4.1 Introduction to Ethernet/802.3 LANs  
  4.1.6 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, understanding latency and its effect on network timing is crucial to determining whether CSMA/CD for detecting collisions and negotiating transmissions 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 receiving 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 along 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 according to which networking devices, whether they are Layer 1, Layer 2, or Layer 3, are added to the path between the two communicating computers.

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-consuming functions. A router must analyze Layer 3 data.