Network Congestion

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Network Congestion Definition

Network congestion refers to a reduction in quality of service (QOS) that causes packet loss, queueing delay, or the blocking of new connections. Typically, network congestion occurs in cases of traffic overloading when a link or network node is handling data in excess of its capacity.

To avoid collapse and reduce the effects of congestion in the network, organizations use various congestion avoidance and congestion control methods. These include:

  • TCP/IP window reduction
  • Fair queueing in network devices such as routers, switches, and other devices
  • Priority schemes which transmit higher priority packets ahead of other traffic
  • Explicit network resource allocation via admission controls toward specific flows

This image depicts network congestion showing: data traveling through network and becoming congetsted on it's way to receivers.


What is Network Congestion?

A highway is congested when it is overloaded with traffic in the form of vehicles. Similarly, a network is congested when it is overloaded with data. And just as is true on the road, network congestion can be the result of temporary circumstances such as high traffic or an attack, or the sign of deeper, chronic problems such as outstanding repairs or misconfiguration—issues that demand more significant solutions.

As far as the end-user is concerned, network congestion feels like slow response times or a “network slow down.” When the internet, the WiFi, or even the computer itself just “feels slow,” that is often the result of network congestion. However, there’s more to network traffic congestion than this.

How to check network congestion and identify issues depends on detecting the effects of congestion in the network:

Bandwidth Usage

Bandwidth is among the most common causes of network congestion. Bandwidth refers to the maximum rate that data can move along a path, or the total capacity of that path. Network congestion happens when there’s just not enough bandwidth to handle the existing amount of traffic. This is the same problem that a road built for 50 cars faces when 200 cars a day try to drive on it.


Latency is the time it takes a data packet to travel from point A to point B. Latency is usually closely connected to other congestion issues such as bandwidth. Back on the road, latency is expressed by the way it takes you 20 minutes to travel from point A to point B on one day during certain conditions and 60 minutes for the same trip under different conditions. The slower time is latency, and it’s a sign of the problem, rather than something that itself leads to network congestion.


Jitter is variation in delay in traffic patterns. Computers, like most humans on the road, prefer predictable, consistent traffic. When traffic is unpredictable or inconsistent, this produces jitter, or variability in delay, causing more network congestion.

On the road, drivers access the highway randomly, which means there may be large bursts of cars trying to merge on at any one time. For networks, such a surge can come from a system user that sends the network large bursts of traffic, consuming excessive bandwidth.

Jitter creates congestion because the computer changes its traffic patterns each time the network tries to adjust. To avoid network collisions, the system pauses sending packets and initiates a random back-off for a period of time measured in milliseconds. This increases congestion as other network transmitters wait before trying again in a cascading effect.

Packet Retransmissions

Speaking of the need to retransmit packets, packet retransmissions can also cause congestion and are typically caused by other congestion issues. Packets that arrive damaged or don’t arrive at all must be resent. Clearly, each time a single packet must be sent two or more times, traffic congestion increases without any incremental benefit. It would be like breaking up a successful carpool.


Often it is packet collisions on the network that trigger the back-off process, described above in relation to jitter. Packet collisions can be caused by poor cabling or bad equipment, and can produce a serious situation, forcing all packets to stop and wait for a clear network to retransmit. This produces even greater congestion and delay, and like with a highway collision, traffic direction is often required.

What Are the Reasons for Congestion in a Network?

Generally, network congestion occurs when a network experiences traffic that is too much for the system. That said, there are five main network congestion causes:

Over-Used Devices

Some devices can handle more traffic than others, by design. Devices such as load balancers, switches, routers, and firewalls are built for network throughput. Additionally, any device’s assigned capacity is theoretical; it may not accurately represent the real-world ability of the device in various scenarios. For this reason, over-utilization is a frequent result of pushing devices to their maximum reported capacity.

Often, structures for using multiple devices are hierarchically designed, with higher-level devices serving lower-level devices. To ensure healthy traffic levels and prevent congestion, it’s critical to ensure within the hierarchy that each level is demanding and receiving appropriate support. Incongruencies between firewalls, routers, switches, and other devices can lead to data bottlenecks.

Back on the road, this kind of problem might look like a freeway transition that was not up to the task of merging multi-lane roads into fewer lanes. The constant use of that device with excess levels of traffic will result in over-utilization of the device, and probably packet drops and high CPU utilization. This in turn produces network congestion.


Over-subscription is usually to blame when a web browsing experience is consistently slower or faster at some times of the day or night. This is because during the day, the network’s peak period, there are more users making demands on network resources than there are at night, the off-peak period for the network. This is like commuting to work on a highway or on a train during rush hour or peak times versus the middle of the day or night when people are all already working or home.

Over-subscription is typically done intentionally to achieve cost savings, which is why it is a common cause of network congestion. For example, a business with 1,000 employees in-house normally needs a 1,000Mbps Internet link. If they’ve moved largely to work from home, they may only need 500Mbps—until a company-wide event strains the system and produces network congestion.

