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Network latency / Ethernet 10BASE-T transmission time

Network latency
4.1.6 This page will help students understand the factors that increase network latency.
Latency, or delay, is the time a frame or a packet takes to travel from the source station to the final destination. It is important to quantify the total latency of the path between the source and the destination for LANs and WANs. In the specific case of an Ethernet LAN, it is important to understand latency and its effect on network timing as it is used to determine if CSMA/CD will work properly.
Latency has at least three sources:
  • First, there is the time it takes the source NIC to place voltage pulses on the wire and the time it takes the destination NIC to interpret these pulses. This is sometimes called NIC delay, typically around 1 microsecond for a 10BASE-T NIC.
  • Second, there is the actual propagation delay as the signal takes time to travel through the cable. Typically, this is about 0.556 microseconds per 100 m for Cat 5 UTP. Longer cable and slower nominal velocity of propagation (NVP) results in more propagation delay.
  • Third, latency is added based on network devices that are in the path between two computers. These are either Layer 1, Layer 2, or Layer 3 devices.
Latency does not depend solely on distance and number of devices. For example, if three properly configured switches separate two workstations, the workstations may experience less latency than if two properly configured routers separated them. This is because routers conduct more complex and time-intensive functions. A router must analyze Layer 3 data.
The next page will discuss transmission time. 

 Ethernet 10BASE-T transmission time 
4.1.7 This page will explain how transmission time is determined for 10BASE-T.
All networks have what is called bit time or slot time. Many LAN technologies, such as Ethernet, define bit time as the basic unit of time in which one bit can be sent. In order for the electronic or optical devices to recognize a binary one or zero, there must be some minimum duration during which the bit is on or off.
Transmission time equals the number of bits to be sent times the bit time for a given technology. Another way to think about transmission time is the interval between the start and end of a frame transmission, or between the start of a frame transmission and a collision. Small frames take a shorter amount of time. Large frames take a longer amount of time.
Each 10-Mbps Ethernet bit has a 100 ns transmission window. This is the bit time. A byte equals eight bits. Therefore, 1 byte takes a minimum of 800 ns to transmit. A 64-byte frame, which is the smallest 10BASE-T frame that allows CSMA/CD to function properly, has a transmission time of 51,200 ns or 51.2 microseconds. Transmission of an entire 1000-byte frame from the source requires 800 microseconds. The time at which the frame actually arrives at the destination station depends on the additional latency introduced by the network. This latency can be due to a variety of delays including all of the following:
  • NIC delays
  • Propagation delays
  • Layer 1, Layer 2, or Layer 3 device delays
The Interactive Media Activity will help students determine the 10BASE-T transmission times for different frame sizes.
The next page will describe the benefits of repeaters.

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