Ethernet Timing

Bit Time

For each different media speed, a period of time is required for a bit to be placed and sensed on the media. This period of time is referred to as the bit time. On 10-Mbps Ethernet, one bit at the MAC layer requires 100 nanoseconds (nS) to transmit. At 100 Mbps, that same bit requires 10 nS to transmit. And at 1000 Mbps, it only takes 1 nS to transmit a bit. As a rough estimate, 20.3 centimeters (8 inches) per nanosecond is often used for calculating the propagation delay on a UTP cable. The result is that for 100 meters of UTP cable, it takes just under 5 bit times for a 10BASE-T signal to travel the length the cable.

For CSMA/CD Ethernet to operate, the sending device must become aware of a collision before it has completed transmission of a minimum-sized frame. At 100 Mbps, the device timing is barely able to accommodate 100 meter cables. At 1000 Mbps, special adjustments are required because nearly an entire minimum-sized frame would be transmitted before the first bit reached the end of the first 100 meters of UTP cable. For this reason, half-duplex mode is not permitted in 10-Gigabit Ethernet. 

These timing considerations have to be applied to the interframe spacing and backoff times (both of which are discussed in the next section) to ensure that when a device transmits its next frame, the risk of a collision is minimized.

Slot Time

In half-duplex Ethernet, where data can only travel in one direction at once, slot time becomes an important parameter in determining how many devices can share a network. For all speeds of Ethernet transmission at or below 1000 Mbps, the standard describes how an individual transmission may be no smaller than the slot time. Determining slot time is a trade-off between the need to reduce the impact of collision recovery (backoff and retransmission times) and the need for network distances to be large enough to accommodate reasonable network sizes. The compromise was to choose a maximum network diameter (about 2500 meters) and then to set the minimum frame length long enough to ensure detection of all worst-case collisions.

Slot time for 10- and 100-Mbps Ethernet is 512 bit times, or 64 octets. Slot time for 1000-Mbps Ethernet is 4096 bit times, or 512 octets. The slot time ensures that if a collision is going to occur, it will be detected within the first 512 bits (4096 for Gigabit Ethernet) of the frame transmission. This simplifies the handling of frame retransmissions following a collision.

Slot time is an important parameter for the following reasons:

• The 512-bit slot time establishes the minimum size of an Ethernet frame as 64 bytes. Any frame less than 64 bytes in length is considered a "collision fragment" or "runt frame" and is automatically discarded by receiving stations.
  
• The slot time establishes a limit on the maximum size of a network's segments. If the network grows too big, late collisions can occur. Late collisions are considered a failure in the network because the collision is detected too late by a device during the frame transmission to be automatically handled by CSMA/CD.

Slot time is calculated assuming maximum cable lengths on the largest legal network architecture. All hardware propagation delay times are at the legal maximum and the 32-bit jam signal is used when collisions are detected. The actual calculated slot time is just longer than the theoretical amount of time required to travel between the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected. See the figure:

For the system to work properly, the first device must learn about the collision before it finishes sending the smallest legal frame size.

To allow 1000 Mbps Ethernet to operate in half-duplex mode, the extension field was added to the frame when sending small frames purely to keep the transmitter busy long enough for a collision fragment to return. This field is present only on 1000-Mbps, half-duplex links and allows minimum-sized frames to be long enough to meet slot time requirements. Extension bits are discarded by the receiving device