Databases Reference
In-Depth Information
Network delay = propagation time + transmission time,
A.2
where
Propagation time = network distance/propagation speed,
A.3
and
Transmission time = packet size/network transmission rate.
A.4
The propagation speed is estimated to be 200,000 km/sec, which is approximately
two-thirds of the speed of light, taking into account normal degradations of transmission
media from the theoretical speed of light of approximately 300,000 km/sec. The network
distance is given in kilometers (km). If the network media you are using has a known
propagation delay, then substitute the known value for 200 km/ms in the equation.
Rough approximations are considered reasonable here to get “ballpark” estimates of
total time.
The transmission time for a packet on a network is the time to get the packet on or
off the network to or from the controlling device in the local computer system. It is
analogous to the transfer time for a block on a disk. The transmission rate is given in
bits/second and packet size is given in bytes. We assume an 8-bit per byte conversion
factor.
For example, the time to send one packet of 1,500 bytes (let's say its one block of
data) over an Ethernet with a transmission rate of 1 Gb/sec in a wide area network
(WAN) at a network distance of 1,000 kilometers would be:
Network time = 1,000 km/200,000 km/sec + 1,500 bytes
×
8 bits/byte/1 Gb/sec
= .005 sec + .000012 sec
= .005012 sec (or 5.012 ms).
For a local area network (LAN) with a network distance of .1 km, we get:
Network time = .1 km/200,000 km/sec + 1,500 bytes
×
8 bits/byte/1 Gb/sec
= .0000005 sec + .000012 sec
= .0000125 sec (or .0125 ms).
Putting these numbers in perspective, higher-speed Ethernet networks have very
small transmission times, and for longer distances of WANs, the total network time is
dominated by the propagation time. In this case, the typical values for network time
