Monthly Archives: November 2007

Measuring Latency – Propagation Delay – 4

Part 3 to this series presented serialization delay, or the delay incurred while placing packets of data on the wire. Part 4 of this series on measuring the latency in messaging systems will focus on the latency of these packets as they travel between the sending and receiving nodes.

Propagation Delay

The speed of light, or circa 186,000 miles per second is clearly the upper limit for the speed which packets can travel between sending and receiving nodes. The material used to wire and connect computer networks, be it copper or fiber, limits the speed at which messages can travel by a factor of the speed of light to roughly 75%.

Copper Cabling

The Telecommunications Industry Association (TIA) has developed standards over the years to address commercial cabling for telecom products and services ensuring minimum quality thresholds are met throughout the world. Cable types are typically characterized with performance attributes like those shown in the table below.

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The table displays the propagation characteristics of that category of cable. The propagation delay is typically quoted for a 100 meters length cable, which represents the maximum recommended distance for cables in a 10/100/1000baseT[X] environment.

To illustrate, at a high-level, how propagation delay affects common messaging applications, imagine a streaming application that generates 2,600 new 100 byte messages per second. Individual messages for this application would require about 3 seconds to travel between nodes in New York and California connected when traveling at T1 speeds. Circa 1.5 seconds of this time (2600messages*100bytes / 192,500bytes/second) would be spent serializing the messages onto the wire, and another 20+ milliseconds(3200 miles/ 75% the speed of light) for the messages to propagate between New York and California. Additional latencies are incurred as each network device routes the packet along the its path. We’ll cover these other latencies in part 5 of this series.

Fiber Optic Cabling

Copper wiring is not the only cable type that is used in modern computer networks. Fiber optic cables make up the backbone wiring technology to many of the world’s computer networks. We read above that various types of copper cables exhibit roughly 5.48 nanosecond propagation delay per meter of cable.   For a practical understanding of the propagation delay characteristics of fiber optic cable, refer to the comments following this post.

Cable Lengths

The minimum packet size for Ethernet networks is closely related to the maximum cable length for segmented nodes in these networks.  Click here for a detailed description of this relationship.

Minimizing Propagation Delay

Low latency sensitive industries, such as the financial services, have relied heavily on tactics such as collocation, which geographically minimizes the distance between nodes along the message path by hosting sender and receiver in the same physical location. As the geographic distance is minimized, so is the propagation delay between nodes.

In part 5 of this series, we’ll wrap up the topic of measuring latency by covering other factors that contribute to latency.

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Measuring Latency – Serialization Delay – 3

Part 2 to this series presented packetization delay, or the delay incurred as all systems, along the message path, create and reshape packets. Part 3 of this series on measuring the latency in messaging systems will focus on serialization delay, or the delay in moving packets from the Network Interface Controller’s (NIC) transmit buffer to the wire.

Minimizing Serialization Delay

Larger bandwidth technologies play a much greater roll in reducing the serialization delay than does changing the packet size. This is because serialization delay is a function of packet size and transmission rate expressed as:

Serialization Delay = Size of Packet (bits) / Transmission Rate (bps)

A packet size of 1500 bytes, transmitted using the T1 technology (1,544,000 bps) would produce a serialization delay of about 8 milliseconds. The same 1500 byte packet using 56K modem technology (57, 344 bps) would result in a 200 millisecond serialization delay, whereas using Gigabit Ethernet technology (1,000,000,000 bps) would reduce the 1500 byte packet’s serialization delay to 11 microseconds.

In part 4 of this series, I’ll cover the third of the three latency delays, namely propagation delay.

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