Understanding Latency and Throughput Requirements in Wireless Networks

 

In today's interconnected world, the success of wireless deployments hinges on two critical factors: latency and throughput. Whether you're designing a network for voice communications, video conferencing, or data transfer, understanding these requirements is crucial for delivering optimal performance. No one would like to work on a slow wireless network and impacts the users productivity.

Following is one of the screenshots from my laptop when I was on a customers site and I would explain in the end, why?



Before we deep dive into the situation, lets understand the Latency Challenges:

Latency, simply put, is delay - the time it takes for data to travel from source to destination. While this might seem straightforward, managing latency in wireless networks is more complex than it appears.

VoIP: A Prime Example

Voice over IP (VoIP) serves as an excellent case study for latency requirements. The general rule is simple:

- Maximum one-way latency: 150ms

- Round-trip time (RTT): Should not exceed 300ms

- Packet loss threshold: Less than 1%

However, real-world scenarios can be more complicated. For instance, asymmetric latency (where one direction experiences different delay than the other) can create significant issues. A 120ms delay in one direction paired with a 180ms delay in the other might maintain the 300ms RTT requirement, but the 180ms leg could still cause noticeable quality issues.

Three Key Areas Affecting Latency
 

1. Wireless Connectivity

   - Signal strength and interference can significantly impact delay

   - Power mismatches between access points (APs) and clients can create asymmetric performance

 2. Wired Network

   - The backbone infrastructure plays a crucial role

   - Network congestion can introduce additional delays

 3. Processing Devices

   - Often overlooked but significant

   - Router overload and encryption processing can add substantial delay

 

Throughput Demands: Beyond Just Speed

Throughput represents the amount of useful data transferred through the network, typically measured in Mbps or Gbps. Different applications have varying throughput requirements. It is always great to see a good throughput when you fire up the Speed Test:


 

Application-Specific Demands

1. Voice Communications

   - Bandwidth: 30-128 Kbps

   - Less demanding on throughput but highly sensitive to latency

   - Consistent performance is crucial

2. Video Applications

   - Bandwidth: 384 Kbps - 20+ Mbps

   - Requirements vary between live and stored content

   - Live streaming needs:

     - Latency: 200-400ms

     - Jitter: ≤30-50ms

     - Loss: ≤0.1-1%

3. Data Applications

   - Variable bandwidth requirements

   - More tolerant of latency

   - Can benefit from <2-5% loss tolerance

Best Practices for Network Design

1. Comprehensive Testing

   - Test the full communication path, not just wireless links

   - Include all network elements in testing (APs, switches, routers)

   - Monitor real-world usage patterns

2. Capacity Planning

   - Calculate total throughput needs based on user count

   - Example: If one user needs 250 Kbps, 50 simultaneous users require 12.5 Mbps

   - Include overhead for future growth

3. Power Management

   - Set AP power levels appropriately

   - Maintain 20-25% coverage overlap

   - Consider RRM (Radio Resource Management) capabilities

 

Special Considerations for Outdoor Deployments

When deploying outdoor wireless networks:

- Check AP channelization along walkways

- Don't rely solely on auto-features for channel settings

- Consider rooftop and airway visibility between APs

- Manual channel setting may be necessary for specific scenarios

 

By carefully considering these factors in your wireless deployment, you can create a network that delivers the performance your users need, whether they're making voice calls, streaming video, or transferring data.

Case Study: 

The Laptop screenshot, I have provided in the beginning, shows the speedtest results of my Laptop and I know it wasn't great! However, if I move to a different AP or if the AP is rebooted, the throughput of my laptop would be lot better ~410 Mbps. 

The real root cause of this issue was due to the high CPU usage on the AP. It was found that the AP was set to run streaming captures which in turn consumes a h=good amount of CPU resources. When we run packet captures using a using a Linux Host (in this case an AP), the packet data is copied from the network card to the kernel buffer and then to user space, which involves CPU cycles. Streaming packet capture further is inherently CPU-intensive, especially under high traffic conditions and therefore can lead to high CPU spike and result into poor packet processing for the Wireless clients. 
When you are trying to solve the throughput issues, look at all possibilities around you. Sometime, it is not always the poor signal strength or the channel bandwidth or the interference. There could be other factors that need an attention! 
 


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