Microwave frequency band in the range of 8 - 12 GHz that corresponds to wavelength from to 3.8 - 2.5 cm. 

Older networking standard that was defined by CCITT. X.25 is a public packet switch network that is run over telephone network. X.25 protocol operates at Layer 1, 2, and 3 of the OSI model.
X.25 has been used for low speed applications such as credit card verifications, automatic teller machine and other financial transactions. But it declines as operators and companies move to later technologies such as Frame Relay and ISDN.

A device that is used to provide additional phone connectors from a single wall phone jack. Also known as phone splitter.

New wireless data communication standard that is targeted for home automation, remote control, sensor system, interactive toy, and device monitoring. ZigBee is characterized by very low power consumption, low data rate, and long range.

Based on IEEE 802.15.4 standard, ZigBee operates in three frequency bands, namely 16 channels at 2.4 GHz, 10 channels at 902 to 928 MHz, and one channel at 868 to 870 MHz. The maximum data rate for each band is 250 kbps, 40 kbps, and 20 kbps respectively. As its modulation scheme, ZigBee uses DSSS and O-QPSK in the 2.4 GHz band. In the sub-1 GHz bands, ZigBee used DSSS and BPSK. ZigBee support three network topologies, i.e. star, mesh, and cluster-tree.

Even though in some cases ZigBee can replace Bluetooth, ZigBee is a complement to Bluetooth. ZigBee is suitable for monitoring static devices, while Bluetooth is suitable for connecting portable computers, mobile devices, and peripherals. A ZigBee device needs no battery recharging for months or years, while a typical Bluetooth device needs recharging after days or even hours of use.

See picture. Also see DSSS, BPSK, QPSK, FFD and RFD.