Psion Teklogix 9160 G2, Руководство пользователя, Страница: 414 / 436

D -64 Psion Teklogix 9160 G2 Wireless Gateway User Manual
Appendix D: Glossary
Packet
• Layer 3, the Network layer, defines the how to determine the best path for information
traversing the network. Packet s and logical IP Address es operate on the network layer.
• Layer 4, the Transport layer, defines connection oriented protocols such as TCP and
UDP .
• Layer 5, the Session layer, defines protocols for initiating, maintaining, and ending
communication and transactions across the network. Some common examples of proto-
cols that operate on this layer are network file system (NFS) and structured query
language (SQL). Also part of this layer are communication flows like single mode
(device sends information bulk), half-duplex mode (devices take turns transmitting
information in bulk), and full-duplex mode (interactive, where devices transmit and
receive simultaneously).
• Layer 6, the Presentation layer, defines how information is presented to the application.
It includes meta-information about how to encrypt/decrypt and compress/decompress
the data. JPEG and TIFF file formats are examples of protocols at this layer.
• Layer 7, the Application layer, includes protocols like hypertext transfer protocol
( HTTP ), simple mail transfer protocol (SMTP), and file transfer protocol (FTP).
P
Packet
Data and media are transmitted among nodes on a network in the form of packets . Data and
multimedia content is divided up and packaged into packets . A packet includes a small
chunk of the content to be sent along with its destination address and sender address.
Packets are pushed out onto the network and inspected by each node. The node to which it is
addressed is the ultimate recipient.
Packet Loss
Packet Loss describes the percentage of packets transmitted over the network that did not
reach their intended destination. A 0 percent package loss indicates no packets were lost in
transmission.
QoS features are designed to minimize packet loss.
PHY
The Physical Layer (PHY) is the lowest layer in the network layer model (see OSI ). The
Physical Layer conveys the bit stream - electrical impulse, light or radio signal -- through the
network at the electrical and mechanical level. It provides the hardware means of sending
and receiving data on a medium, including defining cables,
NIC s, and physical aspects.