NOV/ DE C 20 12 19 ® STEN NORDELL is chief technology officer at DWDM and packet- transport systems vendor Transmode . FEATURE By STEN NORDELL With latency becoming a factor in an increasing variety of applications, there are several steps carriers can take to reduce transmission delays. Network latency – how low can you go? L OW LATENCY has always been an important consideration in telecom networks for voice, video, and data, but recent changes in applications within many sectors have brought low latency to the forefront of those industries. The finance industry and its algorithmic trading is a commonly quoted example of a key market for which low latency is critical. Yet many other industries and datahandling activities such as cloud computing and video services are also now driving ever- lower latency in today’s networks. Meanwhile, as mobile operators start to roll out wireless Long Term Evolution ( LTE) services, latency in the backhaul network will become increasingly important to achieve the quality required for applications like realtime gaming and streaming video. In voice networks, latency must be low enough so the delay in speech does not cause problems with conversation. Here, the latency, which typically needs to be 50 msec or lower, is generated by the voice switches, multiplexers and transmission systems, and the copper and fiber infrastructure. Transmission systems add only a small proportion of the overall latency, and historically latency has not been a large consideration as long as it was low enough. In data networks, low latency has been seen as an advantage but until recently was not a top priority in most cases – again, as long as the latency was not excessive enough to cause problems. A good example is Fibre Channel for SAN/ storage applications, where the throughput drops rapidly once the total latency reaches the point that handshaking between the two switches is not quick enough, a) Queuing on ingress increases latency and adds jitter Service application OAM Flows Egress Switching Queuing MAC GbE PHY SFP 1 SFP 2 SFP 21 SFP n XFP 1 XFP n 10G PHY b) Queuing on egress removes latency and eliminates jitter Service application OAM Flows Egress Shared fair adaptive queuing Ultra fast no jitter Switching MAC GbE PHY Stratum 3 SFP 1 SFP 2 SFP 21 SFP 22 XFP 1 XFP 8 10G PHY FIGURE 1. A comparison of different switch architectures and their effects on latency and jitter. In ( a), the conventional enterprise- oriented design, a generic enterprise switch architecture is not optimized to lower latency and jitter. Conversely, a design based on an Output Buffer Queued architecture, as shown in ( b), can significantly decrease network latency as well as jitter.