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334 CONVENTIONAL DIGITAL AND DATA NETWORKS
simple level of protocol conversion occurs at a network node that examines the
packet for errors and either segments the payload of a large packet into multiple
smaller packets or combines the payloads of multiple smaller packets into a single
larger one. After the resulting packet payload(s) form, the node encapsulates each
with the necessary control data in the form of a header and trailer, modifi es the
addressing scheme as required, and presents each to the target network.
7.5.10 Packet-Switching Hardware
The user of an X.25 packet network may require no hardware other than a PC and
modem, with the packetizing performed at the local X.25 node. Alternatively, a
hardware - or software - based PAD conforming to the X.3 standard can perform that
function at the host. ( Note: The advantage of the hardware PAD is that if it misbehaves,
you can hit it with a hammer.) The PAD performs the packet assembly (i.e.,
aggregation of many individual characters) for the transmitting device and disassembly
(i.e., disaggregation) for the receiving device in order to reconstitute the
data in its native format.
Packet carriers, of course, must invest in packet nodes rather than circuit switches.
Such packet nodes are intelligent devices capable of supporting complex routing
tables, buffering packets in temporary memory, resolving packet errors, and accomplishing
protocol conversions. Where X.25 networks are interconnected or where
protocol conversions of any sort are required, gateways must be deployed.
7.5.11 Packet-Switching Standards
The ITU - T sets standards recommendations for packet switching. Those standards
include the following [8, 10, 21] :
• X.3:
Packet Assembly/Disassembly (PAD) functions
• X.25: Interface between DCE and DTE for public packet networks
Packet Layer Protocol (PLP): Network Layer
Link Access Procedure — Balanced (LAPB): Link Layer
X.21bis: Physical Layer
• X.28: Terminal - to - PAD communications formats
• X.29: Host - to - PAD communications formats
• X.31: Packet - mode services over ISDN
• X.32: Defi nes X.25 synchronous dial - up mode
• X.75: Internetwork call control procedures
7.5.12 Packet-Switching Applications and Futures
X.25 packet switching originally was intended for interactive time sharing, which
involves long connect times and low data volumes. While X.25 still supports such
applications effectively, contemporary applications include online interactive processing
(e.g., reservations systems), messaging (e.g., e - mail), batch fi le transfer
(e.g., data backup), and Internet access.