Figure 1 - SS7 Signaling Points
SS7, also referred to as out-of-band signaling, is the common channel signaling protocol used for call handling within the Public Switched Telephone Network (PSTN). SS7 is the underlying data communications protocol used by telephone networks to control call set-up and call routing, and to provide services such as caller ID and CLASS features. SS7 offers telephone network management functions that are faster and more reliable, by managing voice circuit functions on a separate, fully redundant data network.
SS7 was designed in the late 1970s and early 1980s for exchanging call control information between the various network switches and databases in the PSTN. It replaces an earlier Common Channel Inter-office Signaling system (CCIS, or SS6 in ITU terms) and offers several important advantages, including greater speed and flexibility. SS7 is now widely deployed and is used to support services such as ISDN (Integrated Services Digital Network).
The global SS7 standard defined by ITU allows for national variants. Variants are defined by the national Telecommunications Companies (Telcos). There may be more than one national variant. ANSI is the international SS7 standard for T1 lines and ETSI is the standard for E1 lines.
The SS7 standard allows network elements in the PSTN to exchange information over a digital signaling network to support wireless (cellular) call setup, routing and control.
An SS7 network is a packet network operating in parallel with the public switched telephone network. SS7 networks are designed for extremely high reliability using traditional telephone company techniques, i.e., providing full redundancy. An IP Network (such as the Internet) achieves high availability with routing protocols that route around failures. An SS7 network achieves high availability by having two or more copies of everything - every router and every connection (link). SS7 networks are typically engineered so that no link is loaded to more than 40 percent of its capacity. In the event of a link failure, the remaining link can carry all the traffic with margin to spare.
SS7 network signaling uses relatively small messages that are efficiently packed. SS7 typically runs on 64-Kbps links. Since the messages are extremely compact, a single 64-Kbps-link set (that is, pair of links) is capable of carrying the message traffic for over 50,000 calls per hour.
SS7 allows a single flag byte between packets to denote both the end of the preceding packet and the beginning of the next packet. It requires the constant transmission and reception of small "fill-in" signal units (packets), which can bombard an HDLC processor with interrupts. And, with SS7, one must constantly monitor the error rate on each to identify links that may need to be removed from service. In combination, these features generally require special-purpose hardware and/or firmware rather different from that used with Ethernet, X.25, or traditional IP transports.
The SS7 network is basically a packet network that controls the operation of the PSTN and provides access for new services in the traditional telephone network.
The advantages of using SS7 (over other PSTN signaling systems) are numerous:
A growing numbers of service providers around the globe, from the major Local Exchange Carriers (LECs) and Inter Exchange Carriers (IXCs), to the wireless operators, mobile operators and smaller carriers, are opening up for those vendors who can connect to the SS7 network. In addition, corporate users are gaining indirect access to SS7 networks via gateway services offered by the major IXCs. As an example, a corporation with multiple, geographically dispersed call centers can improve operating efficiency by using their own computers and an SS7 gateway to specify routing for their inbound traffic on a call-by-call basis. SS7 is the interface to connect this new application logic to the traditional circuit-switched network.
The following Telephony service providers commonly use SS7:
SS7 messages are sent at 56-64 Kbps on bi-directional channels called signaling links. Each SS7 signaling link can support up to 4096 calls. Actual numbers depend on the network load and type of equipment. Out-of-band signaling allows for faster call setup and more efficient use of the voice channel. Out-of-band signaling also supports Intelligent Network (IN) services, which require signaling to network elements without voice trunks (e.g., database systems).
Each signaling point is uniquely identified by a numeric point-code. These include:
Each signaling point uses a routing table (e.g., based on prefixes) to select the appropriate signaling path for each message.
Figure 1 - SS7 Signaling Points
A typical SS7 network consists of the following signaling points:
The SSP is a switch that originates, terminates, or tandems (transfers) calls. The SSP is used to set up, manage and release voice channels and to complete calls.
The SSP may send query messages to a centralized database (SCP), e.g. to map a toll-free 1-800/888 virtual number to an actual number. The purpose of these queries is to enhance the functionality of the SSP by providing external information (such as routing information for the 800/888 destination mapping).
A tandem SSP tunnels the call information to the outgoing trunk, while a terminating SSP (which ends the SS7 signaling section of the call) does not. A terminating SSP is for example, when the call continues over a PRI line or an analog line to a customer. An SSP can be terminating for one call and tandem for another destination (the call goes through to another SSP or CO). An SSP can be tandem for all calls.
An STP acts as a hub or router for SS7 messages (including call setup, management
and teardown messages, call information and all database queries). This improves
network utilization and alleviates the need for a meshed network.
An STP can also act as a firewall to screen SS7 messages exchanged with other
networks. For that purpose it may perform global title translation - translating
OPCs and DPCs between SS7 sub-networks.
An SCP provides application services (such as 1-800 mapping). An SCP usually
drives a database to hold the permanent data. The SCP services, known as Service
Control Functions (SCF), provide data in response to queries.
SS7 network elements (such as SCPs, STPs and links) commonly appear in mated pair configurations (e.g., a pair of STPs serve a PBX), so that there is always an alternative route for traffic in the event that one element fails. Elements are often placed at separate geographical locations. Each site is physically connected to at least two other sites. A link-set is the group of links that service the same connection between points A and B. If one of the links (in a link-set) fails, the signaling traffic is rerouted over another link in the set. Traffic is also load-balanced among all links in the link-set. Error correction and retransmission allow for continued service in the event of any signaling point or link failure.
Figure 2 - SS7 Network Design for Availability
Figure 3 - SS7 Signaling Link Types
The following signaling links are based on the SS7 signaling network:
Hardware and software functions of the SS7 protocol are divided into functional abstractions called "levels". These levels map loosely to the Open Systems Interconnect (OSI) 7 layer model.
Figure 4 - SS7 Protocol stack
The message transfer part (MTP) is divided into three levels:
Protocol used to set-up, manage, and release trunk circuits that carry voice and data between terminating line exchanges. Used for both ISDN and non-ISDN calls.
TUP is an older variant of ISUP. In some parts of the world (e.g., China, Brazil),
TUP is used to support basic call setup and tear-down. TUP handles analog circuits
only. In most countries, ISUP has replaced TUP for call management.
Provides connectionless and connection-oriented network services and Global
Title Translation (GTT) capabilities above MTP Level 3. SCCP is used as the
transport layer for TCAP-based services.
Supports the exchange of non-circuit related data between applications across the SS7 network. TCAP is provided over SCCP.
Queries and responses sent between SSPs and SCPs are carried in TCAP messages,
e.g., to determine the routing number associated with a dialed 800 number and
to check the personal identification number (PIN) of a calling card user.
In mobile networks (IS-41 and GSM), TCAP carries Mobile Application Part (MAP)
messages sent between mobile switches and databases to support user authentication,
equipment identification, and roaming.
When a mobile subscriber roams into a new mobile switching center (MSC) area,
the integrated visitor location register (VLR) requests service profile information
from the subscriber's home location register (HLR) using mobile application
part (MAP) information carried within TCAP messages.
Figure 5 - SS7 ISUP Call Flow
The following table describes the ISUP Call Setup messages illustrated above.
IAM Initial address Message - contains the dialed E164 number address.
ACM Address Complete Message - voice channel open and setup is completed.
ANM Answer Supervision Number Message - the destination user answered.
REL Release Message - release the call.
RLC Release Complete Message - call release complete.
Table 1 - ISUP Call Setup Messages