D-Link DWL-8500AP Product Manual - Page 162

D-Link Unified Access Point Administrator's Guide a translation table that maps all internal IP addresses in outbound requests to its own address and converts all inbound requests to the correct internal host. NAT serves three main purposes: it provides security by obscurity by hiding internal IP addresses, enables the use of a wide range of internal IP addresses without fear of conflict with the addresses used by other organizations, and it allows the use of a single Internet connection. communication and addressing. For example, protocols such as CSMA/CA and components like MAC addresses, and Frames are all defined and dealt with as a part of the Data-Link layer. • Layer 3, the Network layer, defines the how to determine the best path for information traversing the network. Packets and logical IP Addresses operate on the network layer. Network Address See IP Address. NIC A Network Interface Card is an adapter or expansion board inserted into a computer to provide a physical connection to a network. Most NICs are designed for a particular type of network, protocol, and media, for example, Ethernet or wireless. NTP The Network Time Protocol assures accurate synchronization of the system clocks in a network of computers. NTP servers transmit Coordinated Universal Time (UTC, also known as Greenwich Mean Time) to their client systems. An NTP client sends periodic time requests to servers, using the returned time stamp to adjust its clock. O OSI The Open Systems Interconnection (OSI) reference model is a framework for network design. The OSI model consists of seven layers: • 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 protocols 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 fullduplex 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). • Layer 1, the Physical layer, identifies the physical medium used for communication between nodes. In the case of wireless networks, the physical medium is air, and radio frequency (RF) waves are a components of the physical layer. • Layer 2, the Data-Link layer, defines how data for transmission will be structured and formatted, along with low-level protocols for 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 162 © 2001-2008 D-Link Corporation. All Rights Reserved.