between New York and Los Angeles. Both links have the same speed (1.54 Mbps), but it takes longer for packets to get from New York to Los Angeles than it does for them to get from New York to Cleveland. New York Cleveland Angeles Figure 19-4. Different propagation delays The discrepancy occurs because Los Angeles is a lot farther away from New York than Cleveland is. This form of latency is called propagation delay. Propagation delay is, to a large degree, a function of physics, and as such cannot be fixed, improved, or otherwise changed (no matter what your boss may want). To oversimplify, the speed at which electrons can transmit elec- trical impulses is limited. The speed at which photons can move in fiber is similarly limited. Telecom Glossary Processing delay Another form of latency is called processing delay, which is the time it takes for a device to process information. In contrast to propagation delay, which usually cannot be changed, processing delay is a function of the speed of the equipment in use. Figure 19-5 shows two links: the top link is a direct connection between two modern Cisco 7609 routers, involving only the latest hardware; the bottom link connects the same two routers, but with a very old Cisco 2501 router in the middle. Cisco 7609 Propogation delay about the same Cisco 7609 Cisco 2501 Cisco 7609 Cisco 7609 Added latency in the form of processing delay Figure 19-5. Processing delay Although the total distance between the two Cisco 7609s is the same from point to point in both cases, adding a Cisco 2501 in the middle of the second link increases the processing delay dramatically. Another example of increasing processing delay occurs when using multilink- PPP. Taking three 1.54 Mbps T1s and bonding them to form one logical 4.5 Mbps link sounds great, and it can be, but the added processing delay when you do SO can be enormous. As an example, notice the delay parameter in this show interface output from a T1 interface: Router# sho int s0/1 Serial0/1 is administratively down, line protocol i