3DES
See ”DES“

AC
Access Client. Radio based communication unit, which must announce itself at the Access Point (? AP). Only after suc­cessful authentication, the access client can send data to the network or receive and/or request data from the network. (? Wireless LAN).

ACK
Acknowledge. A name for a positive acknowledgment of receipt. The ACK is a part of the communication protocols and responsible for the acknowledgment of receipt of the transmission.

Access protocol
Access method that regulates access to the medium. ETHERNET: CSMA/CD Token-Ring: Token FDDI: Append Token WLAN: CSMA/CA

Access method
See access protocol.

ADSL
Asymmetric Digital Subscriber Line. Interface to Wide Area Network.

AES
Advanced Encryption Standard. Encryption standard with 128-, 192- and 256-Bit-keys. This symmetrical encryption standard was developed to replace the earlier ? DES standard.

Aging
Function to update data especially the address buffer. An address is marked ”old“ after expiration of a time and will be deleted at next cycle if it is not learned anew.

AP
Access Point. In wireless networks the access point is the ? bridge to the wired networks. It can be attached directly to ethernet, token ring or atm. The access point is connected with all nodes "access clients" and takes over the central functions like roaming or security. (? Wireless LAN).

API
Application Programming Interface.

CCK
Complentary Code Keying. CCK is used with the 11 Mbps version of the 802.11-LAN (802.11b) and can pack several bits into a symbol. Thus a higher data transmission rate is possible.

CD
Collision Detect.

CENELEC
Comité Européen de Normalisation Elektrotechnique (European Committee for Electrotechnical Standardization). Responsible for the harmonization of electrotechnical standards in the European Union (e.g. EN 50173, …).

CHAP
Challenge Handshake Authentication Protocol. PPP authenti­cation method. Passwords are transmitted after being enco­ded with a random number. Compare with ? PAP.

Cheapernet
Coax cable in accordance with the ETHERNET standard 10BASE2. Synonyms: Thinwire, RG58.

CLI
1. Command Line Interface.
2. Calling Line Idendification

Concentrator
See ”Hub“.

CoS
Class of Service. A network with class of service has the ability to deliver data traffic with a minimum amount of delay in an environment in which many users share the same net­work. CoS classifies traffic into categories such as high, medium, and low (gold, silver, and bronze).

CRC
Cyclic Redundancy Check. Error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and compares the calcu­lated remainder to a value stored in the frame by the sending node. See also FCS.

CSMA/CD
Carrier Sense Multiple Access Collision Detect. Media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices. This collision subsequently delays retransmissions from those devices for some random length of time. CSMA/CD access is used by ETHERNET and IEEE 802.3.

Cut-Through
A device using cut-through packet switching reads, processes, and forwards packets as soon as the destination address is looked up and the outgoing port determined. Also known as on-the-fly packet switching. See also Store & Forward.

DA
See Destination address.

DBPSK
Differential Binary Phase Shift Keying. DBPSK is a modulation procedure of which is used with the ? DSSS transmission method according to standard 802.11 for systems with 1 Mps.

DCE
Data Communication Equipment, e.g. printer, modem. See also DTE.

DES
Data Encryption Standard. Symmetric encryption algorithm. For encryption and decryption the same secret key is used. Thus every station need to know this key in order to encrypt/ decrypt . DES uses a 56 bit key. 3DES consists of three separate DES cryptographic operations, each performed with a different 56 bit key. The key length of 3DES is thus 168 bit.

Destination address
Used with ETHERNET, IP, etc. The address to which a data packet is sent.

DeviceNet
DeviceNet incorporates CAN technology and provides a low­cost industrial network used to connect industrial devices such as limit switches, photoelectric cells, valve manifolds, motor starters, drives, and operator displays to PLCs and PCs.

DHCP
Dynamic Host Configuration Protocol. Provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.

DIX
Digital, Intel, Xerox. Groupement à l’origine de la norme Ethernet à 10 Mbps

DNS
Domain Name System. System used in the Internet for trans­lating names of network nodes into addresses.

Domain
Broadcast domain: Network area which can only be bordered by a router, and through which a Broadcast can freely travel. Collision domain: Network area which is bordered by a switch or router, within which collisions can occur.

