X.25 Packet Switching
X.25 Packet Switching refers to the switching process between the data terminal equipment (DTE, a user machine) and the data circuit terminating equipment (DCE, a network component), using X.25 protocol. In a typical X.25 Packet Switching network, the major components are the customer premises equipment (CPE, e.g., user terminals or host computers), packet assemblers/Dis-assemblers (PADs), concentrators, and X.25 switches.

xDSL: Digital Subscriber Line
xDSL refers to a collection of Digital Subscriber Line(DSL) technologies, which is a modem technology for broadband data access over ordinary copper telephone lines (POTS) from homes to businesses. xDSL refers collectively to all types of DSL, such as ADSL (and G.Lite), HDSL, SDSL, IDSL and VDSL etc. They are sometimes referred to as last mile (or first mile) technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations.

Yellow Alarm
Yellow alarm, also known as Remote Alarm Indication (RAI), is an indication provided to a source device indicating a signal failure condition at a sink device. An incoming yellow alarm indicates that the T1 network element connected to the T1 interface has a problem with the signal it is receiving from the T1 interface.

ZBTSI: Zero Byte Time Slot Interchange
Zero Byte Time Slot Interchange (ZBTSI) is a technique used to ensure pulse density for clear channel capability. ZBTSI is applied to a DS1 frame to ensure that pulse density requirements are met, where bits 2 through 193 of each frame are scrambled to minimize the occurrence of all-zero octets.

Zero-Dispersion Slope
Zero-dispersion slope, in a single-mode optical fiber, refers to the rate of change of dispersion, with respect to wavelength, at the fiber's zero-dispersion wavelength.

Zimmermann Telegram
The Zimmermann Telegram was a telegram dispatched by the Foreign Secretary of the German Empire, Arthur Zimmermann, on January 16, 1917, to the German ambassador in Mexico, Heinrich von Eckardt, at the height of World War I. It instructed the ambassador to approach the Mexican government with a proposal to form an alliance against the United States. It was intercepted and decoded by the British and its contents hastened the entry of the United States into the war.

Zulu Date
Zulu Date refers to the date at Greenwich England. Zulu Date may be ahead of the date in some time zones such as the USA, or behind some other time zones such as Japan, etc.

Zulu Time
Zulu Time, also known as Greenwich Mean Tme (GMT), is the time at the Longitude Zero - Greenwich (England). Zulu Time is 6 hours later than Central Standard Time (US) and 5 hours later than Central Daylight-Savings Time. Zulu Time (GMT) is always the same worldwide. Communication network switches are coordinated on Zulu Time.