Signaling System No. 7


International Telecommunication Union Telecommunication Standardization Sector (ITU-T) developed the different signaling system SS6 in 1997 Caro et al, (2003, pp. 56–63). This was later upgraded to SS7 in 1980 as the international standard by ITU-T. The SS7 replaced SS6 because the former was restrictive thereby had limited functionality and could not be seamlessly connected to the digital systems. However, it is important to note that amongst the early signals system, SSR1, and the signaling system (SSR2) were robust and flexible in nature and are still used. On the other hand, SS5 was retired due to limited functionality and call insecurity (Snow, &, Weiss, 1997, pp. 197-213).

Key words: ITU-T, SS7, call blocking, and factors

Table of Contents

Abstract 2

1- Introduction. 3

1.1Research aim and objectives. 3

1.2 The research objectives include. 3

2-Literature review.. 3

3. Functionality of the SS7. 4

3.1 Infrastructure. 4

3.2- How the Signaling System No. 7 works. 5

3.3 SS7 Link Interface. 6

3.4 SS7 Signaling Link Types. 6

4. History. 7

5. SS7 outages. 8

6. Future of Signaling System No. 7. 9

7- Conclusion. 9

8. References. 10

1- Introduction

According to Fu, Atiquzzaman, &, Ivancic, (2000, pp. 141–48), The PSTN has a system for adding the information required to set up and manage all telephone calls to the public switched telephone network called the Signaling System No. 7 (SS7). The SS7 refers to a set of telephony signaling protocols. Therefore, SS7 enables the PSTN to separate call in different networks and not on the same networks in which the telephone call is running. The singling information in the SS7 is mainly in form of digital packets. The SS7 uses out of band signaling control-to control information traveling over channels. For example, the information may be made to travel on a dedicated 56kpbps channel separate from the line in which the telephone call runs.

1.1Research Aim And Objectives

The aim of this research is to analyze the causes and factors contributing to the outages of SS7 disasters prone areas

1.2 The Research Objectives Include

To analyze the cause of SS7 fluctuations

To determine the cause of SS7 outages in PSTN in major cities

To analyze the effect of SS7 outages and develop solutions to the SS7 outage

To recommend ways to minimize and or eliminate SS7 outages

2-Literature Review

In the past most of the signaling was through in-band signaling. However, with the SS7, it is easy to set up telephone call more efficiently and securely. Additionally, most of the telephone subscribers have benefited from value added services such as call forwarding and roaming services. Currently, the SS7 is considered an international telecommunication standards which means it is accepted in almost all countries with telecommunicate infrastructures. The main services that can be realized by deploying an SS7 include:

Call connection set up and management

Connection termination after completing call

Telephone billing

Managing all incoming and outgoing calls

Caller identity display (both number and name) as well as any other intelligent networks services

The SS7 can also enable toll free calling (800, 888, as well as 900) calls

Finally, SS7 can enable wireless and landline call services. The most common application of the SS7 is the mobile subscriber and personal communication services

SS7 can also enable call routing to other alls. For example if the route to any network point is crowd, SS7 can enable priority 2 or 3 calls to the same network point. Intelligent network services can check if a subscriber to a number is valid and enable call setup to a call. Finally, the other benefit is that it does not allow call system security violations. The current application of SS7 is in integrated services digital network (ISDN) mainly riles on out- of-band signaling and extends it to the end users. End users use the D-channel as the voice and data flows through the B-channels

3. Functionality Of The SS7

3.1 Infrastructure

The SS7 is comprised of a set of reserved channels that signal links between the networks points. The signaling points or network points are of three major types: – the Service Switching Points (SSP), the Signal Transfer Points (STP), and the Service Control Points (SCP). The main function of the SSP is to terminate calla or originate call through the entire SSP network. Their communication with the SCP helps in determining when and how a call can be routed. This functionality also helps in setting up and managing calls plus any other special feature. All the traffics through the SS7 networks are mainly routed through the packet switches known as Signal Transfer Points (STPs) the STP and SCP are responsible for call continuation in the event of service failure between points.

