December 10, 2015

Steering of Roaming: Active Redirection vs Passive Tapping


Steering systems are quickly becoming a de facto tool in a roaming departments toolkit. Without one, Operators are at a significant disadvantage when negotiating IOT discounts with their roaming partners and ensuring a high QoS for roamers. There are many factors to consider when evaluating roaming systems that mainly lie in the capabilities of the application, but one very important decision is that of the system architecture.

Steering can be either SS7 based or OTA based. Some systems, are hybrid based, which provide both SS7 and OTA steering capabilities. No matter the steering methodology in use, both require the inspection of Update Location messages from visited networks. There are two ways in which these registration requests can be monitored, via Active Redirection or via Passive Tapping.

Active Redirection requires integration with the STP (Signalling Transfer Point) that is responsible for processing international traffic. The STP will be configured to detect when an Update Location message is seen, and it will send this message to the steering system. With Active Redirection, the steering system is a separate network element that the STP is aware of.

Passive Tapping simply inspects messages as they are transferred into the home network. This tap has no control over the Update Location message and simply records the fact that it has seen one. It requires no integration with any core network element such as an STP.

There are Pro’s and Con’s to each architecture, let’s examine a few different factors:

Passive-Steering Pros

–    No Core Network Changes
The strongest argument in favour of Passive-Steering is that it is risk averse. With a Passive system, there is no modification of the core network needed (i.e. STP routing rules needing change). This makes a Passive System a Value Added Service as opposed to a core network concern. This may involve less teams being involved in the deployment and less inter-team co-operation needed. The core network team should be familiar in applying these changes however, in addition to this being a once off configuration with no on-going changes needed.

–    Risk Factor
What if the steering system goes down? Will my roamers service be affected? With Passive-Steering, if the system fails, roaming service remains unaffected. This is true. However, it’s equally true for Active-Steering. The STP will be configured to send Update Location messages to the Active-Steering system as a primary destination. If there are problems with the steering system, the STP simply uses it’s secondary/backup route, which will be the HLR. So roaming service is completely unaffected.

Passive-Steering Cons

–    Anti-Steering Vulnerabilities
Passive-Steering is of very high risk to anti-steering from non-preferred roaming partners who wish to boost their inbound roamer revenue. If this roaming partner, monitors and drops error messages from the home network, then the real response from the home HLR will be delivered to the visited VLR and allow the attach to complete. The Passive-Steering system will be powerless to stop this anti-steering methodology as it is not in control of the message flow. And whats worse is that it will have no visibility that this is occurring and will have it’s roamer distribution figures way off. This anti-steering methodology is impossible with an Active-Steering system as the HLR will not send a response to the visited network when rejecting a registration request.

Active-Steering Pros

–    Cost of Signalling
The premise of SS7 Steering is to analyse when an outbound roamer requests to attach to a non-preferred network and reject that registration request. With Active-Steering this Update Location is sent to the steering system, and the system simply drops the message from the network (and optionally sends an error message back to the visited network).

Passive-Steering cannot drop this message as it is simply a tap on the wire only viewing messages. In order to stop this registration request from completing successfully, an error message is injected onto the network destined for the visited network. The hope here is that the error message will reach the visited network before the home HLR responds with an Insert Subscriber Data and Update Location Response message (which would authorise and allow the outbound roamer to attach to the non-preferred network).

So with Passive-Steering, the Update Location is not blocked, but rather an extra error message is inserted onto the network. The entire registration transaction is still completed, but it is simply rejected on the visited VLR. This means that up to 5 signalling units are processed unnecessarily by the home network and up to 4 signalling units are sent over the international links unnecessarily. Sending messages over the international links costs the Operator, it’s the equivalent of throwing away money.

–    Optimised Network
As discussed above, additional unnecessary signalling occurs with Passive-Steering. In addition to the cost of sending international traffic when it is not required, core network elements are needlessly utilised. The HLR will process Update Location messages that are being rejected by the steering system, in addition to creating and sending of Insert Subscriber Data and Update Location Response messages also. This results in unnecessary load on the HLR in addition to costing money in licencing if the HLR has a bandwidth based licencing model. In general it is a poor policy to have redundant messages passing though the network.

–    Steering Accuracy
The premise of steering subscribers using Passive-Steering is that the injected error message reaches the visited VLR before the HLR responds to the original Update Location message. Systems based on timing like this are prone to error. If the HLR response reaches the visited VLR first or if the error message never reaches the visited VLR for whatever reason, the outbound roamer will be permitted to attach to the non-preferred network. What’s worse is that the Passive-Steering system cannot determine if the error message was indeed successful in rejecting the registration request. This will affect both reporting figures in addition to the overall distribution of the outbound roamers in relation to the steering polices defined.

In conclusion, you need to be an active element to take/have control over the signalling flow in order to control your own eco system. Without this level of control, you are not in control of your outbound roamers nor are you in control of the messages that are bouncing around your core network. The Cellusys Steering of Roaming application uses Active-Steering with combined OTA and SS7 methods to achieve highly accurate steering with full control over your network and your outbound roamers. To learn more, please see our Steering of Roaming page or send any questions/comments you may have to

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December 10, 2015