(Accedian Networks') EtherNEWS Blog

Teletimes published this Feature Article in their current edition: download original Teletimes backhaul article.

Mobile Carrier Ethernet backhaul services from network operators, and the advent of packet based mobile systems, are predicted to provide mobile operators with scalable and more cost efficient solutions for handling both the increasing number of mobile devices attached to their networks and the traffic volumes they generate.

A major reason for the increased use of Carrier Ethernet in wireless backhaul applications is the ability to use a diverse physical infra­structure to deliver Carrier Ethernet to the base station. The physical delivery mecha­nisms for Carrier Ethernet include:

• Ethernet over copper
• Ethernet over fiber (both dark fiber and over SONET/SDH)
• Ethernet over bonded copper
• Ethernet over radio (microwave)
• Ethernet over PON (Passive Optical Networks)

Low latency is the key to delivering reliable, high-performance backhaul for 3G and 4G wireless networks. Real-time communications, transactional applications, high-speed roaming, and media streaming are all delay-sensitive. Latency increases of just a few milliseconds can result in dropped calls, garbled voice and unresponsive applications, and can mean significant losses in financial trading.

At times, service providers over-provide bandwidth to keep latency and jitter in check. While increasing bandwidth can sometimes reduce latency, it often has little effect. In packet-based networks the relationship between latency and bandwidth is complex and varied. Consider the four main sources of latency, categorized as:

• Serialization delay: time required for a port to transmit a packet, related to frame size and bit-rate;
• Propagation delay: limita­tions imposed by the laws of physics (speed of light, path length, circuit design);
• Congestion delay: the time a frame idles in the output queue of a network element (NE) while a backlog of packets is being transmitted. Congestion delay can be caused by traffic bursts, larger ingress vs. egress bandwidth (e.g. oversub­scribed aggregation), or due to network congestion resulting in paused trans­mission (flow control).
• Forwarding delay: the time required for the Network Element (NE) to analyze, process and forward a packet in a congestion-free scenario; a function of NE architecture and packet-processing requirements (the number and complexity of operations performed on a packet between receipt and transmission, e.g. service mapping, switching, rate limiting, shaping, etc).

Of these components, serialization delay is the most constant, having only a small influence on end-to-end latency. Propagation delay, typically stable in circuit-switched networks, can be irregular and introduce jitter over routed networks due to path variation; overall, its contribution is usually small, even under heavy utilization.

Packet Delay Sources

The more important sources of latency – congestion and forwarding delay – are not entirely independent: as a NE is subject to heavy load (conges­tion delay), it may need additional queue time to handle and process the increased volume of traffic (forwarding delay). Depend­ing on the NE’s design, forwarding delay can be significant when advanced functions such as traffic shaping and multi-flow Ethernet OAM (Operations Administration & Mainte­nance) are enabled. (more…)