SIP trunking replaced ISDN as the way a business connects its phone system to the public network. It is cheaper and more flexible, but it also moves telephony onto the same network as everything else, with the same security and reliability concerns. Dinstar makes the hardware that sits between the PBX and the carrier – session border controllers and trunk gateways – and this article explains what that hardware does and when you need it.
What a session border controller does
A session border controller (SBC) is a purpose-built device at the edge of the voice network. It terminates SIP signalling and media from the carrier and re-originates them towards the PBX, which gives it control over everything that crosses the boundary. In practice an SBC provides:
- Security – it hides the PBX from the internet, blocks registration attacks and toll-fraud attempts, and enforces TLS and SRTP encryption where the carrier supports them.
- Interoperability – it normalises SIP headers and codecs so that a carrier’s quirks do not have to be accommodated inside the PBX.
- NAT traversal – it solves the one-way-audio problems that come from putting a PBX behind a firewall.
- Call admission control – it caps simultaneous calls so that a burst does not overrun the WAN link.
- Routing and failover – it can route calls to more than one carrier and fail over between them.
Do you need one?
A five-person office with a hosted PBX does not. A company with an on-premises PBX, more than one carrier, a compliance requirement, or a history of toll fraud does. Anything with a PBX exposed to the internet on a public IP should have one. The cost of a mid-range SBC is less than a single month of fraudulent international calls, which is the usual way companies discover they needed it.
Trunk gateways and migration
Dinstar’s digital trunk gateways connect E1/T1 PRI circuits to SIP. During a migration they let a new IP PBX use the existing carrier PRI on day one, while numbers are ported to a SIP trunk over the following weeks. After migration the same gateway can remain as a backup path, so that if the SIP trunk fails, calls fall back to the PRI. For organisations that keep a PRI for regulatory or contractual reasons, the gateway is permanent infrastructure.
Survivability for branches
A branch whose phones register to a central PBX loses telephony if the WAN link fails. Dinstar gateways with local survivability keep the branch’s phones registered to the gateway itself during an outage, route internal calls locally and send external calls out via a local FXO line or GSM channel. When the link returns, the phones re-register to the central PBX. For clinics, retail branches and security control rooms this is the difference between an inconvenience and an incident.
Specifying the hardware
Size an SBC by concurrent sessions with growth headroom, and check the licence model (sessions are usually licensed separately from the chassis). Confirm TLS/SRTP support, the codecs and transcoding you need (transcoding between G.711 and G.729 consumes capacity), high-availability options if the site cannot tolerate a single point of failure, and the management interface your team will use. For trunk gateways, count E1/T1 ports and check whether you need SS7 or only ISDN PRI signalling.
Why Dinstar
Dinstar’s SBC and gateway lines are used by carriers and enterprises across the region. They interoperate with 3CX, Yealink, Snom and Fanvil endpoints and with the major carrier platforms, and the pricing makes an SBC realistic for the mid-market rather than only for enterprises. Firmware is actively maintained, which matters for a security device.
How Zen can help
Zen Interactive Technologies is a Dinstar distributor for the UAE and GCC. We can review your current trunking arrangement, recommend an SBC or gateway with the right session count and ports, and supply it from Dubai stock with technical support. The range is on the Dinstar page.