Switching
Class 4 SoftswitchAlso known as Tandem switch, Transit switch, Wholesale softswitch
Definition
A Class 4 softswitch is a carrier-grade switch that routes large volumes of voice traffic between networks rather than to end users. Named after the Class 4 toll and tandem offices of the old North American hierarchy, it is the core of wholesale VoIP, selecting routes by prefix, cost and quality and recording every call for billing.
Key functions
- Least-cost and quality routing: chooses among vendors per destination prefix, balancing rate against ASR, ACD and PDD.
- Failover: retries the next route when one rejects or times out.
- Capacity controls: concurrent-call and calls-per-second limits per customer and vendor.
- Number translation: prefix manipulation and normalisation.
- Interop: codec negotiation and transcoding, often via an SBC in front.
- Accounting: high-volume CDRs feeding rating and billing.
Class 4 vs Class 5
| Class 4 | Class 5 | |
|---|---|---|
| Serves | Carriers and wholesale customers | End users and businesses |
| Main job | Transit routing between networks | Subscriber features and calling |
| Typical features | LCR, rate decks, CPS limits, vendor management | Voicemail, call forwarding, IVR, hosted PBX |
| Scale | Millions of minutes, many routes | Many subscribers, rich features |
Read more in Class 4 vs Class 5 softswitch or see the Class 5 softswitch entry.
Class 4 switches and mobile gateways
Performance is measured in calls per second (CPS) and concurrent sessions as well as minutes, because wholesale traffic arrives in bursts and a switch that cannot keep up adds post-dial delay or rejects calls.
Wholesale VoIP operators work through licensed carriers and interconnect agreements. Where a licensed operator uses mobile gateways, for example for authorised enterprise or local-presence services, the gateway is simply another route behind the Class 4 switch. GSMCalls is intended for lawful, authorised deployments only; see the acceptable-use policy.
Frequently asked questions
Why is it called Class 4?
The name comes from the AT&T switching hierarchy, in which Class 4 offices were toll and tandem switches connecting local exchanges (Class 5) across the long-distance network.
What is LCR in a Class 4 softswitch?
Least-cost routing: choosing the cheapest vendor route for each destination prefix that still meets quality targets.
Can one platform be both Class 4 and Class 5?
Yes. Many modern softswitches offer both transit routing and subscriber features.