DQPSK
Differential Quaternary Phase Shift Keying. DQPSK is a modulation procedure of which is used with the ? DSSS transmission method according to standard 802.11 for systems with 1 Mps or 2 Mps.

DSC
Duplex straight connector. See also SC.

DSL
Digital Subscriber Line. Provides a technologie, in order to use the internet with 1,5 Mbps (via copper lines).

DSSS
Direct Sequence Spread Spectrum. DSSS is a transmission method according to standard 802.11. The procedure changes the narrow-band by coding to a wide-band signal. In this way the entire frequency band can be used. Thus a higher data transmission rate as well as a lower susceptibility to interference is possible.

DTE
Data Terminal Equipment, e.g. computer. See also DCE. Difference to DCE: Pin assignment.

Dual Homing
Network topology in which a device is connected to the network by way of two independent access points (points of attachment). One access point is the primary connection, and the other is a standby connection that is activated in the event of a failure of the primary connection.

DVMRP
Distance Vector Multicast Routing Protocol. Internetwork gateway protocol, largely based on RIP, that implements a typical dense mode IP multicast scheme. DVMRP uses IGMP to exchange routing datagrams with its neighbors.

DWDM
Dense Wavelength Division Multiplex.

Dynamic DNS
Assigns always the same name also if the IP-address of one client changes. See also DNS.

EANTC
European Advanced Networking Test Center.

EIA
Electronic Industries Association. Standardization body.

ELED
Edge-emitting LED.

EMC
Electromagnetic compatibility. Electromagnetic interferece and electromagnetic emissions, class A/B.

EN
European norm (standard). See also CENELEC.

ESD
Electro Static Discharge.

ETHERNET
The first experimental ETHERNET system was developed in the early 1970s by Bob Metcalfe and David Boggs of the Xerox Palo Alto Research Center (PARC). In 1983, the Institute of Electrical and Electronic Engineers (IEEE) released the first IEEE standard for ETHERNET technology. It was developed by the 802.3 Working Group of the IEEE 802 Committee. The formal title of the standard was IEEE 802.3 Carrier Sense

Multiple Access with Collision Detection (? CSMA/CD)
Access Method and Physical Layer Specifications. ETHERNET has a variable packet length between 64 and 1522 byte included the TAG field.

EtherNet/IP
EtherNet/IP is an ETHERNET implementation designed for industrial applications, built on standard TCP/IP protocol and shares a common application layer with DeviceNet thus facilitating the exchange of information between device-level networks and plant level information systems.

ETHERNET Packet
Term for an ETHERNET data packet. It contains the destination and source address field (DA or SA) apart from the actual payload data, the TAG field (4 bytes, optional) and the length/type field.

FCS
Frame Check Sequence. Checksum at the end of an ETHERNET frame, which is calculated and appended by the transmitter. The receiver recalculates this checksum based on the contents of the frame, and compares the two values. See also CRC.

FDB
Forwarding Data Base. Address table of a switch for the decision at which port to transmit a frame. The table assigns MAC addresses to the port via which the respective device can be reached. The table is updated regularly (? Aging).

FDDI
Fiber Distributed Data Interface. Data network, standardized by ISO 9314 and ANSI X3T9.5 as well as X3T12.

FDX
Full Duplex. Transmission mode of a component: simultaneous transmission and reception is possible. No access control procedure is necessary. See also HDX.

Flow Control
Procedure used when an exit port is overloaded, and data is being lost from the buffer: The incoming port indicates to an end device that the device should stop sending data. In half duplex mode this is achieved by simulating collisions. In full duplex mode, special ”Pause“ frames are used

F/O
Fiber optics

Frame Relay
Modified version of the X.25 protocol used in WANs.

FTP
1. File Transfer Protocol. A layer 5 protocol which runs over TCP. Can also be used across WANs.
2. Foiled Twisted-Pair.

FTTD
Fiber To The Desk.

Full Duplex
? FDX

GARP
Generic Attribute Registration Protocol. A family of protocols used to exchange information between switches at layer 2. Currently the family consists of ? GMRP and ? GVRP.

Gateway
Components above layer 2 of the ISO/OSI reference model. At layer 3 the gateway is usually a router. Converts between protocols like IP to IPX.

GBIC
Gigabit interface converter. See also SFP

Gbps
Gigabits per second, Gbit/s.

