Any goals regarding delay must take into account fundamental physics. Despite sciencefiction stories that say differently, any signal experiences a propagation delayresultingfrom the finite speed of light, which is about 300,000 kilometers per second (186,000miles per second). Network designers can also remember 1 nanosecond per foot. Thesevalues are for light traveling in a vacuum. A signal in a cable or optical fiber travelsapproximately two-thirds the speed of light in a vacuum.Delay is relevant for all data transmission technologies but especially for satellite linksand long terrestrial cables. Geostationary satellites are in orbit above the earth at a heightof about 36,000 kilometers, or 24,000 miles. This long distance leads to a propagationdelay of about 270 milliseconds (ms) for an intercontinental satellite hop. In the case ofterrestrial cable connections, propagation delay is about 1 ms for every 200 kilometers(120 miles).Another fundamental cause for delay is serialization delay, the time to put digital dataonto a transmission line, which depends on the data volume and the speed of the line. Forexample, to transmit a 1024-byte packet on a 1.544-Mbps T1 line takes about 5 ms.An additional fundamental delay is packet-switching delay. Packet-switching delayrefersto the latency accrued when switches and routers forward data. The latency depends onthe speed of the internal circuitry and CPU, and the switching architecture of the internetworking device. Latency also depends on the type of RAM that the device uses.Dynamic RAM (DRAM) needs to be refreshed thousands of times per second. StaticRAM (SRAM) doesn’t need to be refreshed, which makes it faster, but it is also moreexpensive than DRAM. Low-end internetworking devices often use DRAM to keep thecost low.Packet-switching delay can be quite small on high-end switches, in the 5- to 20-microsecond range for 64-byte Ethernet frames. Routers tend to introduce more latency thanswitches. The amount of latency that a router causes for packet switching depends onmany variables, including the router architecture, configuration, and software featuresthat optimize the forwarding of packets. Despite marketing claims by switch salespeople,you should not assume that a router has higher latency than a switch. A high-end router with a fast CPU, SRAM, optimized software, and a highly evolved switching fabric canoutperform many low-end or medium-end switches.Of course, a router has a more complicated job than a Layer 2 switch. In general terms,when a packet comes into a router, the router checks its routing table, decides whichinterface should send the packet, and encapsulates the packet with the correct data linklayer header and trailer. Routing vendors, such as Cisco, have advanced caching mechanisms so that a frame destined for a known destination can receive its new encapsulationquickly without requiring the CPU to do any table lookup or other processing. Thesemechanisms minimize packet-switching delay.Packet-switching speed depends on the type and number of advanced features that areenabled on a packet-switching device. When designing an internetwork fabric, considerthe power that you will need to incorporate into the design to implement quality of service (QoS), Network Address Translation (NAT), IPsec, filtering, and so on. Consider thepolicies that your design customer wants to enforce and the effect they will have onpacket-switching delay.
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