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Different Types of Firewall Firewalls operate in different ways, depending on the layer on which they are implemented. Circuit level firewall: data link layer Various names are used for this type of firewall. It is the type provided whenever NAT and PAT technology are used (see 3.7).

A1. Introduction

Generally, VPN is only worth consideration when collaboration between sites or remote users and sites is long-term relative to the time needed to provision the service. If VPN use is short-term then the overheads associated with service provisioning may not be justified. For example, if the provisioning of a VPN service takes, say, two days, it is not worth using such a service for a 10 minute VoIP phone connection. However, it might be effective to connect several sites for a research trial lasting some six months.

Janet provides its users with a basic transmission service, which is a regular IP best-effort service. Every packet is treated alike, with the same chance of being delayed or dropped if network congestion occurs. However, network applications available to Janet users may benefit from enhanced transport services. For example, multimedia applications like IP videoconferencing and VoIP may benefit from enhanced network performance (for example lower latency and delay variation parameters than are available from IP best-effort) that IP QoS could provide.