X

X Modem
The X modem protocol is a data communications file transfer protocol developed for transmitting files between PCs.

X Series Recommendations
The X Series Recommendations are the ITU-T standards for the DTE to DCE interface. Examples of the X Series are X.25, X.75, X.28 and X.29.

X.25
X.25 is a long-time packet-switching protocol used to transmit information across a wide area. The X.25 interface lies at the OSI model Layer 3, and defines a protocol stack having three layers. Data networks use the X.25 protocol to transmit information across a wide area.

Several concepts are important for understanding X.25 protocols and networks. These include:

• physical circuit: the physical hardware and cabling used to connect devices together
• virtual circuit: the complete path between two communicating devices
• logical channel: the logical connection between the user node and the network

X.25 has been a long-time standard for packet switching. The X.25 interface lies at OSI layer 3, rather than at layer 1. X.25 defines a protocol stack having three layers.

X.500
X.500 is an OSI (Open System Information) based directory services protocol designed by the CCITT (International Telegraph and Telephone Consultative Committee). X.500 provides distributed directory services to network users. The X.500 directory specifies a model for connecting directory services to form one distributed global directory. Each directory service holds a part of the global database and the directory information is made available via a server (called a Directory System Agent — DSA). The database is maintained locally. From the user point of view, the entire directory is accessible from the local server.

X.75
X.75 is a standard that defines the interface between two distinct X.25 networks and is nearly identical to X.25.

X-ON/X-OFF
X-ON/X-OFF is a flow control mechanism used for asynchronous communication. It is used when the receiving device is not capable of receiving data as fast as the sending device can send it. When this occurs the X-OFF signal is sent to notify the sender to stop transmitting data until the receiver can "catch up." When the receiver does catch up the X-ON signal is sent. This technique is also called pacing. Most implementations use the ASCII DC1 code for X-ON and the ASCII DC3 code for X-OFF.

X Window
The X Window System uses the X Window protocol on TCP to draw windows on a workstation's bitmap display. X Window is much more than a utility for drawing windows; it is entire philosophy for designing a user interface.

XNS
XNS stands for Xerox Network Systems. It was the original LAN protocols which became the framework for network operating systems such as Novell NetWare.

Y

Y Modem
Y modem is a data communications file transfer protocol similar to X modem. The basic difference between Y modem and X modem is the block size was increased from 128 byte blocks to 1024 byte blocks with the Y modem protocol. It is compatible with the X modem protocol when sending 128 byte blocks.

Yellow Wire
10Base5 is the IEEE standard for 10Mbps baseband Ethernet over coaxial cable. 500 meters is the maximum distance for this type of cable. Also called "Thicknet" and "Yellow Wire." This type of physical cable is typically used as the backbone media for Ethernet networks.

Z

Z Modem
The Z modem protocol is a data communications file transfer protocol developed for transmitting files between PCs. It is similar to both the X modem and Y modem protocols. Z modem however is implemented using full duplex communication and does not require each block of data to be acknowledged as does the X modem and Y modem file transfer protocols. Z modem continues to send blocks of data until a negative acknowledgment is received (NAK). It then retransmits all blocks prior to the block which was NAKed.

ZIP
ZIP stands for Zone Information Protocol. ZIP supports multiple-zone networks. It supports request/reply packets to help determine a node's network range and zone multicast address and to acquire the list of valid zone names for the node's network.