GMRP
? GARP Multicast Registration Protocol

GVRP
? GARP VLAN Registration Protocol.

Half Duplex
? HDX

HASH
Checksum, securing the integrity of information.

HCS ®
Hard Polymer Cladded Silica. Plastic fiber with a quartz glass core. See also PCF, POF.

HDX
Half Duplex. Transmission mode of a component. Transmission and reception of data are possible, but not simultaneously. Half duplex ETHERNET requires the CSMA/CD access method. See also FDX.

HIPER-Ring
For ETHERNET networks Hirschmann has developed the HIPER-Ring (Hirschmann Performance Redundancy Ring) based on the concept of the Spanning Tree Protocol. The HIPER-Ring significantly increases the availability of the net­work and facility: while with Spanning Tree 30 seconds typi­cally elapse before the failure of a link is compensated, with HIPER-Ring this takes less than half a second. Furthermore the structure is considerably simplified with a possibility of expansion of up to 50 devices.

HiRRP
Hirschmann Router Redundanz Protokoll. Protocol to control a redundant router. If one of the routers fails, within 800 ms the remaining router completely takes over the tasks of the other one

Hop
Passage of a data packet between two network nodes (for example, between two routers).

HSRP
Hot Standby Routing Protocol. Protocol which accommodates redundant routers. See also VRRP.

HTML
Hypertext Markup Language.

HTTP
Hypertext Transfer Protocol. The protocol used by Web browsers and Web servers to transfer files, such as text and graphic files.

HTTPS
? HTTP Secure. Paketwise encrypted HTTP communication.

Hub
Components at layer 1 of the ISO/OSI reference model. Regenerates the amplitude and signal shape of the incoming signals, and transmits them out of all ports. Synonyms: Star coupler, Concentrator.

IAONA
IAONA (Industrial Automation Open Networking Alliance Europe e.V.) Europe was founded in 1999 at the SPS/IPC/ Drives in Nuremberg (with HIRSCHMANN as one of the esta­blishment company) as an alliance of meanwhile more than 130 leading international manufacturers and users of auto­mation systems. It pursues the aim of establishing ETHERNET as the standard application in every industrial environment at an international level. Sense of this is to realise a general, in­terfaceless communication through all levels of an enterprise. This refers to all fields of plant automation, process automati­on, and building automation. More information: http://www.iaona-eu.com/

Internet Protocol
See IP.

IP
Internet Protocol. A layer 3 communications protocol, most widely used (> 80 %).

IPv4: Version 4 = 4 byte addresses

IPv6: Version 6 = 16 byte addresses

IPnG = IPv6

IP address
A logical address, assigned by a network manager. Address format (v4): 4 bytes in decimal code, separated by dots, for

example 192.178.2.1. See also Network Mask.

IPnG
IP next generation. Communications protocol, see IP.

IPsec
IP Security. Standard, which uses encryption to verify the authenticity of the sender and ensure the confidentiality and integrity of the data in IP.

Layer 3 ? VPNs connections are configured with IPSec

(using ? 3DES for instance).

IPv4
IP Version 4. Communications protocol, see IP.

IPv6
IP version 6. Communications protocol, see IP.

IPX
Internetwork Packet Exchange. NetWare network layer protocol used for transferring data from servers to work­ stations. IPX is similar to TCP/IP.

ISDN
Integrated Services Digital Network. WAN communication protocol.

ISO
International Organization for Standardization. International standardization body.

ISO/OSI
? OSI model

ISP
Internet Service Provider.

ITU-T
IInternational Telecommunication Union, Telecommunication Standardization Sector. Standardization body.

Jabber
A faulty ETHERNET frame with more than 1518 bytes.

L2TP
Layer 2 Tunneling Protocol. For configuration of ? VPN-Tunnels on layer 2. See also IPsec.

LACP
Link Aggregation Control Protocol.

LAN
Local Area Network. Local data network, e.g. ETHERNET, FDDI, and Token Ring. See also Wireless LAN.

LAP
Link Access Protocol.

Latency
Time difference between the reception and retransmission of data, mostly between the last received bit and the first retransmitted bit.

LED
Light Emitting Diode.

Link-Aggregation
Combining several physical ports (maximum 4) to create one virtual port. Data is transmitted in parallel, with redundancy in the event of port loss. Standard IEEE 802.3. Also known as Trunking.