3.2- How The Signaling System No. 7 Works

The PSTN was developed before the SS7. However, the demand for better functionality leads to the development of SS7. The SS7 is integrated into the PSTN. Additionally, the 800 portability acts necessitated the installation of the SS7. The SS7 helped the IXC cut the post-call delay, and access ad or egress charges. There were also federal regulation, and the opportunity for better revenue with the realization of the SS7 capability. These are the main reason why the SS7 was deployed into the already existent PSTN.SS7 reduced overall disruption of the networks. Before the SS7 is linked to the PSTN, it is important to ensure that the two systems are compatible. In most cases, the telecoms companies had to purchase additional hardware as well as other software upgrades t help in linking the hardwires to the digital switches (SSP).

However, many telecoms companies either use their existing system if they are compatible with the movement regulations. The SS7 only uses the time slots on the established trunk facilities meaning it was not an expensive venture (Sastry, &, Shahnawaz, 1990, pp. 51 – 57).

The SS7 uses the STP node and the SCP node overlay the existing signaling facilities

3.3 SS7 Link Interface

The SS7 link is deployed over the existing PSTN trunk in each time slot. For example, the T1 and E1 both have an SS7 deployed. This enables the signaling link to travel on the digital trunk transmission medium over the entire network. For each noted, the SS7 interface equipment is expected to extract a link time slot from the digital trunk to enable faster processing. The processing is mainly done in the channel bank or ion the Digital Access and Cross-Connect (DAC). The DAC multiplexes the TDM time slots from each digital, stream thereby enabling the time slots to be processed at a time. The individual SS7 link provides the SS7 messages to the digital switch for processing. While implementations vary, dedicated peripheral processors usually process the lower levels of the SS7 protocol (Level 1, Level 2, and possibly a portion of Level 3); call- and service-related information is passed on to the central processor, or to other peripheral processors that are designed for handling call processing–related messages. Of course, this process varies based on the actual equipment vendor (Conway, 1990, pp. 552 – 558).

3.4 SS7 Signaling Link Types

The SS7 signaling link types are names from A to F based on their functions in the SS7 singling network. For example;

A Link: An “A” (access) link- the main function of the access link to mark the end of an SCP or the SSP to an STP. It signals the STP that the SCP or SSP has reached its end

B Link: A “B” (bridge) link -= this link’s main function is to connect any of the STP to the other STP found in anther network.

C Link: A “C” (cross) link- this is the link responsible for connecting all the STPs that perform similar functions. It is mainly an alternative route for the STP whenever there is a link failure. Most telecom companies deploy SCP in pairs to increase the networks reliability, and survivability

D Link: A “D” (diagonal) link- the D link mainly connect the secondary links to the regional STPs. In such case therefore, the secondary STP in the Sam networks are connected suing he same D link

E Link: An “E” (extended) link connects an SSP to an alternate STP.

Finally, the F Link: An “F” (fully associated) link is threw links used to connect two or more single end points in a networks.

4. History

Common Channel Signaling or common-channel interoffice signaling (CCIS) refers to the transmission of signaling or control information on different channels from data. In most case, it is important to note that the signaling channel is responsible for controlling multiple channels of data. Most of the telephone companies developed the CCS in 1975 (Snow, &, Weiss, 1997, pp. 197-213).

According Enriquez, &, Patterson, (2002), the main mode of functionality of SS5 was limited by in-band signaling and could not accommodate out of and signaling. The in-band signaling call set up was completed by alternating multi-frequency tones into the bearer channels. This contributed to falsing through common channel interoffice singling system (CCIS) because it was not easy t separate the signaling channels from the bearer channels. The signaling system was also expensive, as it required the telecommunication companies to deploy a separate channel to handle the signaling thereby reducing signaling speed. It was also difficult to balance the signaling speed and the holding time of the bearer’s channels. In most cases, the telecommunication companies registered higher fluctuations in the number of signal channels. Combining the data and calling channels in SS7 increased the number of social channel sadly and lead to the development of IP (internet protocol). There are two variants of SS7- the national variant and international variants (Chayanam, 2005).

5. SS7 Outages

Most SS7 outages are experienced further major disasters. For example, during hurricane Katrina, major cities across the US could not receive any communication singles due to station or booster problems. Additionally, after 9/11, most of the communication channels either were blocked by the government or were interfered with by virtue of the magnitude of the blast. While there has never been a better explanation theories indicate that blasts affected the survivability of SS7 is very low. The blasts interfered with the frequency, size as well as duration of the SS7 outage. Additionally, there most calls were blocked ion the days as a security measure by the federal communication commissions. While Bajaj, (2007), argues that the impact of blast and other natural disaster reduces the survivability of SS7, it is a fact that the magnitude of the blast resulted into huge impacts on the booster station. However, it is also unclear if human activity could have contributed to the length of the SS7 outrages (Munjure, Mamunur, &, Islam, (2010).

