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Knowledge Base - SMS Suite Operational Background

The use of SMS for business purposes

Important issues affecting ‘quality of service’


general overview 

SMS (Short Message Service) is a remarkably simple technology that delivers short messages to and from mobile devices. It was developed as a control tool for the voice networks but is rapidly becoming accepted as a mainstream business tool today.


Serious business users must be wary of wild claims from SMS vendors when the networks cannot deliver the promises. Many vendors offer SMS facilities for business, with varying promises of service. However, the business community is largely unaware of the way the networks process and manage SMS traffic and this can lead to a disparity between expected and actual quality of service. In order for a business to implement SMS as a communications tool, it is important to gain an understanding of the real issues as set out in this paper.



As this is a business solution document, SMSs are deemed to be sent from a GSM modem to a cellphone and not from a cellphone to cellphone.





Any Cellphone Network Provider


Any cellphone which is roaming internationally


The GSM modem that sends SMSs from a PC.


The receiver of the SMS


The sender of the SMS. In this document it is the individual on the LAN (at a PC) or an automated server/mainframe etc.


Receiver of a message

Status or Status Message

The message that the network sends back to the sender at various points in the sending process (e.g. “Delivered”, “Failed”). This too is an SMS.


Any SMS. This could be the message that is being sent to the recipient or it could be the status coming back to the sender.


Short Message Service Centre which processes SMSs for the network.  Each of the networks may have several of these.


A message sent back to the Sender by the Recipient. It is also an SMS.


A period specified by sender, after which, the SMS must not be re-tried if it has not already been delivered. Can be any time up to 3 days.

Network Time-out

The time period that the network will queue an SMS for, until the message either expires (i.e. validity period exceeded), or another trigger pushes it out.


An event that causes an SMS queued at the SMSC to be re-tried to the same cellphone. See below.

the network

The network is responsible for the following:

  • Delivering the SMSs to the cellphone anywhere in the world
  • Queueing messages until the cellphone is available
  • Providing statuses back to the sender
  • Timing out messages once they have expired


Companies that offer business SMS facilities merely provides the interface options that enable clients to get the SMS messages into the GSM network and retrieve and consolidate the status reports afterwards. The networks have a complicated set of conditions which determine how messages are delivered and how statuses are sent back to the sender.  See diagram below.


validity and network time-out

SMS messages can be sent with a validity period which is set by the sender software (or sender’s cellphone if sending from a cellphone).  The validity is the maximum time that the network can continue trying to send the SMS if it doesn’t get through immediately. For example, if the validity is set to 1 hour and the recipient’s phone is switched off for 2 hours, then the message will not be sent when the cellphone is back on line.

can be set to any time up to a maximum of 3 days because this is the maximum network timeout. Validity is set by the sender while the network time-out of 3 days is a network setting which does not change. Thus validity can be a maximum of 3 days (any longer and the network time-out comes into effect).


If the 3 day network time out is reached before the SMS can be delivered, then the sender is not notified. However if the validity period is reached and a trigger occurs to cause a re-try, then a “Failed” status message is sent to the sender.


It is important to note that the networks will time out all messages after they have been queued at the SMSC for 3 days. However if the sender sets a validity period on the message, then this becomes the maximum length of time that it will be queued. The principle is best illustrated by an examples:






Sender sends message at 10h00 on 12th January with no validity period set. The recipient’s cellphone is off until 13h00 on 13th January. The network sends the message “Pending” to the sender. As the recipient switches his/her phone on, it registers with the SMSC and the queued SMS is delivered. The network sends the message “Delivered” to the sender.



Sender sends message at 10h00 on 12th January with a 6 hour validity period set. The recipient’s cellphone is off until 13h00 on 13th January. As the recipient switches his/her phone on, it registers with the SMSC which checks the validity time-stamp and then cancels the SMS. The status message “Failed” is sent back to the sender.



Sender sends message at 10h00 on 12th January with no validity period set. The recipient’s cellphone is off until 13h00 on 16th January (ie longer than 3 days).  At 10h00 on 15th January, the SMSC cancels the message and sends a status “Failed” back to the sender.



Sender sends message at 10h00 on 12th January with a 6 hour validity period set. The recipient’s cellphone is off until 13h00 on 12th January (same day). Recipient switches his/her phone on but it does not register with the SMSC (this can happen from time to time). Another SMS is sent to the same recipient at 14h00 on 12thJanuary. It then ‘pushes’ out the first SMS and both are delivered to the recipient.




sms message size 

Each SMS is limited by the networks to a maximum of 160 characters. However, some SMS generators (e.g. SMS Suite Enterprise software) allow the user to type a longer message in which case it is automatically split into more than one SMS. E.g. 200 characters will require 2 SMSs and 321 characters will require 3 SMSs to send.


The recipient of a split message will be informed on the cellphone how many parts there are to the message. A 3-part message will be numbered as follows: 1-3, 2-3 and 3-3 at the start of each part.


status messages

No senders or messages or companies have any higher priority than any other . The network generates a number of status messages at different points in the SMS sending process. Note that the network handles single and bulk messages in exactly the same way and generates the same messages for both types. Likewise, there are no priority advantages for either type of transmission, nor does any sender of SMSs get a higher priority than any other.


