Clarion M10 Series Bedienungsanleitung PDF herunterladen (Seite 5)

C-SPEC CORPORATION WIRELESS PRODUCTS

CLARION MerLAN M10 3

Regeneration of the ethernet preamble and frame sync

Detection of any collision condition for avoiding collision of the download

to the upload and signaling of such to the AUI interface and to the CPU.

M10 CONTRASTED WITH WIRED-MEDIUM

The industry way of transmitting and receiving data over Clarion’s MerLAN

M10 (and many other) networks cause data packets to be frequently lost or

links to be limited to only point-to-point (i.e., only 2 radios per cell). This is

due to the fact that a wireless network does not have the ability to properly

detect collisions like an Ethernet network has. In an Ethernet network

collisions can be detected by the hardware (Ethernet chip) and are

immediately re-transmitted. Ethernet is referred to as CSMA/CD (Carrier

Sense Multiple Access with Collision Detect). Wireless networks are referred

to as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance).

The reason that collisions cannot be detected is because with a radio cannot

receive and transmit at the same time, hence collisions cannot be detected.

While the M10 has been designed to appear electrically as a standard

MAU, it is executing a medium-access protocol appropriate for the wireless

medium. As a result, certain of its behaviors are not reflected in a wired-

medium MAU, and the transparency to the wired-medium MAC in the

attached computer or bridging device cannot be absolute. MAUs for wired

networks have no internal storage; they operate with negligible delay relative

to the signals at the AUI/MAU interface. Because the MAC software in the

attached computer is executing a protocol appropriate for the wired medium,

and the M10 must employ a protocol appropriate for the wireless medium,

the M10 stores frames in buffers in order to isolate the (wired and wireless)

media. This results in important differences from conventional 802.3 MAUs;

these include buffer delay as well as the need for RF retransmissions and for

flow control.

BUFFER DELAY

The protocol operating over a MerLAN M10 link has an inherent delay of two

frames in each direction. When a frame is offered from the AUI port, it is

first saved in a buffer. Subsequently, the frame is transferred over the RF

channel to a receive buffer in the destination MerLAN M10. Only then is the

frame downloaded to the destination AUI port. This need not limit

throughput if the transport protocol properly anticipates the delay. If the

protocol waits for each frame to be acknowledged, then the throughput

achieved will be very low due to the excess delay inherent in the buffering

process. However, if a burst mode is used, then the effect of the delay can be

made negligible.

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Clarion M10 Series Bedienungsanleitung Seite 5