Transrating vs Transcoding in VoIP: Audio Quality and Complexity Explained

When you make a VoIP call, your voice gets turned into digital packets. But what happens when your phone uses a different audio format than the person you’re calling? That’s where transcoding and transrating come in. They’re not the same thing - and mixing them up can cost you quality, bandwidth, or money.

What’s the Difference Between Transcoding and Transrating?

Think of transcoding like translating a book from English to Spanish. You’re taking the original content, understanding it fully, and rewriting it in a new language. In VoIP, that means converting audio from one codec to another - say, from G.711 to G.729. The system decodes the incoming audio, then re-encodes it using a different codec. This is necessary when two devices don’t share a common audio format.

Transrating, on the other hand, is like changing the size of the boxes you ship the same book in. You’re not changing the content - you’re just packing it differently. For example, you might take G.729 audio and change how often it’s bundled into packets - from 20ms to 30ms. The codec stays the same, but the way it’s sent over the network changes.

This distinction matters because one affects quality, and the other affects efficiency.

Transcoding: The Quality Trade-Off

Transcoding is essential for connecting old and new systems. Imagine a call from a legacy PBX using G.711 to a modern smartphone using Opus. Without transcoding, the call won’t connect. But every time you decode and re-encode audio, you lose a little quality. Each conversion adds 10-30ms of delay and introduces noise, distortion, or muffled speech.

Cisco’s data shows that a single G.711 to G.729 transcoding step lowers the Mean Opinion Score (MOS) - a standard measure of voice quality - by 0.3 to 0.5 points. A MOS of 4.5 is crystal clear. A MOS of 4.0 is still good. But drop to 3.8, and users start complaining about robotic voices. Chain two transcoding steps together, and you’re looking at a 0.6-1.0 point drop. That’s the difference between a call that sounds fine and one that sounds broken.

And it’s not just quality. Transcoding eats up CPU and DSP resources. A single DSP chip can handle 120 transrated G.729 channels - but only 30 transcoded ones. That’s four times less capacity. For a business with 5,000 users, that means more hardware, more power, more cooling, and more maintenance.

Then there’s the licensing problem. G.729, once the most common codec for low-bandwidth calls, is patented. Each port you use for transcoding costs $0.50 to $2.00 per year. For a company with 10,000 concurrent calls, that’s $5,000 to $20,000 annually - just for licensing. Many businesses are switching to Opus or G.722 to avoid this entirely.

Transrating: The Bandwidth Hack

Transrating doesn’t change the codec. It changes how the audio is packaged. Smaller packets (like 10ms or 20ms) mean more headers per second, which uses more bandwidth. Larger packets (30ms or 40ms) reduce header overhead - sometimes by up to 33% - without touching the audio quality.

On a congested WAN link, that’s a game-changer. A telecom engineer in Texas reported an 18% drop in bandwidth usage after switching G.729 from 20ms to 30ms packetization on their Cisco SBC. No quality loss. No extra hardware. Just smarter packing.

Transrating also improves resilience. Larger packets mean fewer packets overall. Fewer packets mean fewer chances for one to get lost. Cisco’s tests show transrating G.729 to 30ms improves packet loss resilience by 12%. That’s especially useful on mobile networks or unreliable Wi-Fi.

Latency? Almost nothing. Transrating adds only 1-5ms of delay - barely noticeable. Compare that to transcoding’s 10-30ms. Transrating is low-cost, high-reward.

When Do You Need Each One?

You need transcoding when your endpoints speak different languages. Examples:

• Legacy phone system (G.711) calling a WebRTC app (Opus)
• Mobile network (AMR) connecting to a VoIP PBX (G.729)
• Call center IVR system receiving calls from multiple regions with different codec support

You need transrating when your network is tight, and your endpoints already agree on the codec. Examples:

• Branch office calling HQ over a slow MPLS link - increase packet size to save bandwidth
• Remote workers on mobile data - reduce packet count to avoid timeouts
• Video conferencing with voice - optimize audio packet size to match video frame rates

Here’s the catch: most modern networks need both. A call might start with transrating to save bandwidth on the WAN, then hit a transcoding gateway to connect to a legacy system. But the more transcoding steps you add, the worse the quality gets.

Industry Trends: The Future Is Less Transcoding

Opus is changing everything. It’s open-source, supports 8-48kHz audio, works on everything from smartphones to old landlines, and outperforms G.711 and G.729 in both quality and bandwidth. By 2025, 92% of new enterprise endpoints support Opus. That means fewer transcoding hops.

Transrating, however, is getting smarter. Cisco’s new adaptive transrating adjusts packet size in real time based on network congestion. If packet loss spikes, it automatically switches to larger packets. If bandwidth is free, it drops back to smaller ones for lower delay. This isn’t just optimization - it’s automation.

Analysts predict transcoding usage will drop 15% per year through 2027. Transrating? It’ll be built into 95% of Session Border Controllers by 2026. The goal isn’t to eliminate these tools - it’s to use them smarter.

Implementation Pitfalls to Avoid

Many companies mess this up. Here’s what goes wrong:

• Transcoding everywhere - instead of limiting it to the bare minimum. Best practice: Do it only at network aggregation points, not at every edge device.
• Using G.729 without checking licenses. One infringement lawsuit can cost millions.
• Setting transrating without testing. Changing packet size from 20ms to 40ms might help bandwidth but hurt real-time interaction. Try 30ms first.
• Ignoring DSP capacity. If your SBC is maxed out on transcoding, calls drop or crackle during peak hours. Monitor utilization.
• Assuming transrating fixes codec issues. It doesn’t. If your phone uses AMR and your PBX uses G.711, no amount of transrating will make them talk.

Training matters too. Avaya’s data shows engineers need 40-60 hours of focused training to configure these correctly. Common mistakes? Mismatched packet sizes (28% of support tickets) and under-allocating DSP resources (22%).

What Should You Do Today?

Here’s your action plan:

1. Map your codec usage. What codecs are your endpoints using? Are you still relying on G.729?
2. Replace G.729 with Opus wherever possible. It’s free, high-quality, and widely supported.
3. Use transrating on all your internal links. Set packet size to 30ms for G.722, Opus, and G.711 to cut bandwidth without losing quality.
4. Limit transcoding to only where absolutely necessary - like connecting to legacy PBXs or mobile gateways.
5. Monitor your DSP usage. If you’re hitting 80% utilization during peak hours, you’re one call away from trouble.

The goal isn’t to avoid transcoding and transrating. It’s to use them like tools - not crutches. The best VoIP networks don’t need to transcode often. They just need to transrate well.