Cabletron Systems WPIM-DDS Installationsanleitung PDF herunterladen (Seite 22)

Chapter 2: About the HSIM-W6

2-8 HSIM-W6 Installation Guide

The NAT method allows several DHCP clients on a sub network to connect to WAN clients by

allowing the DHCP clients to share a single public IP address. When the HSIM-W6 uses NAT, the

NAT method modiﬁes the IP headers and addresses, and the selected ﬁelds in upper layer protocol

headers. This is done to replace the hidden local IP addresses from the sub network with one or

more public InterNic assigned IP addresses that can be sent over the outside network on the

HSIM-W6 WAN interfaces. Once the HSIM-W6 is assigned at least one public IP address, over

250 IP clients can share this address simultaneously using NAT. This public IP address is assigned

statically by the Internet Service Provider (ISP).

Frame Relay Protocol

Frame Relay can be deﬁned as a “packet mode” service, organizing data into individually

addressed units known as “frames”. Frame Relay eliminates all Layer 3 processing. Only a few

Layer 2 functions are used, such as checking for a valid, error free frame, but not requesting

retransmission if an error is found. Frame Relay uses a variable length framing structure, which,

depending on user data, can range from a few to more than a thousand characters.

A Frame Relay Network will often be depicted as a cloud, because the Frame Relay Network is

not a single physical connection between one endpoint and another. Frame Relay protocol is based

on the concept of Virtual Circuits (Vcs). Vcs are two-way, software deﬁned data paths between two

ports that take the place of private lines in the network. There are two types of Frame Relay

connections; Switched Virtual Circuits (SVCs), and Permanent Virtual Circuits (PVCs).

Permanent Virtual Circuits, or PVCs, are set up via a network management system, and initially

deﬁned as a connection between two sites, or endpoints. PVCs may be added as the demand arises

for more bandwidth, alternate routing, or more sites. PVCs are ﬁxed paths, not available on

demand, or on a call-by-call basis. Although the actual path through the network may change from

time to time, such as when automatic rerouting takes place, the beginning and end of the circuit

will not change.

Switched Virtual Circuits, or SVCs, are available on a call-by-call basis using the SVC signaling

protocol (Q.933). The network must quickly establish the connection, and allocate bandwidth

based on the user’s request.

In a Frame Relay frame, user data packets are not changed in any way. A two byte header is

appended to the frame. Contained in this header is a 10-bit number called the Data Link

Connection Identiﬁer (DLCI). The DLCI is the “virtual circuit” number which corresponds to a

particular destination. The DLCI allows data coming into a Frame Relay switch to be sent across

the network using a three-step process: Check the integrity of the frame and discard it if it is in

error, look up the DLCI in a table and if not intended for this link, discard the frame. If the frame

passes the previous tests, relay the frame toward its destination out the speciﬁc port speciﬁed in the

table.

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Cabletron Systems WPIM-DDS Installationsanleitung Seite 22