Leased line latency and BTnet's latency commitment

What latency is

Latency is the time it takes for a piece of data to travel from one point to another and back again, usually measured in milliseconds (ms) as a round trip. When you click a link, speak on a call or save a file to the cloud, every exchange of data pays that delay.

Latency is not the same as speed. Bandwidth is how much data the line can carry at once; latency is how long each piece takes. A line can have plenty of bandwidth and still feel sluggish if latency is high or unsteady. These related measures are worth knowing:

• Latency (round-trip time): the time for data to reach a destination and for the reply to come back
• Jitter: how much latency varies from one packet to the next; steady is better than low but erratic
• Packet loss: data that never arrives and has to be resent, or is simply lost on a live call

Distance adds latency because signals take time to travel, and every hop through routers and networks adds a little more. Congestion adds the most unpredictable delay, because packets queue when a link is busy. That is where a dedicated line helps: BTnet bandwidth is uncontended, so your traffic does not queue behind a neighbour's. See contended vs uncontended internet for the background.

Why latency matters to a business

Most business applications are now interactive and cloud hosted, which makes them sensitive to delay and to variation in delay.

• Voice calls. High or unsteady latency causes talk-over, echo and gaps. BT says BTnet is optimised for voice, including its Cloud Voice and SIP services, and that it prioritises voice traffic over its low-latency network.
• Video meetings. Video needs a steady stream in both directions; jitter shows up as frozen frames and broken audio.
• Cloud apps and Microsoft 365. Every click in a web app is a round trip. BT uses Direct Private Peering to reach Microsoft, Apple, Google and Amazon clouds, so that traffic does not cross the public internet.
• Remote desktops and virtual desktops. Keystrokes and screen updates travel back and forth constantly, so delay is felt immediately.
• Payments, bookings and stock systems. Short transactions that happen thousands of times a day add up when each one waits.

If your business is moving phones to the cloud, our guide to a leased line for VoIP covers call planning, and Microsoft 365 and cloud apps covers peering in more detail.

BT's latency commitment for BTnet

BT's service schedule for BTnet includes a network latency service level. Latency is determined by BT, from average round-trip measurements taken at 10-minute intervals over each calendar month, on two defined paths:

If BT does not meet the commitment in two consecutive calendar months, BT's terms provide a service credit of one day's regular charges for the site. Under the current schedule you request the credit within 28 days of BT verifying the incident, giving the reason, the trouble ticket number and your reference. Our BTnet SLA page explains the claim steps and exclusions, which also apply to latency, such as planned engineering works and denial of service attacks.

These are BT's figures, measured on BT's network. They describe how BT's core performs, which is a useful standard to have in writing, but they are not a figure you will see on every test from your desk.

What the commitment does not cover

It helps to be clear about what sits outside BT's measurement, because these are often where slow-feeling connections come from:

• Your office network. Busy switches, long or damaged cabling, overloaded Wi-Fi and firewalls doing heavy inspection all add delay before traffic reaches the line.
• Your access line to BT's PoP. The commitment is measured from BT's selected PoP across its core, not from your desk.
• The wider internet. Once traffic leaves BT's network for another provider's, latency depends on routes and networks BT does not control.
• The far end. A slow server, an overloaded cloud service or a distant data centre adds its own delay.
• Wi-Fi. BT does not test wireless, and BT's terms exclude wireless access from the availability service level.

So if calls or apps feel slow, the fix may be inside your building rather than on the line. That is worth checking before you raise a fault, because BT may charge for investigating incidents it finds are not on its side.

How to check latency on your line

You can get a useful picture of latency with simple tools. For the cleanest result:

1. Use a wired connection. Plug a laptop into the network as close to the router or firewall as you can. Wi-Fi adds its own delay and variation.
2. Test at quiet and busy times. On an uncontended line, results should stay fairly steady through the day. Big swings point to congestion on your own network or on a busy link further on.
3. Ping more than one destination. Compare a nearby target with a distant one, and the services you actually use, to see where delay is added.
4. Look at jitter and loss, not just the average. A speed test that reports jitter and packet loss tells you more about call quality than download speed does.
5. Write it down. Note the time, the test and the result. If you need to raise a fault, a record of what you saw helps.

Our leased line speed test guide covers testing throughput in both directions as well. If results look wrong after you have checked your own network, report it to BT as a fault using BT's procedures, as described on BT leased line support.

Planning for low latency

A dedicated line is a strong foundation, and a few design choices help you get the most from it:

• Size the bandwidth properly. Even on an uncontended line, your own traffic can fill it. Backups, large uploads and updates running during the day can delay calls. Our bandwidth guide helps you plan, and Bandwidth Flex lets you change speed within the bearer later.
• Prioritise real-time traffic on your network. Quality of service settings on your switches, router or firewall keep voice and video ahead of bulk transfers.
• Keep the LAN in good order. Business-grade switches and sound cabling matter as much as the line.
• Think about resilience. Latency is irrelevant if the line is down. A failover or back-up line keeps critical traffic moving.

If call quality or cloud performance is the reason you are looking at BTnet, tell us what you run and we can help you plan the line around it. Contact us to start a quote.