Is USSD Hungry For SS7? (The Answer is No)
When launching new services, mobile operators are often concerned if these services will inhibit existing services or infrastructure. In particular, USSD services are often under suspicion to exhaust radio channel capacity and to induce voice channel breakdown. It is evident that a service exposing operator’s primary asset to a risk should be carefully examined before launch.
This article will discuss these concerns and possible risks.
Following USSD services are technologically possible:
USSD Phase 1 (GSM 02.90) service – a service launched by a mobile subscriber (phone initiated session or USSD Pull)
USSD Phase 2 (GSM 03.90) service – launched by a mobile subscriber or initiated by network (USSD Pull & Push)
Both USSD Phase 1 and 2 services can happen simultaneously with a phone call (Call Related USSD Services) or without voice connection (Call Independent USSD Services).
Call Related USSD services use FACCH (Fast Associated Control Channel) for data transfer, Call Independent USSD services use SDCCH (Stand Alone Dedicated Control Channel).
USSD supports a maximum of 160 bytes of user data per message (the upper limit can be less than 160 bytes depending on the underlying protocol layer).
The speed of the FACCH channel is approximately 140 bytes per second and that of the SDCCH channel is approximately 83 bytes per second.
Thus, any USSD message can be delivered in less than 2 seconds.
Call Related USSD messages are less interesting for commercial use since it is most unlikely that the user will be both calling and using USSD. It is possible, however, that a USSD network-initiated message is delivered while the subscriber is talking. Nonetheless, the risk of a voice channel breakdown is minimal that is supported by a fact that FACCH is not a dedicated channel but an associated one that means that it is implemented by stealing bits from the associated speech channel if one exists. If such bit stealing is allowed frequently and for long periods of time, it will certainly degrade speech quality to some extent but will not prevent the subscriber from speaking. In the case of a significant deterioration of speech quality, the situation can be improved on the side of operator by rearranging FACCH channel utilization priorities.
The situation is more complex in the case of Call Independent USSD services. It is actually a channel resource used for many purposes. These include establishing calls (including authentication), terminating calls, location updates, informing the network that the subscriber is still within the service area. Accordingly, without a free SDCCH channel a mobile phone is not able to access mobile services (i.e. the subscriber is not able to perform a call).
Furthermore, Short Message Service (SMS) and USSD messages are also communicated over SDCCH logical channels. With the above said, an effective use of resources of SDCCH logical channel is crucial for reliable and efficient mobile communication.
According to GSM 05.02 guidelines one cell can have up to 8 SDCCH channels. A typical cell configuration has 4 or 8 SDCCH channels.
One USSD session utilizes one SDCCH channel. It means that 8 simultaneous USSD sessions exhaust SDCCH channel capacity of one cell. Therefore, other subscribers served by this very cell would not be able to perform calls or send / receive SMS messages.
It seems like a serious objection to the use of USSD services. But it is not; in real life this situation does not happen.
A proof can be found in a German research paper “Performance Evaluation of GSM Signaling Protocols on USSD”. According to the paper, it is more probable that new call initiations would block existing SDCCH channels rather than new USSD sessions. It is supported by our 4 year long experience of USSD services with MTS – we actually never had problems of the kind.
Troubles can actually arise with mass-use of USSD in a geographically limited location, i.e. when service requests are concentrated in a small number of cells. This could happen within a network-initiated advertising campaign on a stadium, for example.
Such situations require additional attention on the side of the operator. First, every operator is able to decrease USSD session timeouts and thus decrease the probability of simultaneous service peak loads. One transaction within a dialog session is limited to 30 seconds and overall session length is limited to 600 seconds (it can actually differ on various hardware platforms), so there is some space to change these parameters. Timeout timers are defined in GSM 09.02.
Second, it is possible to define priorities for network services. Thus phone calls may receive the highest priority while SMS/USSD services may have a lower priority.