MDI
Medium Dependent Interface.

MDI-X
MDI-Crossover, see also MDI.

MIB
Management Information Base. Contains a description of the objects and functions of a network device.

MII
Media Independent Interface.

mini-GBIC
Mini gigabit interface converter, see alsor SFP.

MLPPP
Multilink PPP. See also PPP.

MPLS
Multiprotocol Label Switching. Layer-3 protool.

MSB
Most Significant Bit.

MTBF
Mean Time Between Failure.

MTTR
Max Time To Repair.

Multicast
Data packet intended for a group of devices, for example all Hirschmann devices.

Multi-mode fiber
Fiber optic cables that are distinguished through core diameters of comparable size. The typical core diameter for step-index fiber optic cables is 100 µm for glass fibers, 200 µm for PCS/HCS ® fibers and 980 µm for POF fibers. The graded index fibers on the other hand have a typical core diameter 50 or 62.5 µm.. Because of this relatively large core diameter, the light in multi-mode fibers spreads over several paths and modes. See also Single-mode fiber.

NAT
Network Address Translation.

NAT-T
NAT-Traversal. If there is a ? NAT-Gateway inbetween two ? IPsec end points IPsec does not work, as the IP-addresses of the end points are also encrypted. NAT-T solves this problem. NAT-T is enable automatically during the handshake if required (and supported).

NetBEUI
NetBIOS Extended User Interface. Enhanced version of the NetBIOS protocol used by network operating systems such as Microsoft Windows

Network Mask
The network mask marks all bits in an IP address for identifying the network and the subnetwork. See also IP address.

NEXT
Near End Cross Talk.

NIC
Network Interface Card.

NMS
Network Management System.

Node
Participant in a data network (PC, printer, switch, hub, etc.).

NRZ
Non Return to Zero. Signal code. See also NRZI.

NRZI
Non Return to Zero Invert. Signa code. See also NRZ.

NVRAM
Non-Volatile RAM. RAM that retains its contents when a unit is powered off.

ODVA
ODVA (Open Device Vendor Association) is the organization that manages the DeviceNet and EtherNet/IP network technology and standards in addition to promoting their worldwide adoption in industrial automation.

OID
Object ID.

OLE
OLE (Object Linking and Embedding) is a window technology to transfer different datas between devices.

OPC
OLE for Process Control. Protocol used in process control, to provide a standardized method of exchanging data between devices.

OSI
Open Systems Interconnection. International standardization program created by ? ISO and ? ITU-T to develop standards for data networking that facilitate multivendor equipment in­teroperability.

OSI model
A model which describes communication in a network. The functionality of the hardware is divided into seven layers. The lowest layer (Physical Layer) describes the physical media.

OSPF
Open Shortest Path First. Protocol for exchanging routing in­formation between routers. Faster than ? RIP, and suitable for use in large networks.

OTDR
Optical Time Domain Reflectometer. Analyser.

OUI
Organizationally Unique Identifier. The first three bytes of a ? MAC address, indicating the manufacturer of the module.

Packet size
ETHERNET: 64 … 1518 byte (1522 with VLAN tag), FDDI:… 4500 byte.

PAP
Password Authentication Protocol. PPP authentication method. Passwords are transmitted unencoded. PAP is based on user names.

POF
Polymere Optical Fiber. Siehe auch HCS ®, PCF.

POL
Power over LAN.

Port Mirroring
The data traffic of a port (in/out) is copied to another port (mirrored), in order that it can be viewed using a protocol analyser.

Port Trunking
see Link Aggregation.

PPP
Point-to-Point Protocol. Provides router-to-router and host­ to-network connections. PPP works with several network layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP.

PPPoE
Point-to-Point-Protocol over Ethernet.

pps
Packets per Second.

PPTP
Point-to-Point Tunneling Protocol.

Prioritization
Data packets are given precedence, subject to defined criteria.
At layer 2 an additional ? Tag field is inserted into the frame.
At layer 3 the ? TOS field of ? IP is used.

Private Key
? Private/Public Key

Public Key
? Private/Public Key

PVV
Path Variability Value. Designation in bit times.

QoS
Quality of Service. Measure of performance for a transmissi­on system that reflects its transmission quality and service availability. See also prioritization.

