On the left side of this illustration, we see (from top to bottom): a telephone labeled "H1" connected to a link-layer switch connected to a server labeled "H2." On the right side, we see (from top to bottom): a telephone labeled "H3" connected to a link-layer switch connected to a computer labeled "H4." The link-layer switch between H1 and H2 is connected on the right to a router labeled "R1." The link-layer switch between H3 and H4 is connected on the left to a router labeled R2. These routers connect to each other by a LAN labeled "1.5 Mbps link."

A dark blue data packet travels from H1 to its link-layer switch, and a light blue packet travels from H2 to the same switch. Inside R1 we see queued packets, both light and dark blue. A dark blue packet and a light blue packet travel towards R2, the dark blue packet slightly head. A dark blue packet travels upwards towards H3, while a light blue packet travels downward toward H4.