Clarion M10 Series Bedienungsanleitung PDF herunterladen (Seite 6)

C-SPEC CORPORATION WIRELESS PRODUCTS

RF RE-TRANSMISSION PROTOCOL

RF signals transmitted from the M10 are not simultaneous with the

corresponding AUI activity. Although there are MAC-level retransmissions

in wired networks when a collision is sensed, the buffering of frames in the

M10, added to the fact that failure of an RF transmission cannot be sensed at

the transmitter, means that the wired-side MAC cannot provide

retransmissions. Thus, in order to avoid long delays for retransmission by

the level-four Transport protocol, e.g., TCP or IPX, the use of RF-MAC-level

retransmissions is important for high throughput.

The re-transmission protocol at the RF MAC layer provides enhanced

reliability. The 32 bit CRC, checked by the 802.3 MAC layer to provide the

final data reliability, is also used to support re-transmission protocol. The

use of the RF MAC level re-transmission protocol, recommended by the

802.11 draft standard, is important for high throughput. Re-transmission via

the level four transport protocol must be avoided, if at all possible, because of

the long time-out typically employed. Prior to the re-transmission, the

MerLAN M10 recognizes and memorizes the source MAC address of an

ethernet device connected to the unit through the MAU port. The download

occurs only when the MerLAN M10 recognizes complete matching of the

memorized source MAC address and destination MAC address written in a

received radio frame. Once the MerLAN M10 memorizes the source MAC

address, no update of the source MAC address is performed in the unit unless

power is turned off. This means that applications that lead to frequent

change of the source MAC address of the ethernet frame coming from

multiple ethernet devices through the MAU interface are not suitable for

making the re-transmission protocol effective. In the case of such

applications, it is recommended that the MerLAN M10 be configured without

the re-transmission option.

The re-transmission protocol is based upon the recognition of

acknowledgments generated by a MerLAN M10 unit that received a radio

data frame from another unit. A radio frame that carries the

acknowledgment is called an ACK frame which only affects the RF MAC level

protocol. The contents of the ACK frame never appears at the MAU port.

The ACK frame is transmitted immediately after the completion of receiving

the radio frame. The re-transmission protocol is not available for Broadcast

frames (destination ethernet address of all 1 bits) because no ACK frame can

be transmitted for those type of frames.

There are two different causes that will result in the re-transmission of

radio frames. One is the missing of an ACK frame and the other is a missing

data frame. In either case, the sending MerLAN M10 recognizes that there

was no return of the ACK frame from the receiving MerLAN M10 within the

allotted time-out period, therefore, the sending unit initiates the re-

transmission of the radio frame. In the case of a missing ACK frame, the

radio frame being re-transmitted is recognized by the receiving unit as a

duplicate frame and filters it out, such that the download of the frame does

not occur. This is called the Duplicate Filter function. The Duplicate Filter is

only functional when a single ethernet device is connected to the MerLAN

M10. This function will be updated in the future to make it available for

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