SIP Proxy Resource Calculator
A Session Initiation Protocol (SIP) proxy is the central component of most VoIP and unified communications (UC) deployments. It routes signaling messages, enforces policies, and can provide loadbalancing, NAT traversal, and security services. Proper capacity planning is essential to guarantee call quality, low latency, and high availability. This page explains how to calculate the CPU, memory, storage, and network resources required for a SIP proxy based on realistic traffic patterns.
Why a Resource Calculator Matters
- Predictability: Estimate the hardware you need before you purchase or provision cloud instances.
- Cost control: Avoid overprovisioning (wasted spend) and underprovisioning (service outages).
- Scalability: Identify the scaling point at which you must add more nodes or enable clustering.
- Compliance: Many regulated environments require documented capacityplanning procedures.
Key Metrics for SIP Proxy Planning
Every calculation begins with a few core traffic figures:
- Concurrent Calls (CC): The number of calls that are active at the same instant.
- Call Setup Rate (CSR): New calls per second (CPS) that the proxy must handle during peak hours.
- Average Call Duration (ACD): Typical length of a call in seconds.
- Message Ratio (MR): Average SIP messages per call (INVITE, 100/180/200, BYE, ACK, reINVITE, etc.).
- Transport Protocols: UDP, TCP, TLS, or SCTP each has different processing overhead.
CPU Capacity Estimation
The CPU load is driven primarily by the number of SIP messages that need to be parsed, processed, and forwarded. A common approach is to derive the Message Processing Cost (MPC) in CPU cycles per message.
StepbyStep CPU Calculation
- Measure or obtain a baseline MPC for your proxy software. For many opensource proxies (e.g., Kamailio, OpenSIPS) a typical value ranges from 2,000 to 5,000 cycles per UDP message. TLS adds roughly 30% overhead.
- Calculate the Peak Message Rate (PMR):
PMR = (CSR MR) + (CC / ACD MR)
The first term covers new call setups; the second term represents ongoing traffic from existing calls (e.g., keepalives, reINVITEs). - Compute the required CPU cycles per second:
CyclesPerSec = PMR MPC
- Convert cycles to GHz (or cores). A modern core can execute roughly 210 cycles per second (2GHz). Apply a safety factor of 1.52.0 to accommodate spikes.
RequiredCores = (CyclesPerSec / 2000000000) SafetyFactor
Example: A call centre expects a peak CSR of 120CPS, an ACD of 180seconds, and an MR of 10 messages per call. Using MPC=3,500 cycles/message and a safety factor of 1.7:
PMR = (120 10) + ( (120 180) / 180 10 ) = 1,200 + 1,200 = 2,400 msgs/sCyclesPerSec = 2,400 3,500 = 8,400,000 cycles/sRequiredCores = (8.4M / 2B) 1.7 0.0071 cores
Even with a generous safety factor the load is low, suggesting that a single lowpower virtual CPU can handle the traffic. However, realworld deployments also need to consider cryptographic work, NAT handling, and background tasks, so a 1core allocation is a realistic baseline.
Memory Sizing
Memory consumption consists of three major parts:
- PerCall State: Data structures storing dialog information, timers, and authentication credentials.
- Cache & Routing Tables: Registrations, ACLs, and loadbalancing maps.
- Operating System Overhead: Kernel buffers, network sockets, and page cache.
PerCall Memory
Most SIP proxies allocate between 200bytes and 1KB per active dialog. For a conservative estimate, use 800bytes.
MemoryForCalls = CC 800bytes
Static Memory
Configuration, modules, and OS footprint typically require 128MB to 256MB. Add an additional 10% headroom for caching.
Example
With 2,500 concurrent calls:
MemoryForCalls = 2,500 800B = 2,000,000B 1.9MBStaticMemory 200MBTotal 202MB Round up to 256MB for safety.
Network Bandwidth Requirements
The SIP signaling itself is lightweight (usually < 1KB per message). The dominant factor is the media stream (RTP/RTCP) that travels through the proxy if you are using a backtoback user agent (B2BUA) configuration or a media relay.
Signaling Bandwidth
SignalingBps = PMR AverageMessageSize
Assuming 1KB per SIP message and a PMR of 2,400 msgs/s:
SignalingBps = 2,400 1,024 2.45Mbps
Media Bandwidth
For a typical G.711 call (64kbps) with RTCP overhead (~5kbps), each call consumes ~69kbps. Multiply by the concurrent call count.
MediaBps = CC 69kbps
With 2,500 concurrent calls:
MediaBps = 2,500 69kbps 172.5Mbps
Include a 20% safety margin for jitter buffers and occasional higherbitrate codecs (e.g., Opus). The total network requirement would be roughly 210Mbps, comfortably handled by a 1GbE interface.
Storage Considerations
Even though a SIP proxy does not store media, it may need persistent storage for:
- Call detail records (CDRs) typically 200bytes per call.
- Registrations and authentication databases.
- Log files (debug, error, audit).
A rough estimate for CDR storage over 30days:
DailyCalls = CSR 3600s/h 24h = 120 86,400 10.4M callsCDRSize = 10.4M 200B 2GB per month
Adding logs (5GB/month) and database files (1GB), a 20GB SSD gives ample headroom while providing fast write performance.
Putting It All Together A Sample Calculator Table
| Parameter | Value (example) | Result |
|---|---|---|
| Concurrent Calls (CC) | 2,500 | |
| Call Setup Rate (CSR) | 120 CPS | |
| Average Call Duration (ACD) | 180s | |
| Message Ratio (MR) | 10 msgs/call | |
| Message Processing Cost (MPC) | 3,500 cycles/msg (UDP) | |
| Peak Message Rate (PMR) | 2,400 msgs/s | Calculated |
| CPU Cores Needed | Safety factor 1.7 | 0.01 allocate 1 core |
| Memory for Calls | 800B per call | 2MB |
| Total RAM | + 200MB OS/Cache | 256MB (rounded) |
| Signaling Bandwidth | 1KB/message | 2.5Mbps |
| Media Bandwidth (G.711) | 69kbps/call | 172Mbps |
| Total Network (incl. margin) | +20% | 210Mbps |
| Storage (30days CDR + logs) | 7GB | Allocate 20GB SSD |
