Switching to VoIP

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Security in Traditional Telephony

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Access Control

The PSTN permits network access via the physical loop component—the cable con-

nection from the CO to the customer premises. This means that a person who has

access to the customer’s phone lines can place calls as though he is that customer. A

friend comes over to your house, picks up your phone, and makes a call. The tele-

phone company assumes he is authorized because he is there. While primitive, this is

the basis of access security on the PSTN.

By comparison to a modern data network, this access control approach seems lax,

but it’s the way the PSTN has always done it. Indeed, even on PBXs and high-capac-

ity voice circuits, physical logistics is still the most common method of controlling

access to legacy telephony apps.

To overcome this weakness, some CO switches and PBXs can require users to dial a

password of DTMF digits before a call can be placed or before certain telephone area

codes can be dialed (for a quick review on phone numbers, refer to Chapter 4). Or

the phone company can be made to force you to use a long-distance code before you

can dial LD calls. Some telephone companies offer what’s called a receive-only phone

line, which controls outbound calling by not allowing it at all. Lots of PBXs let you

limit outgoing calls ...