Leased data connections

Link Types

Leased WAN connections can be established using different link types:

  • Physical Links
  • Packet-switched Links
  • Packets-routed Links

Physical Links

Physical links exist as dark fiber and gray fiber. Dark fiber is a passive optical fiber cable with lengths up to 80km; In contract to dark fibers, grey fibers come with additional optical or electrical elements that combine signals onto a single fiber (Wavelength-division multiplexing, WDM) or add timing and level regeneration. These additional elements connecting the individual fibers can add latency and jitter to the signal, so that the parameters needs to be assessed before using such a connection in production.

Packet-switched Links

Use a technique such as Multiprotocol Label Switching (MPLS). A virtual point-to-point connection is established. These connections can be set up conforming to defined bandwidth and QoS parameters, but the physical infrastructure underneath is shared.

Packet-routed Links

Access to the network is established using techniques such as Digital Subscriber Line (DSL) and uses IP packet routing. The path is shared and no guarantees for bandwidth or QoS are given. Multicast traffic forwarding is usually not available.

Analysis

Broadly speaking the physical links provide the best performance at the highest cost, while the packet-routed links provide the least performance for the lowest cost. Choosing the right WAN connection for your application depends both on budget and requirements, which are also determined by the capabilities of the devices you finally connect to the link.

 

Bandwidth and QoS 

First, determine how much traffic you will need to transport at peak times, e.g. 4 J2K compressed video streams with a data rate of 100Mbit/s each, equaling a total net data rate of 400Mbit/s. Consider that there is overhead incurred by the packaging of the audio / video data into e.g. SMPTE ST2022-6. If you cannot measure the final data rate, calculate with approximately 5% overhead.

Data streams originating from the V__line are marked “Expedite Forwarding” (EF; see above). Ask your provider how much “EF” traffic he can guarantee on the line (e.g. 90% of a 1GBit/s line). The remaining traffic is handled as “best effort” and comes with no guarantees in regards to latency, packet delay variation and packet loss.

If the bandwidth guarantee is below 100%, ask your provider about how “bursty” traffic is handled.

Choose the bandwidth of the connection based on the comparison of the needed bandwidth and the bandwidth guarantee of the provider.

 

Latency and Packet Delay Variation 

Query the packet delay variation (“PDV”, aka “jitter”) and the packet loss rate for the “EF” traffic on the line. Packet loss is often qualified as “Packet Loss Ratio” describing the number of packet losses per time unit, e.g. one packet loss per 10 days. Be aware that “bursty” (as opposed to evenly distributed) packet loss has more impact on the resulting video quality and can lead to a degraded experience even on lines rated with relatively low loss.

When renting two lines for Seamless Protection Switching (“SPS”, aka hitless merge), the latency difference (“offset”) must be below the maximum data buffer size including a reserve to compensate for the Packet Delay Variation on the lines.

 

Measurement

Once you have the leased line available and a V__line unit connected to each end, you can use the V__line “Sounding” feature to measure the line characteristics. For details see the V__line manual.

 

Sample line characteristics

LAN

For LANs, the error rate should be below 10-8 (Copper) 10-12 (Fiber).

PDV is expected to be below 10µsec.

WAN

For WANs, the error rate should be below 10-3.

PDV is expected to be below 20ms.

 

Lawo device performance 

The V__line has been tested successfully with 30ms of PDV using RAW video (SMPTE ST-2022-6) and 150ms of PDV using J2K-encoded video (2x2 mode; currently only 8ms for 4x4 mode).

 

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