3Com Megahertz
Site Survey Administrator Guide
Theory of Operation 
 

Previous Main Next

Theory of Operation

To improve AP management and performance, you should understand basic AP functionality and configuration options. The AP includes features for different interface connections and network management.

The AP provides MAC layer bridging between its interfaces. The AP monitors traffic from its interfaces and, based on frame address, forwards the frames to the proper destination. The AP tracks the frames sources and destinations to provide intelligent bridging as wireless clients roam or network topologies change. The AP also handles broadcast and multicast message initiations and responds to wireless client association requests.

MAC Layer Bridging

The AP listens to all packets on all interfaces and builds an address database using the unique IEEE 48-bit address (MAC address). An address in the database includes the interface media that the device uses to associates with the AP. (The AP internal stack interface handles all messages directed to the AP.) The AP uses the database to forward packets from one interface to another. The bridge forwards packets addressed to unknown systems to the default interface (either Ethernet or PPP).

Each AP stores information on destinations and their interfaces to facilitate forwarding. When you send an Address Resolution Protocol (ARP) request packet, the AP forwards the request over all enabled interfaces (Ethernet, PPP, or radio), except over the interface on which the ARP request packet was received. (Radio-received ARP request packets echo back to other APs over radio.) Upon receiving the ARP response packet, the AP database keeps a record of the destination address along with the receiving interface. With this information, the AP forwards any directed packet to the correct destination. The AP forwards packets for unknown destinations to the Ethernet interface.

The AP removes from its database destinations or interfaces not used for a specified time. The AP refreshes its database when it transmits or receives data from these destinations and interfaces.

Filtering and Access Control

The AP provides facilities to limit the wireless clients that associate with it and the data packets that can forward through it. Filters provide network security or improve performance by eliminating broadcast/multicast packets from the radio network.

The Access Control List (ACL) contains MAC addresses for wireless clients allowed to associate with the AP. This provides security by preventing unauthorized access.

The AP also uses a disallowed address list of destinations. This feature prevents the AP from communicating with specified destinations. This can include network devices that do not require communication with the AP or its wireless clients.

Depending on the setting, the AP can keep a list of frame types that it forwards or discards. The Type Filtering option prevents specific frames (indicated by the 16-bit DIX Ethernet Type field) from being processed by the AP. These include certain broadcast frames from devices unimportant to the wireless LAN but which utilize bandwidth. Filtering out unnecessary frames also improve throughput.

DHCP Support

The AP uses Dynamic Host Configuration Protocol (DHCP) to obtain a leased IP address and network configuration information from a remote server. DHCP is based on BOOTP protocol. DHCP can coexist or interoperate with BOOTP. An AP sends out a DHCP request searching for a DHCP server to acquire the network configuration and firmware filenames. Because BOOTP and DHCP interoperate, the one that responds first becomes the server that allocates information. The DHCP client automatically sends a DHCP request to renew the IP address lease as long as the AP is running. (This parameter is programmed at the DHCP server. For example, Windows NT servers typically are set for 3 days.)

The AP can optionally download two files when a boot takes place, the firmware file and an HTML file, because firmware versions 4.00-31 and above support Web servers. Users can program the DHCP or BOOTP server to transfer these two files when a DHCP request is made.

When the AP receives a network configuration change or is not able to renew the IP address lease the AP sends out an SNMP trap.

Media Types

The AP supports bridging between Ethernet, radio, and serial media.

The Ethernet interface fully complies with Ethernet Rev. 2 and IEEE 802.3 specifications. The AP supports 10BASE-T wired connections and full-speed filtering. The data transfer rate over radio waves is 11 Mbps. The Ethernet interface is optional for single-cell or PPP-connected networks.

The radio interface conforms to IEEE 802.11 HR specifications. The interface operates at 11 Mbps using direct-sequence radio technology. The AP supports multiple-cell operations with fast, transparent roaming between cells. With the direct-sequence system, each cell operates independently. Each cell provides a 11 Mbps bandwidth. Adding cells to the network provides increased coverage area and total system capacity. The AP supports wireless clients operating in Power Save Polling (PSP) mode or Continuously Aware Mode (CAM) without user intervention.

