Telecoms glossary • B
Bandwidth
How much your line carries at once. For voice, the number that runs out first is the upload.
What is bandwidth?in VoIP and business telephony
1. Bandwidth is the maximum rate at which a connection can carry data, measured in megabits per second. It is a ceiling rather than a guarantee: a 100 Mbps line carries up to 100 megabits each second, and everything on it – calls, backups, video meetings, cloud files – draws from the same ceiling.
2. Bandwidth is not speed and not quality. A call breaks up on a fast line when packets arrive late, out of order or not at all, which is why latency, jitter and packet loss are measured separately from it.
How much a voice call actually uses
Cisco’s per-call bandwidth reference puts a G.711 call at 87.2 kbps over Ethernet and a G.729 call at 31.2 kbps, both assuming a 20 ms sample and 50 packets per second. Those totals include the IP, UDP and RTP headers, which is why they are higher than the 64 kbps and 8 kbps the codecs are named for.
Multiply by concurrent calls, not by staff. The figures below are the sustained capacity voice needs in each direction, before anything else on the line.
| Concurrent calls | G.711, each direction | G.729, each direction |
|---|---|---|
| 5 | 0.44 Mbps | 0.16 Mbps |
| 10 | 0.87 Mbps | 0.31 Mbps |
| 20 | 1.74 Mbps | 0.62 Mbps |
| 50 | 4.36 Mbps | 1.56 Mbps |
Voice is small. Twenty simultaneous calls on the uncompressed codec need under 2 Mbps upstream, which any business fibre line clears without noticing. The reason offices still have call quality problems is almost never the size of the pipe.
Why upload is the number that matters
A call is symmetrical: you send as much as you receive. Most South African consumer fibre is not, giving a large download figure and a much smaller upload, because households mostly pull data in. Business fibre is normally sold symmetrical, with upload matching download, and is contended more lightly.
That is where offices come unstuck. A 100/25 home-grade line has plenty of room for ten calls on paper. Add a cloud backup or a large file sync running in office hours and the 25 Mbps upstream fills, voice packets queue behind the upload, and every call degrades at once. The download figure on the invoice never moves.
Our guide to business fibre against home fibre sets out the contention ratios and upload shapes to expect. Where copper is still in the mix, see moving off Telkom lines.
When more bandwidth will not fix it
Buying a bigger line is the standard response to bad calls and it is usually the wrong one. Three faults look like a bandwidth shortage and are not.
- No prioritisation
- Voice and bulk data are treated equally, so a single upload can queue every call behind it. QoS on the router fixes this at no cost. Doubling the line only moves the point at which it happens.
- Wi-Fi in the path
- The line can be idle while the wireless segment drops packets. Test on cable before blaming capacity.
- Delay, not capacity
- Above roughly 150 ms of one-way delay, per ITU-T G.114, people start talking over each other. That is a routing and distance problem, and a fatter line does nothing for it.
The full diagnostic sequence is in our guide to VoIP quality, internet speed and network requirements.
How to size a line properly
Count concurrent calls, not extensions. Pull the call records for the busiest hour of the busiest day and take the peak number of simultaneous calls. In most offices it lands between a quarter and a third of headcount, which is why a 40 person business rarely needs 40 channels.
Then leave room for everything else. Take your peak voice figure, add what the office actually uses for cloud apps, video and backups, and treat the total as the minimum upload. Sizing on the voice number alone is how a line that tested clean fails at month end.
About this entry
Definitions are written for South African business buyers and checked against primary sources – provider documentation, standards bodies and ICASA – not vendor marketing. Reviews are independent and sponsors are always disclosed. Read our editorial policy and scoring methodology.
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