Link usually refers to the transfer of data from one device to another device of a physical medium, there are no other devices. In data communications, two communication devices must be in some way connected, a direct link is the easiest way to connect. It includes: point-to-point links and visited links.
Point to point links
Point-to-point link is when two devices directly on the physical level through physical media connection. This connection can be either wired or wireless.
For example: when the modem Internet access, between a computer and a modem, watching TV between TV and the remote control are point to point links.
Multi-point access link
Multi-point access link is when two or more devices are sharing the same physical link.
The limitations of direct links
Limited number of host connections. Point to point links connect only two communication devices, and more access to a link connection sharing media, the same can not connect many devices.
The limited geographical scope of the connection. If the device is too far from, laying line cost is huge.
Link number, structure is too complex.