The DB-9, 9-pin, RS-232 serial port provides a User Interface (UI) or a Point to Point Protocol (PPP) connection. The UI provides basic management tools for the AP. The PPP provides a link between APs using a serial connection. The serial link supports short haul (direct serial) or long haul (telephone line) connections. The AP is a Data Terminal Equipment (DTE) device with male pin connectors for the RS-232 port. Connecting the AP to a PC requires a null-modem cable; connecting the AP to a modem requires a straight-through cable.

Bridging Support

The AP PPP interface, accessible from the serial port at the rear of the AP, provides two types of bridging operations: Internet Protocol (IP) bridging between an AP and a computer, and between two APs (with one AP connected to a LAN). To establish an Internet Protocol bridge with an AP, ensure that the computer includes the appropriate Telnet software with PPP and TCP/IP protocols. Using Telnet, a remote computer can connect to any AP on an Ethernet network, as long as data transfers through IP packets.

A PPP link provides the option of using a direct serial link or modem to extend wired Ethernet topologies. Once in PPP mode, the AP automatically attempts to communicate with the other device using the Data-Link Bridging (DLB) protocol. An AP using DLB communicates on the MAC level, and receives and transmits Ethernet frames.

If the other device does not support DLB, the AP attempts to communicate using Internet Protocol Control Protocol (IPCP). An AP using IPCP communicates on the IP level, and receives and transmits IP packets.

The PPP implementation in the AP uses the Link Control Protocol (LCP) and Network Control Protocol (NCP) as described in:

(RFCs are Requests For Comments used in Internet Communities.)

The AP database dynamically tracks wireless clients and APs on the PPP interface. Packets forward to the PPP link after the AP determines their destination.

The PPP implementation in the AP uses the NCP as described in RFC 1220: PPP Extensions for Bridging to encapsulate packets at the Ethernet level. The PPP provides IP bridging control as defined by RFC 1172 and MAC-level bridging. The protocol provides support for PPP negotiations conforming to RFC 1661. Users cannot plug a non-AP node directly into the AP serial port, only AP-to-AP PPP links.

For detailed information, refer to RFC 1171: The Point to Point Protocol and RFC 1220: PPP Extensions for Bridging.

PPP Connection

Connecting an AP and a computer with a direct serial link requires the use of a null-modem serial cable.

Null-modem serial cable pinout:

     

Connecting an AP and computer with modem devices requires the use of straight-through cables between the APs and modems. Using modems requires a telephone line for as long as the link remains active.

Straight-through cable pinout:

     

When using a modem connection, one AP represents the originating AP and the other represents the answering AP. When using a PPP link, do not use the serial port to access the UI. Access to the UI requires establishing a Telnet session with the AP.

Direct-Sequence Spread Spectrum

Direct Sequence Spread Spectrum (DSSS) uses a high-speed, non-information bearing signal to spread the transmitted information over a segment of the radio frequency band or spectrum. The AirConnect access point uses DSSS for radio communication.

Direct-sequence systems communicate by continuously transmitting a redundant pattern of bits called a chipping sequence. Each bit of transmitted data is mapped into chips by the access point to find the chipping sequence corresponding to the output signal.

Wireless clients receiving a direct-sequence transmission use the same chipping sequence to recreate the original data transmitted by the access point. Intercepting and decoding a direct-sequence transmission requires a predefined algorithm to associate the chipping sequence used by the transmitting access point to the receiving wireless client. This algorithm is established when the access point and wireless client are configured. The bit redundancy within the chipping sequence enables the receiving wireless client to recreate the original data pattern, even if bits in the chipping sequence are corrupted by interference.

Wireless Client Association Process

APs recognize wireless clients as they associate with the AP. The AP keeps a list of the wireless clients it services. Wireless clients associate with an AP based on the following conditions:

The following table gives the wireless client support rates:

Data Rate

Requirement

11 Mbps

Optional

5.5 Mbps

Optional

2 Mbps

Optional

1 Mbps

Required

Wireless clients perform preemptive roaming by intermittently scanning for APs and associating with the best available AP. Before roaming and associating with APs, wireless clients perform scans to collect AP statistics and determine the direct-sequence channel used by the AP.

Scanning is a periodic process where the wireless client sends out messages on all frequencies defined by the country code. The statistics enable a wireless client to reassociate by synchronizing its frequency to the AP. The wireless client continues communicating with that AP until it needs to switch cells or roam.

