TXC-06880BIOG Transwitch Corporation, TXC-06880BIOG Datasheet - Page 70

• Manufacturer Part Number: TXC-06880BIOG
• Manufacturer: Transwitch Corporation

Specifications of TXC-06880BIOG

• Operating Supply Voltage (typ): 1.8/3.3V
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Lead Free Status / RoHS Status: Not Compliant

Envoy-CE2 Device
DATA SHEET
TXC-06880
7 0 o f 12 2
6.4.2 Carrier Sense
6.4.3 Collision Detection
6.4.4 Alternate BEB Truncation
6.4.5 Excessive Collisions
Following the transmission of an Ethernet frame from the Envoy-CE2, the MAC times the
Inter Packet Gap (IPG) count as programmed in the Back-to-Back IPG configuration register
for the specific port. The MAC monitors the Carrier Sense status for the duration of IPG time
before transmitting a new frame for the port. Envoy-CE2 provides an optional IPG mechanism
where the IPG time is divided into a two-thirds/one-third ratio. During the first two-thirds of the
IPG, the IPG timer is reset and Carrier Sense status is monitored or sensed. During the final
one-third of the IPG, the Carrier sense status is ignored and the port begins transmission
once the IPG timer has expired. This optional mechanism enhances system robustness. The
configuration bits defining the two-thirds/one-third values are IPG1 and IPG2.
In the event the Envoy-CE2 MAC detects a collision for a port when the device is transmitting
an Ethernet frame, the MAC outputs the 32-bit jam sequence for the port. The jam sequence
is made up of several bits of the CRC, inverted to guarantee an invalid CRC upon reception of
the frame. The port then backs off transmission of the frame (retry) based on the “Truncated
Binary Exponential Backoff” (BEB) algorithm. Following this backoff time, the frame is retried.
When enabled, the “No Backoff” configuration bit (Register 0x400C Bit 17) retransmits the
frame without a backoff, following a collision. This option needs to be enabled with caution.
The backoff time following a collision is a controlled randomization process called “truncated
binary exponential backoff”. It is defined as an integer multiple of the slot times. The number
of slot times to delay before the n
distributed random integer r in the range: 0
collision, the port will backoff either 0 or 1 slot times. After the fifth collision, the port will
backoff between 0 and 32 slot times. After the tenth collision, the maximum number of slot
times to backoff is 1024. By setting the Alternate BEB Enable bit (Register 0x400C Bit 19),
the truncation point can be changed from min(n,10) to min(n,m) where m is set in the
Alternate BEB Truncation register (Register 0x400C Bits 20-23).
Upon collision, the port attempts re-transmission of the frame. Per the IEEE 802.3
specification, a frame has excessive collisions if 15 re-transmission attempts have occurred.
The number of retransmission attempts for excessive collisions is configurable. In the event a
frame has been excessively deferred, the frame is discarded and will not be transmitted. It is
possible to configure the Envoy-CE2 not to discard an excessively deferred frame.
-
Operation
th
retransmission attempt is chosen as a uniformly
-
r
2
k
where k = min(n,10). So, after the first
PRELIMINARY TXC-06880-MB, Ed. 4
February 2005