Unneeded Traffic

Unneeded traffic such as streaming video on a work system is another common cause of network congestion. Other examples of unneeded traffic eating up bandwidth include junk VoIP phone calls or unsolicited traffic like advertisements. Use the network management console to identify unneeded traffic.

Antiquated Hardware and Faulty Devices

Hardware, devices, and the ethernet cables and wire connections between them might all require updates or replacements as the organization’s needs change. The data speed and other metrics for each piece of the network should be analyzed as part of a network performance assessment for network congestion.

Security Attack

Various security attacks can cause network congestion, including worms, viruses, and Denial of Service (DoS) attacks.

Deficient Design or Misconfiguration

Poor design or device misconfiguration is a more serious cause of network congestion. Each network must be designed to handle the right loads and configured to meet that organization’s needs. An optimized network connects all segments while maximizing performance across each of them.

A broadcast storm is a good example of this issue. This problem causes severe performance degradation when the network experiences a large mass of broadcast or multicast traffic in a short time. Broadcasts are contained inside subnets, so a broadcast storm can have more serious effects on larger subnets. Designing a network that has large subnets without giving proper consideration to broadcast storms can cause network congestion. To avoid this problem, create subnets near where large amounts of data will be stored to allocate performance where it’s needed.

How to Solve Network Congestion Issues?

Monitor and Analyze Network Traffic

The starting point for solving most network congestion issues, especially for too many devices, over-utilized devices, or an insufficient network design, is monitoring and analyzing network traffic. This will help identify where congestion may exist and highlight under-utilized regions that are ripe for re-allocation to improve performance. With deeper insights into network traffic, it’s possible to take intelligent steps toward reducing network congestion.

Monitor during heavy traffic times to diagnose network congestion, especially during peak hours when many devices are connected, or during company wide events. The right network discovery tool can help reveal the source of network congestion. Scan cloud servers, virtual networks, and all other wireless devices and networks with a network discovery program to identify servers, devices, and even users eating up too much bandwidth.

After identifying the issues with bandwidth usage, update the network infrastructure to allocate it more effectively during peak times.


Speaking of bandwidth, network congestion is less likely when the network can transmit more data, making increasing bandwidth an obvious solution. However, a network, like a chain, is only as strong as the weakest—or in this case the slowest—component.

Segmenting and Prioritizing

Traffic monitoring produces an additional benefit: the capacity to design or re-design a bespoke, optimized network for any business. To do that, segment the network into smaller sub-networks to create space for practical priorities and increase efficiency. This permits more accurate monitoring as it produces a more viable network, increasing or reducing data traffic as needed to impact the areas most affected by network congestion.

Prioritization means placing appropriate emphasis or priority on key network processes over less- or non-essential traffic to reduce network congestion. Prioritizing must be done carefully to avoid the wrong design or configuration—which can exacerbate the problem it is meant to resolve.

Business critical traffic can be a mix of typical business network traffic types, including multicast traffic for real time media streams, broadcast traffic for network operation, and unicast traffic to support everyday voice, data transfer, and video functions. Network devices cannot automatically distinguish which of this intermixed traffic should get a priority share of bandwidth without special configuration. This is the realm of Quality of Service (QoS) protocols.

Quality of Service allows traffic to share the same network, but classified and forwarded in an unequal way based on preset rules. QoS is like a police escort that helps real-time applications and business critical traffic through network congestion.

Assess Your Devices

The number, type, and bandwidth usage of network devices affects the whole network’s data processing. In some cases, some network users might accidentally be incorrectly using devices, while other users could be using “legacy devices” that are not well-supported. Older and inefficient device usage both contribute to network congestion, so assess each device to reduce or even prevent network congestion.

Assess Your Network Architecture

Network architecture should be built to provide each user with the appropriate network bandwidth. The wrong network architecture can cause network congestion.

For example, a large company is more likely to deploy a “client/server” network architecture than a “peer-to-peer” network which can provide too much access and bandwidth to users. Instead, allocate access according to needs-based, specific “tiers” for all users. IT or the C suite can ultimately control processing speeds, access levels, and other network permissions to reduce the risk of network congestion.

Other areas to explore when considering how to reduce network congestion include:

  • Optimize the TCP/IP settings to balance the packet send/request speed
  • Use a CDN (Content Delivery Network) that will place more requests on edge servers to optimize resources
  • Use choke packets to prevent network congestion by reducing sender device output
  • Choose multi-hop routing for traffic so whenever the default route starts queueing traffic will be routed over a different path
  • Assess security attacks and attack attempts in your internet connection logs and elsewhere
  • Use a VPN to bypass congestion
  • Using redundancy models
  • Conduct LAN performance network congestion tests

Does VMware NSX Advanced Load Balancer Offer Network Congestion Monitoring Tools?

Yes. VMware NSX Advanced Load Balancer’s comprehensive traffic monitoring platform distributes network traffic across multiple servers to ensure no single server triggers network congestion as it bears too much demand. By creating and distributing an even workload, application responsiveness, availability, and security all increase. Learn more about our approach to network congestion management here.

For more on the actual implementation of load balancing, security applications and web application firewalls check out our Application Delivery How-To Videos.