Wireless clients perform scans at start-up. In a scan, a wireless client uses a sequential set of channels as the scan range. For each channel in range, the wireless client tests for Clear Channel Assessment (CCA). When a transmission-free channel becomes available, the wireless client broadcasts a probe with the wireless LAN service area and the broadcast HSA_ID. An AP-directed probe response generates a wireless client Acknowledgment (ACK) and the addition of the AP to the AP table with a proximity classification. An unsuccessful AP packet transmission generates another wireless client probe on the same channel. If the wireless client fails to receive a probe response within the time limits, it repeats the probe process on the next channel in the sequence. This process continues through all channels in the range.

A wireless client can roam within the coverage area by switching APs. Roaming is transparent and virtually instantaneous in high-level applications. Roaming occurs when:

A wireless client selects the best available AP and adjusts itself to the AP direct-sequence channel to begin association. Once associated, the AP begins forwarding any frames it receives addressed to the wireless client. Each frame contains fields for the current direct-sequence channel. The wireless client uses these fields to resynchronize to the AP.

Mobile IP

The Internet Protocol identifies the wireless client point of attachment to a network through its IP address. The AP routes packets according to the location information contained in the IP header. If the wireless client roams across routers to another subnet, the following situations occur:

Mobile IP enables a wireless client to communicate with other hosts using
only its home IP address after changing its point-of-attachment to the internet/intranet.

Mobile IP is like giving an individual a local post office forwarding address when leaving home for an extended period. When mail arrives for the individual home address, it is forwarded by the local post office to the current care-of-address. Using this method, only the local post office requires notification of the individual current address. While this example represents the general concept of Mobile IP operation and functionality, it does not represent the implementation of Mobile IP used.

A tunnel is the path taken by the original packet encapsulated within the payload portion of a second packet to some destination on the network.

A Home Agent is an AP acting as a router on the wireless client home network.
The home agent intercepts packets sent to the wireless client home address and tunnels the message to the wireless client at its current location. This happens as long as the wireless client keeps its home agent informed of its current location on some foreign link.

A Foreign Agent is an AP acting as a router at the wireless client location on a foreign link. The foreign agent serves as the default router for packets sent out by the wireless client connected on the same foreign link.

A care-of-address is the IP address used by the wireless client visiting a foreign link. This address changes each time the wireless client moves to another foreign link. It can also be viewed as an exit point of a tunnel between the wireless client home agent and the wireless client itself.

The AirConnect Mobile IP (roaming across routers) feature enables a wireless client on the Internet to move from one subnet to another while keeping its IP address unchanged. The scanning and associating process continues for active wireless clients. This allows the wireless clients to find new APs and discard out-of-range or deactivated APs. By testing the airwaves, the wireless clients can choose the best network connection available.

Set the wireless client for Mobile IP as specified in the AirConnect Network Interface User Guide.

Security

Security involves two distinct areas: authentication and privacy. Authentication ensures that only authorized users access the wireless network. Privacy ensures that communication between authenticated users and the network cannot be intercepted or overheard. The Access Control List provides authentication using the wireless LAN service area and a system administrator-supplied list of all the wireless client MAC addresses authorized to access the Home Service Area. Privacy is ensured by enabling the 40-bit WEP encryption option.

Supporting CAM and PSP Stations

Continuously Aware Mode (CAM) stations leave their radios on continuously to hear every beacon and message transmitted. These systems operate without any adjustments by the AP.

A beacon is a uniframe system packet broadcast by the AP to keep the network synchronized. A beacon includes the wireless LAN service area, the AP address, the Broadcast destination addresses, a time stamp, Delivery Traffic Indicator Maps, and the Traffic Indicator Message (TIM).

Power Save Polling (PSP) stations power off their radios for long periods. When a wireless client in PSP mode associates with an AP, it notifies the AP of its activity status. The AP responds by buffering packets received for the wireless client. The PSP-mode wireless client wakes up to listen to the AP beacon every nth Beacon Interval where n is a PSP-mode value from the 1 to 5-range; the Beacon Interval is set on the wireless client. When the wireless client wakes up and sees its bit set in the TIM, it issues a poll request to the AP for packets stored for it. The AP sends them to the wireless client and the wireless client goes back to sleep. A DTIM field, also called a countdown field, informs wireless clients of the next window for listening to broadcast and multicast messages. The AP sends the messages following the nth beacon where n is the DTIM interval defined in the AP. When the AP has buffered broadcast or multicast messages for associated wireless clients, it sends the next DTIM with a DTIM Interval value. This value decreases by '1' with each successive beacon. The AP sends broadcast and multicast messages immediately following the beacon where the DTIM value is '0.' To prevent a PSP-mode wireless client from sleeping through a DTIM notification, select a PSP mode value less than or equal to the DTIM value. PSP-mode wireless clients hear the beacons and awaken to receive the broadcast and multicast messages.

