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latency_bandwidth.md

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Speed as a Feaure; Latency and Bandwidth

Speed is a feature

  • Faster sites lead to better user engagement.
  • Faster sites lead to better user retention.
  • Faster sites lead to better higher conversions.

Latency: the time from the source sending a packet to the destination receiving it

  • Propagation Delay: Amount of time required for a message to travel from the sender to receiver, which is a function of distance over speed with which the signal propagates.
    • usually within a small constant factor of the speed of light
  • Transmission Delay: Amount of time required to push all the packet’s bits into the link, which is a func‐ tion of the packet’s length and data rate of the link.
    • determined by the available data rate of the transmitting link
  • Processing Delay: Amount of time required to process the packet header, check for bit-level errors, and determine the packet’s destination.
    • once the packet arrives at the router, the router must examine the packet header to determine the outgoing route and check other properties for the data
  • Queuing Delay Amount of time the incoming packet is waiting in the queue until it can be pro‐ cessed.
    • if the packets are arriving at a faster rate than the router is capable of processing, the packets are queued inside an incoming buffer

Total latency is the sum of all of the delays listed above.

Cause -> Effect Relationships

  • farther distance between source and destination +> INCREASED propagation delays
  • more intedrmediate routers along the way => higher processing and transmission delays for each packet
  • the higher the load of the traffic along the path => higher the likelihood of queueing delays

Bandwidth: maximum throughput of a logical or physical communication path

The ratio of the speed of light and the speed with which a packet travels in a material is known as the refractive index of the material. The larger the value, the slower light travels in that medium. * The typical refractive index value of an optical fiber, through which most of our packets travel for long-distance hops, can vary between 1.4 to 1.6 * rule of thumb is to assume that the speed of light in fiber is around 200,000,000 meters per second, which corresponds to a refractive index of ~1.5

Content Delivery Networks: by distributing content around the world and serving that data from a nearby location to the client, we can significantly reduce the propagation time of all the data packets

Last-Mile Latency: it is often the last few miles, not the crossing of oceans or continents, where significant latency is introduced. To connect your home/office to the Internet, the local ISP must route the cables throughout the neighborhood, aggegrate the signal, and forward it to a local routing node.

Latency, not bandwidth, is the performance bottleneck for most websites!

Traceroute is a simple network diagnostics tool for identifying the routing path of the packet and the latency of each network hop in an IP network.

$ traceroute google.com

Optical fibers have a distinct advantage when it comes to bandwidth because each fiber can carry many different wavelengths (channels) of light through a process known as wavelength-division multiplexing (WDM). Hence, the total bandwidth of a fiber link is the multiple of per-channel data rate and the number of multiplexed channels.

We must construct protocols with explicit awareness of the limitations of available bandwidth by

  1. reducing round trips
  2. moving data closer to the client
  3. building applications that hide latency through caching, pre-fetching and other techniques (to be covered...)