this decision.
Mobile Switching Centers (MSCs) directly evolved from the wireline Class-5 switches
with a slight modification to the basic call model in order to route calls based
on information regarding subscribers roaming in and out of their home area.
Base Station Controllers (BSCs) integrated with the switching hierarchy as remote
switching units to the MSCs. SS7 was chosen even for BSC to MSC signaling, for the
purpose of relaying radio interface measurement and control messages, and text messages,
through a connection oriented interface, called the A-Interface, over the SCCP layer.
The roaming service was accomplished by implementing the notion of home and visiting
areas into Home Location Registers (HLR) and Visitor Location Registers (VLR), which
are SCP class equipment - transacting between themselves and MSCs using SS7 TCAP.
Subsequently, SS7 was used for store-and-forward short text messaging service, known
as SMS. This service not only used the TCAP layer to track down a roaming subscriber,
but went to the extent of using SS7 to transport the SMS messages between the Short
Messaging Service Centers (SMSC) and the subscribers via MSCs.
Last but not the least, SS7 became the easy choice to enable Location Based Services
(LBS). The new network components designed for this purpose, the Serving Mobile
Location Center (SMLC) and the Gateway Mobile Location Center (GMLC) use SS7 to
interact with the BSC and the VLR and between each other to compute in real-time
the precise coordinates of a subscriber for location sensitive services.
Future Signals...
Today, SS7 is the incumbent signaling technology and architecture allowing the participating
network elements to collectively deliver the services we discussed, creating a hosting
environment for value-added, programmable applications.
As networks evolve to deal with such notions as broadband access, multimedia content
and mobile data, the nature of network services and the role of signaling will not
drastically change. Nevertheless they will evolve to deliver additional value and
efficiencies while providing interoperability with and smooth migration from today's
networks to NexGen networks.
The core network services enabled by signaling can be grouped to these general categories:
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MOBILITY: Recognition and
authorization of subscriber access to network capabilities from anywhere within
wider network coverage area. |
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LOCATION: Detection and
delivery of precise subscriber positioning information to enable customized, location-specific,
emergency or commercial services. |
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PAYMENT: Real-time measurement
and processing of network utilization to support various payment policies. |
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SWITCHING: Capability
to establish and control (virtual) connections between multiple points in a network
for transmission of content.
|
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MESSAGING: Creation, storage
and forwarding of voice, text or multimedia content in the form of discrete messages. |