Forecasting SS7 Traffic

SS7-traffic stability is crucial to subscribers' quality of service (QoS). Make sure your network is prepared to handle the demands of subscriber growth and enhanced services.

Most wireless-service providers today spend the majority of their time deploying the latest revenue-generating applications to both acquire customers and retain the ones they already have. However, most of these new services (and some not so new) require close attention to the basic SS7 traffic being generated by them. The most common services are roaming and basic wireless-call delivery. But new services based on SMS and rapid subscriber growth can be devastating to the SS7 networks and the nodes that send and receive this traffic.

SS7 comprises the basic underlying network and transport layers for most mobile telecommunications traffic. Any network based on GSM, ANSI-41 and even PDC (personal digital cellular in Japan) technology requires SS7 to perform all of the signaling required for mobility management, call delivery and feature application. This signaling is based on mobile application part (MAP) protocol transactions that are sent primarily among the MSCs, HLRs and voice-location registers (VLRs) in the wireless network.

Mobility management provides the functions of registration, identification, service qualification, location management and authentication for each subscriber in the network. Each time a mobile phone is powered on, a series of SS7 transactions occur that identify the subscriber as valid, authenticate and register that subscriber, enabling wireless service. Each time a mobile call is made, SS7 transactions are used to qualify the subscriber for service and allow the call. These transactions are based on the SS7 transaction capabilities application part (TCAP).

SS7 ISUP signaling is used to establish the call, set up voice trunks, manage the call while it is in progress and finally tear it down when it is completed. Each time a mobile call is received, SS7 transactions are used to query the HLR and serving MSC/VLR to deliver the call to the subscriber at the proper location. ISUP also is used to set up and tear down the voice trunks for the call.

With more than 100 million subscribers in the United States today, the amount of SS7 traffic becomes alarming based on the number of calls made and received by wireless subscribers. Add to this SS7 signaling traffic new SMS service traffic (which is transported using MAP-over-SS7 signaling). As an example, the most common SMS application message traffic in GSM is based on transactions used to notify subscribers of message-waiting status for short messages or voice mail. Again, add other SMS transport-based application services such as mobile e-mail and other custom enhanced services. It now becomes readily apparent that proper SS7-traffic engineering and forecasting is necessary to retain a high QoS for wireless subscribers.

To perform proper SS7-traffic engineering for a given number of subscribers, it is important to understand some basic SS7-traffic-engineering terms:

- SS7 signaling link. A bi-directional transmission path for signaling, consisting of two data channels operating together in opposite directions at the same data rate. A signaling link consists of a physical signaling data-link terminal together with its transfer-control functions. The signaling link provides reliable transfer of signaling messages between nodes in the SS7 network.

- SS7 traffic. The aggregate of all SS7-message-signal units sent and received by the SS7 signaling points that service the network.

- Traffic capacity. The maximum traffic-per-unit time that the SS7 communications system can carry under specified conditions.

- Traffic intensity. A measure of the average occupancy of an SS7 transmission-link facility during a length of time, normally a busy hour, measured in erlangs and defined as the ratio of the time during which the facility is occupied continuously or cumulatively to the time that the facility is ready and available for occupancy. A traffic intensity of one erlang means the continuous occupancy of a facility during the time span under consideration, or the time-span measurement, regardless of whether or not information actually is transmitted.

- Traffic load. The total traffic carried by a trunk or trunk group during a specified time interval.

- Erlang. A dimensionless unit of average traffic intensity typically made during the busy hour (the 60-minute period during which the traffic load is greatest for the day.) The number of erlangs is the ratio of the cumulative time the facility is occupied to the time it is available for occupancy. Erlang B is used when calls are made randomly and when lost calls are cleared, not held. This is the case for mobile subscribers. If they can't connect to the system, the call is cleared, not held.

- Busy-hour call attempts (BHCAs). The (estimated) number of calls attempted from an average subscriber during the busiest hour of the busiest day of a normal week.

SS7-traffic intensity is measured in erlangs. The number of SS7 signaling links required can be based upon a traffic-engineering model. The assumptions of the traffic model include the following basic parameters:

1. Total number of subscribers supported initially and growing to some maximum.

2. Amount of roaming traffic. Typically about 10% of all subscribers are roaming at any given time.

3. Number of BHCA per subscriber.

4. Link load factor (based on the amount of traffic per signaling link to determine the number of signaling links).

5. Number of transactions per hour per subscriber (based on the number of signaling transactions required by an average user). A transaction is defined as all SS7 signaling messages related to a given query. For example, a location update usually involves one transaction of two SS7 messages each.

These parameters need to be determined by the wireless-service provider and can be obtained by interpolating network-traffic-engineering measurements today. (See Figure 1 on page 62.)

Note that in Figure 1 the primary assumption is the number of BHCAs. One BHCA per subscriber is probably very high and varies from provider to provider. Also, this model does not include the application of additional call features such as call waiting and call forwarding. These parameters can be added in based on the number of feature invocations per subscriber in the busy hour.

SS7 networks are designed to be the most robust and reliable networks in existence. They are highly over-engineered to maintain this robustness, hence the engineering of the network to 20% of its capacity (i.e., the link-loading factor). This parameter can be as high as 40%, but anything over this figure can put the network in danger during extremely high-peak traffic loads.

Because of the rapid growth of wireless-market penetration, wireless-service providers need to be aware of the effects of the additional traffic on the SS7 network. Proper traffic modeling will lead to efficient and safe engineering of the SS7 network.