For long-distance network links, a different type of equipment
is frequently used, which is based on a standard named X.25.
Many so-called Public Data Networks, like Tymnet in the U.S., or
Datex-P in Germany, offer this service. X.25 requires special
hardware, namely a Packet Assembler/Disassembler or PAD. X.25
defines a set of networking protocols of its own right, but is
nevertheless frequently used to connect networks running TCP/IP
and other protocols. Since IP packets cannot simply be mapped
onto X.25 (and vice versa), they are simply encapsulated in X.25
packets and sent over the network. Frequently, radio amateurs
use their equipment to network their computers; this is called
packet radio or ham radio. The protocol used by ham radios is
called AX.25, which was derived from X.25. Other techniques
involve using slow but cheap serial lines for dial-up access.
These require yet another protocol for transmission of packets,
such as SLIP or PPP, which will be described below.