RADIUS
Remote Authentication Dial In User Service. A RADIUS Server authenticates a client, who registers for access with a name and password. The password is transmitted encoded.

RAM
Random Access Memory. Volatile memory.

RARP
Reverse Address Resolution Protocol. Obtains the IP address associated with a specified MAC address. See also BOOTP and DHCP.

RAS
Remote Access System.

Repeater
Layer 1 component which regenerates a signal. Regenerates amplitude, signal edge and clock. Repeater with more than two ports are also known as hubs.

RFC
Request For Comments. Quasi-Standard for Internet, Proto­cols and Applications, published by the IETF. See 6.3.

RG58
Coax cable with 50 ? resistance. Also known as Thinwire or 10BASE2.

RIP
Routing Information Protocol. Used to exchange routing in­formation between routers on a LAN. There are two versions: RIP V1 and RIP V2. See also OSPF.

RJ45
Connector for Twisted Pair. Usually for ? ETHERNET and ? ISDN.

RMON
Remote Monitoring.

Router
Component at layer 3 of the ? ISO/OSI reference model. Connects networks at layer 3. Offers additional features such as choosing the best path through a network based on criteria such as path cost.

RS 232 C
Recommended Standard. Serial interface, also known as V.24. Actually an extension of V.24 acc. ? CCITT.

RSTP
Rapid Reconfiguration Spanning Tree Protocol.

RSVP
Resource Reservation Protocol. Reserves bandwidth in a ? WAN.

RTCP
Realtime Transport Control Protocol.

RTP
Real Time Protocol.

Rx
Receive.

SA
Source Address.

SAN
Storage Area Network. Network for connecting servers and storage sub-systems, such as disks, RAID and Tape Systems. Mostly based on Fibre Channel.

SAP
1. Service Access Point. 2. Service Advertising Protocol.

SC
Straight Connector. See also DSC.

SCADA
Supervision Control And Data Acquisition. Process visualiza­tion system for process control and visualization. Based on Windows.

SD
Starting Delimiter.

SDH
Synchronous Digital Hierarchy. European standard that defines a set of rate and format standards that are transmitted using optical signals over fiber. SDH is similar to SONET, with a basic SDH rate of 155.52 Mbps, designated at STM-1.

SFD
Start Frame Delimiter.

SFP
Small form-factor pluggable. A ? transceiver for 1 Gbps networks that converts serial electric signals to serial optical signals and vice versa. see also GBIC.

SHA-1
Secure Hash Algorithm 1. See also Hash.

SLA
Service Level Agreement.

SLIP
Serial Line Internet Protocol. Standard protocol for point-to­point serial connections using serial interface (e.g V.24) for IP communication.

SMON
Switch Monitoring.

SMTP
Simple Mail Transfer Protocol. Internet protocol providing e-mail services.

SNTP
Simple Network Time Protocol. Protocol for time synchroni­sation, based on NTP, with a precision of 1 to 50 ms. For higher precision ? PTP (Precision Time Protocol acc. to IEEE 1588) is used.

SNAP
Subnetwork Access Protocol.

SNMP
Simple Network Management Protocol. Network manage­ment protocol definied by ? IETF used almost exclusively in ? TCP/IP ? LANs to monitor and control network devices, and to manage configurations, statistics collection, perfor­mance, and security.

SOHO
Small Office Home Office. Networking solutions and access technologies for offices that are not directly connected to large corporate networks.

Spanning-Tree
Protocol which automatically blocks network loops. Allows the installation of redundant paths, to improve resilience in case of connection failures. Recovery time between 30 to 60 seconds.

SQE
Signal Quality Error. Transmission sent by a transceiver back to the LAN controller (processor) to let the controller know whether the collision circuitry is functional. Also called heart­beat.

SSH
Secure Shell. Allows an encrypted communication via un­secured networks with authentication of the communicaton partners, integrity and confidentialy of the exchanged data.

Star coupler
For active star couplers, see Hub. A passive star coupler is a component used in fiber technology with x entrances and y exits without amplifying the signal.

Store & Forward
A switching mechanism in which the complete packet is saved into a buffer, and then retransmitted. Also see Cut-Through.