There are however two priorities for SMS messages: High for sending messages and Standard for processing status messages.

The following table indicates the message statuses that are generated by the network:






Message received by the sender after sending the SMS to the network, but before the network has either delivered of failed the SMS.

The SMSC generates this message as soon as it received the SMS.

The sender’s modem must be enabled to receive status messages.


Message received by the sender when the SMS has been delivered. Delivery can only take place within the validity period and, if not set, within 7 days.

The 3 day time-out is a network function. No SMS can be sent after this time-out.

The validity period is set by the sender (e.g. 1 hour, 6 hours, 24 hours)


Message received by the sender when the network cannot deliver. It would generally be if:

the number does not exist on any of the networks or

the validity period is reached and a trigger causes a re-try.

Unless either of these two conditions are met, the original message (i.e. “Pending”) will remain on the sender’s modem.





SMS sending process

The following diagram illustrates the key processes with annotations in the table below.









Modem sends message to the network (SMSC)




Status message received back from network


This is before the network has delivered or failed the message.


Network first locates the cellphone and then sends the message.


If the phone cannot be located, the message is queued for up to 3 days at the SMSC, unless pushed out to the cellphone based on triggers as listed below.


Network generates status message depending on result

“Delivered” if it gets through before the network times it out or before validity period is reached.


The cellphone will send the “Delivered” status to the network.


Network sends status message back to sender

“Delivered” if it gets through

“Failed” if the cellphone does not exist.

If not delivered, not failed, not expired (validity) and not timed-out (network) then no further messages will be sent to the sender which will still have the original message, i.e.  “Pending”.


Recipient sends a reply or forwards it to another cellphone


The Reply back to the sender or forwarded to another phone is the start of another SMS message.


Reply sent back to sender


The reply itself is an entirely new SMS

Cellphones don’t always sign onto the SMSC when they are switched on, 

meaning that queued messages aren’t always delivered on time

tries and re-tries

In general, the SMSC always tries to send the SMS at least once. Included in this ‘try’ is the effort to locate the cellphone in order to send the SMS. If it cannot locate the cellphone, it cannot send the message and therefore the ‘try’ is over.


The message is then queued for a maximum of 3 days at the SMSC and will only be re-tried if:

  • The validity period on the message has not been reached and
  • The SMSC 3-day timeout has not been reached.


If both of these conditions are met, then one of the following common triggers will cause the SMSC to re-try the message:

  • Cellphone was off and is now switched back on AND it alerts the SMSC (this does not always happen);
  • Cellphone was out of GSM coverage and it now comes back into coverage AND it alerts the SMSC;
  • Another SMS is sent to the same cellphone;
  • A voicemail SMS is sent to the same cellphone.











SMS replies always go back to the SIM that sent them. If the SIMs are sending one SMS at a time (i.e. not bulk sending) then the replies will be received as normal and will slot in between the SMSs being sent. However, if the SIMs are being used for broadcast (bulk) SMS delivery then they will be busy sending an SMS every 5-6 seconds and very few replies will be received by the sending SIM.  As a result the replies will be queued by the network until the delivering SIM is free to receive.


Thus, when sending broadcasts, time must be allowed after all messages have been sent to receive all the replies before attempting to view them in a log or process them for automation.

Replies cannot be sent to a SIM which is different from the one that sent the original SMS.


speed of delivery

Speed of delivery means different things to different people and we must therefore clarify the different parts of the transmission process. There are 3 distinct legs in the overall transmission:




SMS Software

The SMS software used to send messages has a bearing on how fast messages are sent to the GSM modems. If for example the software only scans its input files every 60 seconds, then this overhead will be added to the overall delivery time. Likewise if it takes 30 seconds to process 1000 names in a list, then a list of 10000 names will take 300 seconds to process, before it gets to the GSM modem.

GSM Modem

A good GSM modem will deliver an SMS into the network every 5 to 6 seconds (about 10 per minute). This is a GSM network and not a modem limitation. If a message automatically splits into multiple SMSs, then each SMS will take about 6 seconds to send. Thus a message of 10 parts, will take about a minute to deliver to the network.


he network delivery speed is entirely dependent on how long it takes to locate the cellphone (if it is on) or how long it takes the cellphone to be switched on (if it is off at the time of sending) AND be recognised by the SMSC. Worst case scenario will be up to 3 days for delivery, after which time the message will be cancelled.




Each network runs SMSCs (Short Message Service Centres) where SMSs are consolidated and delivered to cellphones via the network. SMSs may arrive at the SMSC via terrestrial link (e.g. Internet or Diginet) or via the GSM network itself (i.e SIM originated). It is important to note that irrespective of how the message gets to the SMSC, there are no speed or service improvements of one over the other.

The SMSCs have 2 priorities: High (for sending all SMSs) and Standard (for processing replies).



Users of PC based or automated SMS systems for business must be aware of the limitations of the network infrastructure as described herein and should not expect more out the system than is possible. SMS is ideal for non- critical alerting (e.g. credit card purchase or bank withdrawal alerts) however, if it is to be used for mission critical applications, then it should be backed up by other alerting mechanisms (e.g. email or voice calls).