Does receiving of sms affects the traffic bandwidth of gsm modem

Question

There is a hardware system that uses a gsm shield to have an access to the internet. The system should be reliable and all ddos attack vectors are taken into account.

We afraid of sms ddos attack, that can be realized via sending too much sms messages (or especially malformed sms/invisible sms/binary sms). The question is does the receiving of sms hampers the traffic sending for gsm shield/modem? What happens with traffic sending at the moment when the sms is received? How many time the shield /modem is out of sending traffic service per each sms?

Answer 1

SMSs are usually transmitted over a control channel, like the SDCCH (Stand-alone Dedicated Control Channel) or the FACCH (Fast Associated Control Channel). These are the same channels which are used to initiate and hold a call. It is possible to receive SMS while a call is in progress by temporarily reallocating part of the data channel to a control channel (e.g. SDCCH) and using this bandwidth to transmitt/receive an SMS. If DDOS is possible, I think, depends more on the bandwidth allocation policy of the mobile network operator.
Of course, since the free air used for signal transmission is a shared medium for everyone in a cell, it is always possible to generate heavy load by gathering a group of people all sending SMS and making phone calls at the same time in close proximity of e.g. your GSM shield, effectively staging a DDOS attack if it's done on purpose.