jack
A device used generally for terminating the permanent wiring of a circuit, access to which is obtained by the insertion of a plug.

J-carrier
The Japanese standard of the PDH, a time-division multiplexed, digital transmission system. J-carrier is a PCM system that uses a 64Kbps per channel as the basis of the hierarchy. Higher levels reflect aggregation of the 64Kbps channels. J-1 is a 1.544Mbps 24-channel communications circuit; J-2 is a 6.312Mbps 96-channel communications circuit; J-3 is a 32.064Mbps 501-channel communications circuit; J-4 is a 97.728Mbps 1,527-channel communications circuit; and J-5 is a 397.200Mbps communications circuit.

jitter
The slight movement of a transmission signal in time or phase that can introduce errors and loss of synchronization for high-speed synchronous communications.

JPEG (Joint Pictures Expert Group)
An international standard used primarily for still image compression.

JTACS (Japanese Total Access Communication Systems)
A Japanese wireless system that operates in the 800MHz to 900MHz band.

jumper
A patch cable or wire used to establish a circuit, often temporarily, for testing or diagnostics.