CableLabs Low Latency DOCSIS® Technology Launches 10G Broadband into a New Era of Rapid Communication
Remember the last time you waited (and waited) for a page to load? Or when you “died” on a virtual battlefield because your connection couldn’t catch up with your heroic ambitions? Many internet users chalk those moments up to insufficient bandwidth, not realizing that latency is to blame. Bandwidth and latency are two very different things and adding more bandwidth won’t fix the internet lag problem for latency-sensitive applications. Let’s take a closer look at the difference:
- Bandwidth (sometimes referred to as throughput or speed) is the amount of data that can be delivered across a network over a period of time (Mbps or Gbps). It is very important, particularly when your application is trying to send or receive a lot of data. For example, when you’re streaming a video, downloading music, syncing shared files, uploading videos or downloading system updates, your applications are using a lot of bandwidth.
- Latency is the time that it takes for a “packet” of data to be sent from the sender to the receiver and for a response to come back to the sender. For example, when you are playing an online game, your device sends packets to the game server to update the global game state based on your actions, and it receives update packets from the game server that reflect the current state of all the other players. The round-trip time (measured in milliseconds) between your device and the server is sometimes referred to as “ping time.” The faster it is, the lower the latency, and the better the experience.
Interactive applications, where real-time responsiveness is required, can be more sensitive to latency than bandwidth. These applications really stand to benefit from technology that can deliver consistent low latency.
As we’ve alluded, one good example is online gaming. In a recent survey we conducted with power users within the gaming community, network latency continually came up as one of the top issues. That’s because coordinating the actions of players in different network locations is very difficult if you have “laggy” connections. The emergence of Cloud gaming makes this even more important because even the responsiveness of local game controller actions depends on a full round-trip across the network.
Queue Building or Not?
When multiple applications share the broadband connection of one household (e.g. several users performing different activities at the same time), each of those applications can have an impact on the performance of the others. They all share the total bandwidth of the connection, and they can all inflate the latency of the connection.
It turns out that applications that want to send a lot of data all at once do a reasonably good job of sharing the bandwidth in a fair manner, but they actually cause latency in the network when they do it, because they send data too quickly and expect the network to queue it up. We call these “queue-building” applications. Examples are video streaming and large downloads, and they are designed to work this way. There are also plenty of other applications that aren’t trying to send a lot of data all at once, and so don’t cause latency. We call these “non-queue-building” applications. Interactive applications like online gaming and voice connections work this way.
The queue-building applications, like video streaming or downloading apps, get best performance when the broadband connection allows them to send their data in big bursts, storing that data in a buffer as it is being delivered. These applications benefit from the substantial upgrades the cable industry has made to its networks already, which are now gigabit-ready. These applications are also latency-tolerant – user experiences are generally not impacted by latency.
Non-queue-building applications like online gaming, on the other hand, get the best performance when their packets don’t have to sit and wait in a big buffer along with the queue-building applications. That’s where Low Latency DOCSIS comes in.
What is Low Latency DOCSIS 3.1 and how does it work?
The latest generation of DOCSIS that has been deployed in the field—DOCSIS 3.1—experiences typical latency performance of around 10 milliseconds on the access network link. However, under heavy load, the link can experience delay spikes of 100 milliseconds or more.
Low Latency DOCSIS (LLD) technology is a set of new features, developed by CableLabs, for DOCSIS 3.1 (and future) equipment. LLD can provide consistent low latency (as low as 1 millisecond) on the access network for the applications that need it. The user experience will be more consistent with much smaller delay variation.
In LLD, the non-queue-building applications (the ones that aren’t causing latency) can take a different path through the DOCSIS network and not get hung up behind the queue-building applications. This mechanism doesn’t interfere with the way that applications go about sharing the total bandwidth of the connection. Nor does this reduce one application's latency at the expense of others. It is not a zero-sum game; rather, it is just a way of making the internet experience better for all applications.
So, LLD gives both types of applications what they want and optimizes the performance of both. Any application that wants to be able to send big bursts of data can use the default “classic” service, and any application that can ensure that it isn’t causing queue build-up and latency can identify its packets so they use the “low latency” service. Both then share the bandwidth of the broadband connection without one getting preference over the other.
Incorporating LLD Technology
Deploying Low Latency DOCSIS in a cable operator’s network can be accomplished by field-upgrading existing DOCSIS 3.1 CMs and CMTSs with new software. Some of the low latency features are even available to customers with older (pre-DOCSIS 3.1) CMs.
The technology includes tools that enable automatic provisioning of these new services, and it also introduces new tools to report statistics of latency performance to the operator.
DOCSIS equipment manufacturers are beginning to develop and integrate LLD features into software updates for CMTSs and CMs, and CableLabs is hosting Interoperability Events this year and next year to bring manufacturers together to help iron out the technology kinks.
We expect these features to become available to cable operators in the next year as they prepare their network to support low latency services.
LLD provides a cost-effective means of leveraging the existing hybrid fiber-coaxial (HFC) network to provide a high-performance network for latency-sensitive services. These services will help address customers’ requirements for many years into the future, maximizing the investments that cable operators have made in their networks. The cable industry is provisioning the network with substantial bandwidth and low latency to take another leap forward with its 10G networks.
For those attending the SCTE Cable-Tec Expo in New Orleans, Greg will be presenting the details of this technology on a SCTE panel “Low Latency DOCSIS: Current State and Future Vision”