Where did that SMS go ?

Firstly, i think some apologies are called for…Sorry Whiteopal… i’ve been ignoring you a bit, and have had no time to post for a week or so. But i have been really busy- with my minor project, which absolutely refuses to show any signs of being alive :) , and with  making up for all the lost time with a precious person…

Have you ever had that intensely disturbing feeling when a SMS you sent is not delivered ? Or when apparently, it is delivered, but you never got a reply from the other end. If you did, take heart. This happens to all of us. I think the phenomenon of text messaging has created with it a whole new psychology. Instant replies are good, but things stop being so hunky-dory when not getting a reply begins to put you off… Know what? – i made a shocking discovery the other day.

The SMS or text message that you send to people, is not always received by them. I mean, well, that is a bit obvious, you would say- its data and communication , right ? So obviously there may be some loss- but what stunned me was, this may happen even though you get a DELIVERY REPORT. I decided to investigate a bit further into how exactly a SMS is sent, and will tell you now what i learnt… i’ll try to make this discussion as non-technical as possible…

A Mobile Network, or a Cellular Network, is basically a collection of cells…yeah,imagine actual hexagons forming a matrix spread over a particular area. Each cell has a transmission tower at its center. When you move from one cell to another, your mobile automatically changes the tower it is in contact with. Now how this happens is dependent on each network, and we’ll leave that discussion for some other time.

A Cellular Network

A Cellular Network

So even if you are not talking on your cell phone, your phone is constantly sending and receiving information. It is talking to its cell phone tower over a pathway called a control channel. The reason for this chatter is that the cell phone system should know which cell your phone is in, and so that your phone can change cells as you move around. Every so often, your phone and the tower will exchange a packet of data that lets both of them know that everything is OK.

This control channel also provides the pathway for SMS messages. SMS stands for Short Messaging Service. ‘Short’  refers to the maximum size of each page, which is just 160 characters ( Open up your cell phone’s ‘New Message’ option…it’ll show you 450 characters remaining , and the first page will get over when 291 characters are remaining).

But how do SMS messages actually get to your phone? Your mobile telephone network has a element in it called a Short Message Service Center (SMSC) . When a friend sends you an SMS message, the message flows through the SMSC, then to the tower, and the tower sends the message to your phone as a little packet of data on the control channel. In the same way, when you send a message, your phone sends it to the tower on the control channel and it goes from the tower to the SMSC and from there to its destination. The data format consists of the length of the message, the time stamp, the characters and stuff. Now the use of a SMSC has certain advantages. Suppose the person you are sending a message to hasn’t switched on his mobile, or is out of coverage area, then your message gets stored at the SMSC. The storage time alotted to each message is network dependent. It may be as much as 72 hours to something like 12 hours. So when the SMSC gets a signal from the receiver indicating that it is ready to receive the SMS, the SMSC forwards your message.

There are some hitches in this though…due to heavy load of data, messages often get misplaced, or lost by the SMSC. Recall the Diwali, New Year or Holi messages you sent, but which never got delivered. Usually there are 2 types of confirmations that the Networks use to inform the sender whether or not his message has been put through-

  1. Message Submission Reports: After leaving the mobile phone, an SMS message goes to the SMS center. When it reaches the SMSC , the SMS center will send back a message submission report to the mobile phone to inform if the message has been sent. If the mobile phone does not receive the message submission report after a period of time, it concludes that the message submission report has been lost. The mobile phone may then re-send the SMS message to the SMS center.Usually flags are set in this resent message, thus preventing the sending of the same SMS message to the recipient multiple times.  These reports are often indicated by ‘ticks’ in the Outbox.
  2. Message Delivery Reports:After receiving an SMS message, the recipient mobile phone will send back a message delivery report to the SMS center to inform whether there are any errors or failures (eg. unsupported SMS message format, not enough storage space, etc). This process is SUPPOSED to be transparent to the mobile user. If there is no error or failure, the recipient mobile phone sends back a positive delivery report to the SMS center. Otherwise it sends back a negative delivery report to the SMS center.

But again, there are failings in this. Sometimes the receiving mobile sends a false ‘positive delivery report’ , though it hasn’t received the message completely or at all. This is so because on the input side, it gets the message from the SMSC, but for some reasons, it fails to completely interpret the data bytes being received. Despite this, it checks the parity or verification bits in the data, and since they happen to be correct, it feels that it has received all the data. The result being, the SMS sender receives a delivery report, but the receiver doesn’t receive the message !

So now you see….if  you sent your friend a message, and he didn’t reply to it, its quite likely he didn’t get your message at all. The probability of this happening increases exponentially each day- the increase in number of  subscribers far outweighs the capacity of the networks..We are all PROFIT MAKING MACHINES right ? – who gives a shit for any ethics or moral responsibility towards consumers. Actually, i think the cell-cos should really be sued right and left , but i am not sure the existing Indian laws allow us to do that. Yeah, but is you are not from India and reading this, then maybe you should consider doing so.

Take my advice- next time, if you don’t get a reply from a friend you totally expected to hear from, stop feeling shitty and ignored- just pick up the phone and give a ring ! ;-)