STP
1. Shielded Twisted Pair. Two-pair wiring medium. STP cab­ling has a layer of shielded insulation to reduce EMI. See also PIMF ,UTP. 2. ? Spanning Tree Protocol.

Switch
Component at layer 2 of the ? OSI reference model. Synonym: Bridge. Unlike a hub, a switch only forwards data to the port where the destination device is connected. This results in separation of segments. No access control mecha­nism is required between two switches connected in full duplex mode. There are also switches known as Layer 3 and Layer 4 switches, in which some functionality of these layers has been implemented.

SX
Short Wavelenth (Gigabit-Ethernet).

Tag field
Optional field in an ETHERNET frame, inserted after the source address.

TCO
Total Cost of Ownership.

TCP
Transmission Control Protocol. Connection-oriented trans­port protocol on layer 4 of the TCP/IP protocol stack. See also UDP.

TCP/IP
Transmission Control Protocol/Internet Protocol. Most widely used protocol family from layer 3 upwards. Standardized by the IETF. Protocols included in this family are: Layer 3: IP Layer 4: TCP, UDP Layer 5: TFTP, SMTP, FTP, … Layer 5 contains layers 5 to 7 of the OSI model.

Telnet
Virtual terminal program, using the TCP/IP stack for remote access to a device’s user interface over a network.

TFTP
Trivial File Transfer Protocol. Layer 5 protocol, uses ? UDP as the transport protocol., therefore use in ? LANs.

TIA
Telecommunications Industry Association. Standardization body.

Token-Ring
Data network, standardized by IEEE 802.5, and also a system proprietary to IBM.

TOS
Type Of Service. Field in the IP packet used for ? prioritization.

TP
Twisted-Pair. Data cable.

Transceiver
Transmits data signals from an AUI interface on to a medium, for example Twisted Pair. New components already have a transceiver implemented. For older components, there are plug-on transceivers for multi-mode, Twisted Pair or coax.

Trunking
See Link Aggregation.

URL
Universal Resource Locator. Standardized addressing scheme for accessing hypertextdocuments and other services using a browser. Hirschmann URL: www.hirschmann.com

UTP
Unshielded Twisted-Pair. Cable with unshielded twisted pairs, mostly 4 pairs. See also STP.

VLAN
Virtual LAN, built with switches. Target: Restrict broadcasts only to the part of the network where they are required. Also used to divide up networks for security reasons.

VPN
Virtual Private Network. A VPN connects several separate private networks (subnets) together via a public network, e.g. the Internet, to form a single joint network. A cryptographic protocol is used to ensure confidentiality and authenticity. A VPN thus offers an economical alternative to using dedicated lines to build a nationwide corporate network.

VRRP
Virtual Redundant Router Protocol. Protocol to control a redundant router. See also HSRP.

WAN
Wide Area Network. Public data and transport networks for joining LANs. Transmission protocols: ISDN, Frame-Relay, X.21 SDH, SONET, ATM.

WDM
Wavelength Division Multiplex.

WEP
Wired Equivalent Privacy. WEP is a coding procedure in Wire­less LANs according to 802.11 for the protection of the trans­ferred data.

WFQ
Weighted Fair Queuing. Procedure for processing prioritization queues in a switch. For example, the highest queue receives 50 % of the bandwidth, the next 25 % , … .

WiFi
Wireless Fidelity. WiFi is a certifying of Wireless LANs (WLAN) according to standard 802.11which is accomplished by the WECA (Wireless Ethernet Compatibility Alliance). With this certifying interoperability of the wireless LAN products are confirmed. See also http://www.wi fi.net

Wireless LAN
Lokale Netze, die ohne Kabelverbindungen arbeiten.

Wire-speed
Processing packets at the highest physically possible speed.

WLAN
Wireless ? LAN. Acc. IEEE 802.11, .15, .16 (Bluetooth).

WWDM
With WWDM-system (Wide Wavelength Division Multiplex) networks with limited fiber can increase channel capacity of the fiber by between two locations. A optically multiplexes some single mode optical signals into one composite optical signal. Using the same fiber optic pair, multiple point-to-point applications can be satisfied. This greatly reduces the cost of intalling more fiber.

WWW
World Wide Web.

X.25
Data Packet Control Protokoll, used for example by Datex-P.

XML
Extended Markup Language.

XNS
Xerox Network Systems.