Network Latency, Jitter and Loss Statistical multiplexing assumes
Network Latency, Jitter and Loss Statistical multiplexing assumes that everything will be okay if the average bit rate of all the inbound packet streams does not exceed the capacity of the outbound link. Or alternatively stated, most of the time most packets do not arrive at the same time and will not collide with each other s need for the outbound link . Of course, in reality, packet arrivals do coincide. When multiple inbound packets arrive at the same instant for the same outbound link, the packets are queued up and transmitted one after the other. We will refer to this situation as transient congestion. As previously noted, transmitting a single packet on a physical link introduces a finite serialisation delay proportional to the link s speed. Consequently, any packet queued up for transmission on a particular link will experience additional latency due to the serialisation delays of every packet in the queue ahead of it. We refer to this as queuing delay. Queuing delays appear under many guises in everyday life. Teller service at your local bank, or check-in at your favourite airline, involve queues to cope with customer arrival patterns that are bursty and that often exceed the processing capacity of the available tellers or check-in agents. The delay you personally experience can be short or long, depending on how many people arrived just before you and how fast the tellers (or check-in agents) are processing previous customers. In a typical consumer environment, queuing delays are seen when multiple computers on a home LAN try to send packets out through the same cable modem or ADSL modem. When outbound packets converge on the broadband router they will be queued up, waiting their turn to be transmitted on the upstream link to the ISP (which is usually ten to a hundred times slower than the local LAN link). Another form of queuing delay occurs on shared links where only one host can transmit at a time, and a link access protocol operates to share transmission opportunities amongst attached hosts. A modern example involves 802.11 b/g wireless LANs (so-called WiFi networks). The Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism and four-way handshake protocol (to avoid hidden-node problems) create access variable delays that depend on the traffic load on the wireless network (number of clients and/or number of packets per second being sent). For example, 802.11 b networks have been shown experimentally to add 50 to 100 ms to the RTT when heavily loaded by bulk TCP file transfers [NGUYEN04]. 5.2.4 Sources of Jitter in the Network As noted in Chapter 4, the actual path taken by a stream of packets can vary over time. When a route change occurs, the new path may be shorter or longer (in both kilometres and number of hops). Packets sent immediately after the route change will still get to their destination and yet experience a different latency. Route changes are usually uncommon, but can create a noticeable change in lag between a game client and server. On links that introduce noticeable serialisation delay, we can experience jitter due simply to the variations in size between consecutive packets sent over the link. This relates directly to congestion-induced queuing delay. Queuing delay depends entirely on the statistical properties of other traffic sharing a congested outbound link not just when and how fast the competing packets arrive, but their size distribution too. Because transient congestion depends on the vagaries and burstiness of entirely unrelated traffic, the queuing delay seen by any particular flow of packets can seem entirely random.
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