Understanding the network.pdf - Back to Home

the operating rate of the MAC increases, the bit-period is reduced in actual time. For
example, with standard 10Mbps Ethernet, it takes 1MHz or 1,000 nanoseconds to
move 1 bit of data across the transmission medium. Therefore, for standard
Ethernet, which operates using a 10MHz bandwidth, a bit-period is 0.1 µsec or 100
nanoseconds. The concept of time and its proper interpretation is vitally important
for all data transmission protocols, in particular, those that use collision detection
for transmission medium access. In order for the Ethernet collision detection
mechanism to work correctly, each 802.3 MAC implementation has defined three
"time periods" for managing access to the transmission medium. These time periods
are slot-time, InterFrame Gap (IFG), and jam-period.
Slot-time represents the minimum period of time that a transmitting station needs
access to the transmission medium to send the smallest legal frame size. Under the
Ethernet CSMA/CD rules, all end-stations connected to the medium segment
"listen" to all the frame transmissions on the wire, but only the destination accepts
the packet. After the destination receives the packet, it checks it for errors, and if
there are none, the data is handed to the ULP for further processing. If the frame is
corrupt, it is discarded. When an end-station wants access to the media to transmit
a frame, it listens to the wire for a period of one slot-time for carrier signal or active
frame transmission. If no carrier signal is heard after this period, the wire is
considered clear and the station can begin its transmission. The original IEEE 802.3
standard states that all Ethernet MACs have a 512 bit-period slot-time. When
Gigabit Ethernet was introduced, the slot-time was extended to 4,096 bit-periods to
accommodate for the increased transmission rate. MAC implementation's slot-time
is also the basis for determining the physical size of the collision domain. Under
Ethernet's CSMA/CD implementation, all stations on the segments must be able to
"hear" the frame transmission before it is completed, in order to avoid a collision.
To ensure that the transmitted frames are distinguishable from one another,
Ethernet's CSMA/CD implementation enforces a mandatory "dead time" between
each frame called the IFG. The IFG is a 96 bit-period "space" of inactivity on the wire
between each transmitted frame. Depending on the Ethernet NIC implementation,
after the end-station has detected a "clear" wire, the end-station needs to wait an
additional 96 bit-periods to ensure the wire is clear, effectively increasing the
slot-time an additional 96 bit-times. The IFG period is also the basis for "defer time"
clock used by the NIC when a station, after the slot-time has elapsed, still detects a
carrier signal on the wire. The IFG, although not technically part of the Ethernet
frame, needs to be accounted for as a percentage of the utilized bandwidth on the
transmission segment. When determining the segment's effective utilization, each
IFG represents a finite period of transmission inactivity on the medium, which
detracts from the total available bandwidth used to transmit Ethernet frames. For
this reason, when looking at segment Frames Per Second (FPS) rates, the IFG is
added to each frame transmitted.