dictionary of multimedia and internet applications: a guide for developers and users. francis botto

Print ISBN 0-471-98624-0 Online ISBN 0-470-84178-8

Packet filtering A method used to promote network security, where

packets are filtered according to predefined criteria. Packet filtering is possible using screening routers and security gateways.

(See Screening router and Security gateway.)

Packet-switched network A data transmission and reception technique

where data streams are divided into packets coded with origination and destination information. The packets may be interleaved with different data transmissions. For instance, packets that may be providing a two-way audio communication link in IP telephony might be interleaved with other streams such as videoconferencing data. Packets may follow dissimilar routes over a network, and are directed over what are perceived as the quickest and least congested routes. If available routes or logical channels are congested, then packets are buffered before transmission. The buffer is a FIFO (first in, first out) storage, where the first packet placed in the buffer

Buffer

a1 a2

Packet b2

b1 b2 c2 switch

c1 a1 b1 c2 b2 a2

system

(PSS) b1

c1 c2

Packet switching buffer

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Paintbrush

is the first to be retrieved and transmitted when the appropriate virtual channel is available. The X.25 protocol standard dictates that a packet may contain between three and 4100 octets or bytes. (See X.25.) Up to 4095 logical channels may be accommodated on a single physical link (1997). The logical channel followed by a packet is determined by its header in-formation. There is also error correction, where the receiver might request that a particular packet is re-transmitted. The original packet switching standard for public data networks is CCITT X.25. This is a multi-tiered recommendation embodying everything from physical connectors to data formatting and code conversion. Packet switching is rather like the logistics involved in shipping a motor car part by part. The disassembled parts are sent and then assembled at the factory of destination. Equally, if a part is damaged, the factory will request that it is sent again. The packets may have one of two identities:

● multicast packets (or items of transmitted information) can be delivered

to more than one destination

● unicast packets have one destination only. Packet-switched networks (that use IP) are currently displacing switched networks in the telecommunications industry, and drive the growing use of IP telephony or Internet telephony. The comparative advantages of IP tele-phony include reduced costs and reduced cost of ownership for telcos and corporations running IP-compatible networks such as intranets. The reduced costs are largely brought by the fact that IP and Internet traffic is unregulated. Corporations and government departments may experience savings in the cost of voice traffic that might reach as much as 80 per cent. Flexibility is also an advantage. IP telephony makes better use of bandwidth. For example, Australian telecommunications giant Telstra introduced the virtual second line in the late 1990s. This allows subscribers to its ISP (Telstra Big Pond) to receive incoming calls while connected to the Internet.

(See Internet telephony.)

Paintbrush A Windows application used for editing and drawing.

PaintShop Pro A popular graphics capture, and editing program. It has numerous special effects.

PAL (Phase Alternating Line) A video standard developed in West Germany, delivering 50 fields/25 frames per second on the 625 line system. Used in the UK, much of Europe, Africa, Australia and South America.

PalEdit An 8 bit colour palette editing program supplied with Video for

Windows and other programs that include Asymetrix ToolBook 3.0.

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