Media Access Management
rier is deasserted. During the first part deferral
(InterFrameSpacingPart1 - IFS1), the Am79C978 con-
troller will defer any pending transmit frame and re-
spond to the receive message. The IPG counter will be
cleared to 0 continuously until the carrier deasserts, at
which point the IPG counter will resume the 9.6 ms
count once again. Once the IFS1 period of 6.0 ms has
elapsed, the Am79C978 controller will begin timing the
second part deferral (InterFrameSpacingPart2 - IFS2)
of 3.4 ms. Once IFS1 has completed and IFS2 has
commenced, the Am79C978 controller will not defer to
a receive frame if a transmit frame is pending. This
means that the Am79C978 controller will not attempt to
receive the receive frame, since it will start to transmit
and generate a collision at 9.6 ms. TheAm79C978 con-
troller will complete the preamble (64-bit) and jam (32-
bit) sequence before ceasing transmission and invok-
ing the random backoff algorithm.
The basic requirement for all stations on the network is
to provide fairness of channel allocation. The IEEE
802.3/Ethernet protocols define a media access mech-
anism which permits all stations to access the channel
with equality. Any node can attempt to contend for the
channel by waiting for a predetermined time (Inter
Packet Gap) after the last activity, before transmitting
on the media. The channel is a multidrop communica-
tions media (with various topological configurations
permitted), which allows a single station to transmit and
all other stations to receive. If two nodes simulta-
neously contend for the channel, their signals will inter-
act causing loss of data, defined as a collision. It is the
responsibility of the MAC to attempt to avoid and
recover from a collision and to guarantee data integrity
for the end-to-end transmission to the receiving station.
Medium Allocation
TheAm79C978 controller allows the user to program
The IEEE/ANSI 802.3 standard (ISO/IEC 8802-3 1990)
requires that the CSMA/CD MAC monitor the medium
for traffic by watching for carrier activity. When carrier
is detected, the media is considered busy, and the
MAC should defer to the existing message.
the
IPG
and
the
first-part
deferral
(InterFrameSpacingPart1 - IFS1) through CSR125. By
changing the IPG default value of 96 bit times (60h),
the user can adjust the fairness or aggressiveness of
the MAC on the network. By programming a lower
number of bit times than the ISO/IEC 8802-3 standard
requires, the MAC engine will become more aggres-
sive on the network. This aggressive nature will give
rise to the Am79C978 controller possibly capturing the
network at times by forcing other less aggressive com-
pliant nodes to defer. By programming a larger number
of bit times, the MAC will become less aggressive on
the network and may defer more often than normal.
The performance of the Am79C978 controller may de-
crease as the IPG value is increased from the default
value, but the resulting behavior may improve network
performance by reducing collisions. TheAm79C978
controller uses the same IPG for back-to-back trans-
mits and receive-to-transmit accesses. Changing IFS1
will alter the period for which the MAC engine will defer
to incoming receive frames.
The ISO 8802-3 (IEEE/ANSI 802.3) standard also
allows optionally a two-part deferral after a receive
message.
See ANSI/IEEE Std 802.3-1993 Edition, 4.2.3.2.1:
Note: “It is possible for the PLS carrier sense indica-
tion to fail to be asserted during a collision on the me-
dia. If the deference process simply times the inter-
frame gap based on this indication, it is possible for a
short interframe gap to be generated, leading to a po-
tential reception failure of a subsequent frame. To en-
hance system robustness, the following optional
measures (as specified in 4.2.8) are recommended
when InterFrameSpacingPart1 is other than 0:
1. Upon completing a transmission, start timing the in-
terrupted gap as soon as transmitting and carrier
sense are both false.
CAUTION: Care must be exercised when altering
these parameters. Adverse network activity could
result!
2. When timing an inter-frame gap following reception,
reset the inter-frame gap timing if carrier sense be-
comes true during the first 2/3 of the inter-frame gap
timing interval. During the final 1/3 of the interval,
the timer shall not be reset to ensure fair access to
the medium. An initial period shorter than 2/3 of the
interval is permissible including 0.”
This transmit two-part deferral algorithm is imple-
mented as an option which can be disabled using the
DXMT2PD bit in CSR3. The IFS1 programming will
have no effect when DXMT2PD is set to 1, but the IPG
programming value is still valid. Two part deferral after
transmission is useful for ensuring that severe IPG
shrinkage cannot occur in specific circumstances,
causing a transmit message to follow a receive mes-
sage so closely as to make them indistinguishable.
The MAC engine implements the optional receive two-
part deferral algorithm, with an InterFrameSpacing-
Part1 time of 6.0 ms. The InterFrameSpacingPart 2 in-
terval is, therefore, 3.4 ms.
During the time period immediately after a transmission
has been completed, the external transceiver should
generate the SQE Test message within 0.6 to 1.6 ms
after the transmission ceases. During the time period in
TheAm79C978 controller will perform the two-part de-
ferral algorithm as specified in the Process Deference
section. The Inter Packet Gap (IPG) timer will start tim-
ing the 9.6 ms InterFrameSpacing after the receive car-
Am79C978
67
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