What internet connection do I need for VoIP?
Bandwidth per concurrent call, why jitter and packet loss break calls before bandwidth does, and how to configure QoS properly.
By the Telappliant team
VoIP needs surprisingly little bandwidth and is surprisingly fussy about everything else. A call uses less capacity than a photograph attachment, but it will not tolerate jitter, loss or a saturated upload.
How much bandwidth a call really uses
Budget roughly 100Kbps in each direction per concurrent call with the common G.711 codec, including overheads, and around a third of that with G.729. Twenty simultaneous calls is therefore about 2Mbps each way. Almost any modern circuit can carry that; the question is whether it can carry it consistently while everything else is running.
The three metrics that decide call quality
- Jitter: variation in packet arrival, which should stay under about 30ms
- Packet loss: should be under 1 percent, and audible above that
- Latency: one-way delay under about 150ms keeps conversation natural
Why upload is the usual culprit
Your voice leaves the building on the upload path, and asymmetric broadband gives you very little of it. One person uploading a large file, or a cloud backup starting at 3pm, can fill the upstream and break every call in the office. Symmetrical services remove the problem outright; where you cannot have one, quality-of-service configuration is essential rather than optional.
Configure the network properly
- Prioritise voice traffic on the router or firewall with DSCP marking honoured end to end
- Put phones on their own VLAN so broadcast traffic does not touch them
- Disable SIP ALG on the firewall, which causes more one-way audio problems than anything else
- Schedule backups and patching outside working hours
- Use wired connections for busy desks; Wi-Fi is fine for movement, not for a full day of calls
What to buy
- Small office, under 20 concurrent calls: FTTP business tariff with QoS configured
- Contact centre or customer-facing site: a symmetrical leased line, so calls never compete with data
- Any site where calls are the business: add 4G or 5G failover and confirm calls fail over cleanly
- Home workers on the phone all day: a business FTTP line and a managed router, not the family broadband
Prove it before you blame it
Run a week of monitoring on the circuit and pull the call quality statistics from the phone platform. Between them they will tell you whether you have a bandwidth problem, a configuration problem or a hardware problem, which is usually not the one people assume.
