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Reliability of SMS

Overview

When an SMS message is sent to a recipient, it is expected that it will deliver to the handset within seconds. However, delivering an SMS is a relatively complex process, and an SMSC takes several measures to ensure a reliable service.

SMS Reliability Features

An SMS may fail to deliver to a handset on its first delivery attempt for many reasons. When an SMSC is notified of a temporary problem either from HLR status information or an MSC acknowledgement, it can perform one or both of the following actions.

Re-send when handset is available
An SMSC may send a command (MAP_REPORT_SM_DELIVERY_STATUS) to a destination network’s HLR, requesting to be notified of when a subscriber handset is available to receive SMS.

A HLR knows that a subscriber is ready to receive SMS from one of the following actions:

  • If a subscriber was out of network coverage, and then comes back into coverage, the HLR is notified with a Ready-for-SM command.
  • If a subscriber handset was turned off or out of coverage for a long time, a Location Update command is sent to the HLR when the subscriber connects to the network.
  • If the handset experiences a temporary problem such as memory full, then when the subscriber deletes messages to free up memory, a Ready-for-SM command is sent to the HLR.

After receiving one of these commands, the HLR will send an AlertSC command to each SMSC that has requested to be notified that a subscriber is ready to receive SMS. It is this process that results in SMS being delivered to a handset when it is turned on.

Time-based retry schedule
In addition to sc-alert notification, an SMSC may attempt to deliver an SMS according to a timed schedule. The schedule that is used is usually dependent on the error that was received. Temporary errors such as communication failures would be retried more frequently, for example every minute for 5 minutes, then every 10 minutes for 1 hour, then every hour for 24 hours.

Longer lasting errors such as absent-subscriber may be retried every hour for 7 days.

Delivery reporting

The sender is able to determine whether their SMS message delivered to a recipient or not based on an SMSC generated delivery report. If a permanent failure error code is received by the SMSC after attempting to deliver an SMS, the SMSC can send a failed report back to the user, including the error code. Similarly, if an SMSC cannot deliver a message after retrying according to the retry schedule, and the message reaches the end of its validity period, an Expired report is generated.

A delivered report is generated only after the subscriber handset confirms receipt of an SMS.

There are several names for delivery reports.

CardBoardFish Provides Added Reliability

CardBoardFish has developed many industry-leading technologies to both pro-actively avoid message loss or delay, and also to quickly detect destination network or SMSC problems.

Monitoring Platform
Through its commitment to provide an unrivalled level of quality, CardBoardFish has pioneered the most sophisticated SMS route monitoring platform available in the industry today. Capable of quickly and reliably detecting route downtime and delays, it can immediately re-route messages to provide a consistently high level of service 24/7. The platform also employs several techniques to reduce false alarms and is capable of alerting CardBoardFish staff, as well as suppliers as soon as a problem is detected. Not only does it track message delivery time, the platform also monitors how long it takes for delivery receipts to be returned and can make routing decisions based on this information too.

A selection of graphs to clearly illustrate the performance of every SMS route, and to identify any patterns such as recurring periods of slower delivery times are also generated in real time. These allow the quality of an SMS route to be easily assessed both in terms of message delivery time, and delivery receipt return time.

An industry grade GSM modem gateway and fixed GSM antennas provide a robust infrastructure for the monitoring platform to receive SMS. Typically used for reliable wireless SMS transmission, and capable of receiving signal from mobile base stations over 20 miles (32km) away, CardBoardFish utilises the gateway solely for SMS route testing to 20 international GSM operators.

Delivery Analysis
In addition to monitoring the delivery of test messages, a further measure is taken to ensure that client’s messages deliver as expected. This analysis system identifies the following possible problems:
  • Low delivery rates affecting individual destination networks or SMSC’s
  • Destination network blockings
  • Unsupported destination network prefixes
  • New unknown prefixes

Networks Management and Verified Coverage
CardBoardFish believes it is essential to maintain up to date and accurate information about all 700+ mobile networks worldwide in order to guarantee comprehensive coverage, and to provide high quality customer support.

The following data sets are meticulously maintained:

  • Mobile network names, alternative names, abbreviations, default operator logo text, and network technology type
  • Multiple MCC and MNC’s for each network
  • Network prefixes optimised for efficient routing by minimum length and sorted by order of popularity
  • Mobile number minimum and maximum lengths per prefix
  • Test mobile numbers per network and per prefix
  • SMS route coverage per network

As well as collecting the above data, CardBoardFish goes one step further by verifying the data in the following ways:

  • Confirming prefixes and mobile number lengths with national telecoms regulators
  • Performing HLR lookups on test numbers to verify MNC codes and to validate prefixes are in use and number length information is correct
  • Sending SMS to test numbers to confirm coverage either by checking with the recipient or using delivery confirmations

Although there are various numbering plan resources available on the Internet, none of them specialise in providing up to date and accurate information solely for mobile networks. Furthermore, these resources have little incentive to guarantee the accuracy of their data (because they may not use it themselves) and, in most cases, do not have the ability to verify the data in the ways that CardBoardFish can (via HLR lookup, SMS with delivery confirmations, and via communication directly with mobile operators).

Infrastructure
Rackspace UK was selected to host CardBoardFish’s core services due to its industry leading 100% uptime network guarantee, specialist expertise, and award winning level of service. For more information about Rackspace, please visit http://www.rackspace.co.uk.

This infrastructure, in combination with CardBoardFish’s 24/7 technical support and monitoring, and a huge selection of failover SMS routes, ensures that customers will always be able to send their SMS and expect instant delivery, at any time.