Snom Prepaid Services, 4S Frequently Asked Questions Manual
F REQUENTLY
A SKED
Q UESTION
Prepaid Services
Date: Oct-28-2003
Author: Christian Stredicke
Document: faq-03-10-28-cs
2 snom technology AG
[ F R E Q U E N T L Y A S K E D Q U E S T I O N ]
Prepaid Services 3
[ F R E Q U E N T L Y A S K E D Q U E S T I O N ]
Prepaid calling cards are becoming more
and more popular in SIP-based VoIP telephony.
This FAQ show how the snom 4S can be used
to implement a prepaid service.
Limited Trust
Prepaid calling cards are often sold
without exact knowledge about the user’s
credit, bank account or credit card number.
This implies that abuse of calling cards must
be avoided as there is no way to charge costs
beyond the already paid amount. Specically,
that means when the calling cards becomes
empty, the active call must be terminated.
In order to avoid surprises, the user should
get an indication shortly before the calling
card gets empty ("Your calling card is almost
empty!").
When the user dials into the calling card
system, he is also interested in how much
credit is left on this calling card. An IVR system
should be able to indicate this credit.
The system must also be robust against
cheating attempts like calling the account
twice at the same time, bypassing a proxy or
"pulling the plug" before the end of the call (no
BYE message). In many cases it is desirable
to hide the true destination of the call, may it
be a gateway or another Internet Telephony
Service Provider (no Record-Route on the
client side).
This all makes it necessary that the
calling card service provides both SIP and RTP
data streams. By handling RTP, the system
becomes able to play announcements and mix
messages into the conversation.
Eavesdropping PINs
In many cases in SIP, it is not secure
to choose "pin codes" for authentication.
Eavesdroppers that monitor the SIP trafc
can easily try 10,000 or even 100,000
combinations with all valid pins to guess the
pin for this account.
MS
Full Cone
NAT
Symmetrical
NAT
A
B
C
GW
Client
Side
Service
Side
Public
IP
Loading...