SRTLA Bonding: Aggregate Multiple Connections for Reliable Streaming

What is SRTLA?

SRTLA (SRT Live Association) is a protocol extension that bonds multiple network connections into a single SRT stream. It was developed to solve a fundamental problem in mobile live streaming: no single cellular connection is reliable enough for broadcast-quality video.

The idea is simple. Instead of relying on one 4G or 5G modem, you use several (each on a different carrier) and distribute the SRT packets across all of them. If one connection drops or degrades, the others continue carrying the stream. The aggregate bandwidth is the sum of all connections, and the reliability exceeds any single connection.

Vajra Cast includes a built-in SRTLA receiver, making it a complete server-side solution for bonded streaming workflows.

How SRTLA Bonding Works

The Sender Side

The SRTLA sender (running on a device like a BELABOX at the venue) takes a standard SRT stream from an encoder and distributes its packets across multiple network interfaces:

Encoder -> SRT -> SRTLA Sender
                    |-- Interface 1 (Carrier A, 4G) --->
                    |-- Interface 2 (Carrier B, 5G) --->  Internet
                    |-- Interface 3 (Wi-Fi)         --->
                    |-- Interface 4 (Ethernet)      --->

Each packet is sent on whichever interface has available capacity. The sender monitors the throughput and latency of each connection in real-time and distributes packets proportionally. Faster connections carry more packets.

The Receiver Side

The SRTLA receiver (running in Vajra Cast) collects packets from all paths and reassembles them into a single SRT stream:

                    ---> Interface 1 packets --\
Internet            ---> Interface 2 packets ---+--> SRTLA Receiver -> SRT Stream -> Routing Engine
                    ---> Interface 3 packets --/
                    ---> Interface 4 packets -/

From the routing engine’s perspective, the SRTLA input looks like a normal SRT input. It has a bitrate, packet statistics, and connection state. The bonding is transparent to the rest of the pipeline.

Packet Distribution

SRTLA does not duplicate packets across connections (that would waste bandwidth). Instead, it distributes them:

• Packet 1 -> Connection A
• Packet 2 -> Connection B
• Packet 3 -> Connection A
• Packet 4 -> Connection C
• Packet 5 -> Connection B

The distribution adapts in real-time. If Connection C slows down, it receives fewer packets. If Connection A loses signal entirely, its packets are redistributed to the remaining connections.

This is distinct from link aggregation (LAG) at the network layer, which operates at the frame level and requires both ends to be on the same switch. SRTLA works across different ISPs, different technologies (cellular, Wi-Fi, wired), and different geographic paths.

Aggregate Bandwidth

The total available bandwidth with SRTLA is approximately the sum of all connection bandwidths, minus overhead:

Total ~ (BW_1 + BW_2 + ... + BW_n) x 0.85

The 15% overhead accounts for SRTLA control packets, SRT retransmissions, and the inherent inefficiency of distributing packets across connections with different latencies.

Example:

This is enough for a 1080p60 stream at high quality (12-15 Mbps) with substantial headroom for retransmissions and variability.

Handling Asymmetric Connections

Real-world connections are never equal. A 5G modem might deliver 25 Mbps while a 4G modem manages 5 Mbps. SRTLA handles this gracefully by sending more packets through faster connections. The slow connection still contributes (it carries some packets and provides redundancy) but it does not bottleneck the overall stream.

If a connection’s bandwidth drops below the minimum useful threshold, SRTLA stops sending payload packets through it but keeps it warm for recovery. If bandwidth recovers, packets resume automatically.

BELABOX Integration

BELABOX is the most widely used SRTLA sender hardware. It is a purpose-built device that manages multiple USB cellular modems and runs the SRTLA sender software.

Typical BELABOX Setup

Camera -> HDMI/SDI -> BELABOX
                       |-- USB Modem 1 (SIM: Carrier A)
                       |-- USB Modem 2 (SIM: Carrier B)
                       |-- USB Modem 3 (SIM: Carrier C)
                       |-- Ethernet (venue Wi-Fi or wired)

The BELABOX encodes the video (H.264 or H.265), wraps it in SRT, and bonds it across all available connections via SRTLA.

Connecting BELABOX to Vajra Cast

1. On Vajra Cast: Create an SRTLA input on a dedicated port (e.g., UDP 5000).
2. On BELABOX: Set the SRTLA destination to your-server:5000.
3. Start streaming. BELABOX connects and begins bonding.

Vajra Cast’s SRTLA receiver reassembles the bonded stream and presents it as a standard SRT input to the routing engine. From there, you can route it to any output: SRT push, RTMP to YouTube, HLS, recording, or all of the above.

Connection Monitoring

Vajra Cast displays per-connection statistics for SRTLA inputs:

• Number of active connections (modems)
• Bandwidth per connection
• Packet distribution ratio
• Connection health (latency, loss per path)

This lets you see at a glance which modems are performing well and which are struggling, without needing physical access to the BELABOX.

Mobile Production Use Case

SRTLA bonding enables professional live streaming from locations where traditional connectivity is impossible or impractical:

Live News and Events

A reporter at a breaking news scene has no time to set up satellite equipment or negotiate venue internet access. With a BELABOX and three cellular modems, they have a reliable streaming link in minutes.

Outdoor Sports

Streaming a cycling race, a marathon, or a sailing regatta from a moving vehicle or boat. Each cellular modem connects to different towers as the vehicle moves. SRTLA handles the constant handoffs and bandwidth variations.

Music Festivals and Outdoor Events

Venue Wi-Fi is overcrowded. A single cellular connection is unreliable due to tower congestion from thousands of attendees. SRTLA bonds multiple carriers together, and the diversity across carriers reduces the impact of any single tower being overloaded.

Houses of Worship

Many churches and synagogues stream weekly services from buildings with limited internet infrastructure. SRTLA bonding over cellular provides a reliable streaming connection without the cost of dedicated fiber installation.

Latency Considerations

SRTLA introduces additional latency compared to a direct SRT connection because:

1. Path variance. Different connections have different latencies. The receiver must wait for slower paths to deliver their packets before the stream can be assembled in order.
2. Jitter buffering. The SRT latency buffer must be large enough to accommodate the worst-case jitter across all connections.

Typical end-to-end latency for an SRTLA bonded stream:

These latencies are higher than a direct SRT connection over fiber (200-500ms) but are acceptable for most live streaming applications. The trade-off is reliability: you get a connection that works where nothing else would.

SRTLA vs. Other Bonding Solutions

SRTLA’s advantage is its tight integration with SRT. There is no protocol translation overhead, no separate VPN tunnel, and no subscription fees for server-side software. Vajra Cast includes the SRTLA receiver at no additional cost.

Reliability Design

For maximum reliability with SRTLA, follow these principles:

1. Use at least 3 connections. Two connections provide redundancy; three provide redundancy with headroom. Four or more add further resilience.
2. Diversify carriers. Use different cellular carriers on different modems. Do not put all SIMs on the same carrier.
3. Include a non-cellular connection when available. Venue Wi-Fi or wired Ethernet supplements the cellular connections and often provides lower latency.
4. Set SRT latency conservatively. Start at 3000ms and reduce only after testing in the actual location.
5. Combine with multi-input failover. Use the SRTLA bonded feed as one input in a Vajra Cast priority chain, with a venue wired connection as a higher-priority input if available.

Next Steps

• Return to the SRT Streaming Gateway Guide for the full architecture overview
• Learn about SRT Redundancy for combining SRTLA with other failover patterns
• Explore SRT Latency Tuning for optimizing latency on bonded connections