6. Future Of Signaling System No. 7

The SS7 has been fully adopted as the backbone of both national and international communications system. However, the fact that many companies are moving from the legacy network such as G2, and G3 (SS7 protocol stack) to 4G LTE, it is important to note that the SS7 will increasingly be used as the backbone of 4G LTE operation. In this regards, the telecoms companies should focus on how to bring the cost of communication down. For example, it has been establish that SS7 can enable telecoms companies achieve 4G LTE without having to invest further in the purchasing and orsubscritoytion.SS7 through the 4g LTE.

7- Conclusion

SS7 outage is caused by various factors for example, most of the outages are caused by human activity, equipotent fail tire a quell as fiber cuts. Human activity l causes A-Link loss while anklets 40% of the outages are caused by SS7 switch process. The longest SS7 outage was caused by the fiber cuts and equipment failures. The fiber costs cause point-to-point loss in A-Link. The isolated access lines and blocked calls were caused mainly by equilemnt failtires and direct ncasues were as aresult to point to point A-Link and SCP failure. Therebiore, it ismimportant to note tht SS7 outage can be caused by factors. It is important to analyse th oputage and understand the SS7 conneciton becaue the Common Channel Signaling System No. 7 (SS7 or C7) is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector. Whiletmeh SS7 has helped in m,porivng tyr overla network optreaiosn, it has also helpin provividn value added sseries to network operators and users. It is therfiore impoortant tom determine how SS7 can be used effectively to help impore the deployment and staility of The 4G LTE.

8. References

Bajaj, G. (2007). A Pre and Post 9-11 Analysis of SS7 Outages in the Public Switched Telephone Network. (Electronic Thesis or Dissertation). Retrieved from

Snow, A. and M. Weiss, “Empirical Evidence of Reliability Growth in Large-Scale Networks.”, Journal of Network and Systems Management 5, no.3 (June 1997):197-213

P. Enriquez, A. Brown, and D.A. Patterson, “Lessons from the PSTN for dependable computing”, In Workshop on Self-Healing, Adaptive, and SelfManaged Systems (SHAMAN), New York, June 2002.

Chayanam, Kavita, “Analysis of Telecommunications Outages due to power loss”,MS Thesis. Ohio University, June 2005

A. L. Caro et al., “SCTP: A Proposed Standard for Robust Internet Data Transport,” IEEE Comp., vol. 36,

no. 11, Nov. 2003, pp. 56–63 S.

Fu, M. Atiquzzaman, and W. Ivancic, “Effect ofDelay Spike on SCTP, TCP Reno, and Eifel in a WirelessMobile Environment,” 11th Int’l. Conf. Comp. Com

A. Jungmaier, “Performance Evaluation of the Stream Control Transmission Protocol,” Proc. IEEE Conf. on High Perf. Switching and Routing, Heidelberg, Germany, June 2000, pp. 141–48

John G. Van Bosse and Fabrizio U. Devetak (2007). Signaling in telecommunication networks (2nd ed.). John Wiley and Sons. p. 111

C. Breen and C. A. Dahlbom “Signalling systems for control of telephone switching”, Bell Syst. Tech. J., vol. 39, pp.1381 -1444 1960

A. E. Ritchie and J. Z. Menard “Common channel interoffice signalling: An overview”, Bell Syst. Tech. J., vol. 57, pp.221 -250 1978

Conway, A.E. “Queueing network modeling of signaling system No.7”, Global Telecommunications Conference, 1990, and Exhibition. ‘Communications: Connecting the Future’, GLOBECOM ’90., IEEE, On page(s): 552 – 558 vol.1

Sastry, A.R.K.; Shahnawaz, I. “A simulation model for performance evaluation of ISDN user part basic call control procedures”, Military Communications Conference, 1990. MILCOM ’90, Conference Record, A New Era. 1990 IEEE, On page(s): 51 – 57 vol.1

Munjure Mowla, Mamunur Rashid, &, Shohidul Islam, (2010). Signaling System No. 7 (SS7) in Telecommunication Networks.



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