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Xerox network system (XNS)

Building on the Ethernet standard, further protocols up to the application layer were developed by the Xerox company with the XNS protocols, Xerox NetworkSystem (XNS). The first XNS protocols that reached up to the transport layer were introduced in 1980. Almost all manufacturers built their first LAN devices exclusively on these protocols. Because of the early development date, XNS became a de facto standard for the network layer and the transport layer before the TCP/IP protocols. XNS was developed specifically for Ethernet LANs and is primarily used in office communications

. The IPX protocol developed by Novell is based on the original XNS specifications. The XNS protocols are divided into so-called levels

. There are levels 0, 1, 2, 3 and 4Level 0 protocols are located in the physical layer and the link layer and serve the various transmission mechanisms and the media access method

(MAC)

XNS Layer Model

XNS Layer Model

Level-1 protocols support the functions of the network layer and are referred to as the first XNS layer transport protocols. The XNS IP protocol located at this layer performs the functions of addressing and routing

and enables datagrams to be sent. TheXNS protocols located at the transport layer are referred to

asLevel 2

protocolsor second XNS layer transport prot

ocols.There are five protocols located at this level: Echo

Protocol, Error Protocol, Internetwork Packet Exchange

Protocol(IPX), Sequenced Packet Exchange

Protocol(SPX), and the Routing Information Protocol

(RIP)Level 3

protocols, called Control Protocols in XNS terminology, are the protocols

that reside at thecommunication control layer and the presentation layer

. Because protocols above the transport layer were not published by Xerox, each vendor had to implement its own Level 3 protocols. XNS protocols at the application layer are called Level 4 protocols, or Application Protocols. Since Xerox never published protocols in this layer, there are many implementations for them.

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Englisch: Xerox network system - XNS
Updated at: 12.12.2014
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