A TIM is a compressed virtual bitmap identifying the AP associated wireless clients in PSP mode that have buffered directed messages. wireless clients issue a poll request when APs issue a TIM. A beacon with the broadcast-indicator bit set causes the wireless client to note DTIM Count field value. The value informs the wireless client of the beacons remaining before next DTIM. This ensures the wireless client turns on the receiver for the DTIM and the following BC/MC packet transmissions.

HTTP, HTML Web Server Support

Hypertext Transfer Protocol (HTTP) is the native language of the World Wide Web. The HTTP protocol makes requests from browsers to servers and responses from servers to browsers. This function provides the user with a Web-based format for configuration and firmware download.

Web pages are written in Hypertext Markup Language (HTML). HTML allows the user to create Web pages containing text, graphics, and pointers or links to other Web pages or elsewhere on the page or document. Pointers are known as Uniform Resource Locators (URLs). A URL is essentially the name of the Web page. The URL consists of three parts:

  1. Protocol (or Scheme)
  2. Fully Qualified Domain Name (FQDN), the machine where the page is located
  3. Local name that identifies the page (usually the HTML file name)

The HTML language describes how to format the document, indication which fonts to use, much like a copy editor describes which fonts to use, such as the location, color, header size and text.

Management Options

Managing AirConnect includes viewing network statistics and setting configuration options. Statistics track the network activity of associated wireless clients and data transfers on the AP interfaces. Configuration involves, among other things, setting system operating parameters and filters used in bridging.

The AP requires one of the following to perform a custom installation or maintain the AirConnect network:

Changing one AP does not affect the configuration of other APs on the network. Make configuration changes to APs individually. Each AP requires an individual IP address.

Programmable SNMP Trap Support

The SNMP protocol defines the method for obtaining information about networks operating characteristics and changing router and gateway parameters. The SNMP protocol consists of three elements:

Nodes can perform as hosts, routers, bridges or other devices that can communicate status information. An SNMP Agent is a node that runs the SNMP management process to systematically monitor and manage the network. The management station performs network management by running application management software.

An SNMP trap is an alert to all configured management stations of some significant event that occurred on the network. The management station queries all stations for details of each specific event, including what, when and where the event took place and the current status of the node or network. The format or structure is defined in the SNMP protocol. The MIB defines what and who monitors the variables.

Using SNMP

The AP includes SNMP agent versions accessible through an SNMP manager application (HP Open View or Cabletron Spectrum MIB browser). The SNMP agent supports SNMP versions 1 and 2, MIB II, 802.11 MIB and one proprietary 3Com Management Information Base (MIB). The SNMP agent supports read-write, read-only or disabled modes. The AP supports traps that return to the SNMP manager when certain events occur. The Wireless LAN Installation and Utilities disk packaged with wireless clients contains the MIB.

Increased MIB Support

The MIB defines what the management station needs to understand and which objects the station manages. The MIB has ten categories defined with approximately 175 variables.

Using the User Interface

The User Interface (UI) is a text-based maintenance tool integrated into the AP. It provides statistical displays, AP configuration options, and firmware upgrades. Access to the UI requires one of the following

Method

Description

Telnet Client

Gain access to the AP built-in Telnet server from any AP interface including remote Ethernet connections.

Direct Serial Connection

Acts as a DTE device to connect directly to a DTE device with a null-modem serial cable. The direct serial access method requires a communication program with ANSI emulation.

Dial Up Access

The dial-up access method requires a communication program with ANSI emulation on the remote terminal or PC. The terminal or PC dials to an AP with a modem connection. The AP supports connection to a Hayes-compatible 28,800-baud or faster modem.

SNMP Via a MIB Browser

Gain access to the AP SNMP function via a MIB Browser.

Web Browser

Gain access to the AP built-in Web server from any AP interface including remote Ethernet connections.