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Half-Duplex Ethernet is the traditional form of Ethernet that uses the CSMA/CD (Carrier Sense Multiple Access/Collision Detect) protocol. With CSMA/CD two or more stations share a common transmission medium. To transmit a frame, a station must wait for an idle period on the medium when no other station is transmitting. It then transmits the frame by broadcasting it over the medium such that it is "heard" by all the other stations on the network. If another device tries to send data at the same time, a "collision" is said to occur. The transmitting station then intentionally transmits a "jam sequence" to ensure all stations are notified the frame transmission failed due to a collision. The station then remains silent for a random period of time before attempting to transmit again. This process is repeated until the frame is eventually transmitted successfully.
The basic rules for transmitting a frame are as follows:
The signals transmitted by Ethernet stations encounter delays as they travel through the network. These delays consist of the time required for signals to travel across the cable segments, and the logic delays encountered when the signals pass through electronic components in Network Interface Cards (NICs) and repeating hubs. The longer the cable segments and the more hubs in the network, the longer it takes for a signal to propagate from one end of the network to the other. The time it takes a signal to travel between the two stations that are furthest apart in the network is known as the maximum "propagation delay" of the network.
For a station to detect that the frame it is transmitting has encountered a collision, its signal must propagate across the network to another station that detects the collision. This station must transmit a jam sequence to indicate a collision has been detected. The jam sequence must then propagate back across the network before being detected by the transmitting station. The sum of a network's maximum "round trip propagation delay" and the time required to transmit a jam sequence are the components that define the length of the Ethernet slot time.
Slot time is an important parameter for the following reasons:
The backoff algorithm implemented in Ethernet is officially known as "truncated binary exponential backoff". Following a collision, each station generates a random number that falls within a specified range of values. It then waits that number of slot times before attempting retransmission. The range of values increases exponentially after each failed retransmission. For the first attempt the range is 0 to 1; for the second attempt, 0 to 3; for the third, 0 to 7 and so on. If repeated collisions occur, the range continues to expand until after 10 attempts when it reaches 0 to 1023. After that the range of values stays fixed from 0 to 1023. If a station is unsuccessful in transmitting after 16 attempts, the MAC function reports an "excessive collision error". The frame being transmitted is then dropped, requiring that application software detect its loss and initiate a retransmission.
Binary exponential backoff results in minimum delays before retransmission when traffic on the LAN is light. When traffic is high, repeated collisions cause the range of numbers to increase, thus lessening then chance of further collisions. In a network where the traffic is extremely high, repeated collisions will begin to cause excessive collision errors to be generated. Excessive collision errors are an indication that the traffic load has increased to the point that it can no longer be efficiently handled on a single Ethernet network.
A simple example of the capture effect consists of two stations that have a lot of data to send and can send data as fast as allowed. They both collide on their first transmission attempt and choose a backoff of 0 or 1. Station A chooses 0, and station B chooses 1. Station A gets to transmit while station B waits for one slot time. After Station A completes its transmission and the interframe gap passes, both stations are ready to transmit again and another collision occurs. This is station A's first collision for this frame, so it chooses a backoff of 0 or 1. However, this is station B's second collision for this frame, so it chooses a backoff between 0 and 3. Thus station A has a higher probability transmitting while station B waits again. If they happen to pick the same number and collide again, then the odds for station B get even worse.
The same scenario can repeat over and over again possibly ending only when station A's queue is finally empty or when station B finally reaches 16 attempts. After 16 attempts station B will reset its collision counter allowing it to compete more aggressively again. But it also discards the frame it was attempting to transmit, requiring that it be queued for transmission again by software.
In 1994 a new backoff algorithm called "binary logarithmic arbitration method" (BLAM) was proposed to alleviate the capture effect problem. An IEEE 802.3w working group was formed to add BLAM as an optional feature of the Ethernet standard. Although simulation results proved that BLAM offered a definite improvement over the binary exponential backoff algorithm, the work to incorporate it into the Ethernet standard was never completed due to a shift in focus to full-duplex Ethernet and a lack of interest in updating the half-duplex hardware.
The release of the IEEE 802.3x standard defined a second mode of operation for Ethernet, called "full-duplex", that bypasses the CSMA/CD protocol. The CSMA/CD protocol is "half-duplex". This implies that a station may either transmit data, or receive data, but never both at the same time. Full-duplex mode allows two stations to simultaneously exchange data over a point to point link that provides independent transmit and receive paths. Since each station can simultaneously transmit and receive data, the aggregate throughput of the link is effectively doubled. A 10 Mb/s station operating in full-duplex mode provides a maximum bandwidth of 20 Mb/s. A full-duplex 100 Mb/s station provides 200 Mb/s of bandwidth.
Full-duplex operation is restricted to links meeting the following criteria:
Full-duplex operation offers several major advantages:
For example, assume a full-duplex link that connects two devices called "Station A" and "Station B". Suppose Station A transmits frames at a rate that causes Station B to enter into a state of congestion (i.e. no buffer space remaining to receive additional frames). Station B may transmit a PAUSE frame to Station A requesting that Station A stop transmitting frames for a specified period of time. Upon receiving the PAUSE frame, Station A will suspend further frame transmission until the specified time period has elapsed. This will allow Station B time to recover from the congestion state. At the end of the specified time period, Station A will resume normal transmission of frames.
Note that the PAUSE frame protocol is bi-directional. Station A may send frames to pause Station B, and Station B may send frames to pause Station A. A PAUSE frame is the one type of frame that a station is allowed to send even if it is currently in the paused state. Support for PAUSE frames is optional among devices that implement the full-duplex protocol (the use of PAUSE frames is not supported in a half-duplex environment). It is valid for a device to support only half of the protocol; i.e. it may transmit PAUSE frames without having the capability to decode them on the receive side, and vice-versa. Devices use the Auto-Negotiation protocol to discover the PAUSE frame capabilities of the device at the other end of the link. This permits interoperability between devices that do or do not support one or both halves of the protocol.
The format of a PAUSE frame is illustrated below. It conforms to the standard Ethernet frame format but includes a unique type field and other parameters as follows:
| Preamble (7-bytes) | Start Frame Delimiter (1-byte) | Dest. MAC Address (6-bytes) = (01-80-C2- 00-00-01) or unique DA |
Source MAC Address (6-bytes) | Length/Type (2-bytes) = 802.3 MAC Control (88-08) |
MAC Control Opcode (2-bytes) = PAUSE (00-01) |
MAC Control Parameters (2-bytes) = (00-00 to FF-FF) |
Reserved (42-bytes) = all zeros |
Frame Check Sequence (4-bytes) |
"Link Aggregation", or "Trunking", is another Ethernet feature that applies only to the full-duplex mode of operation. It provides for increased link availability and bandwidth between two Ethernet stations by allowing multiple "physical" links to be combined to operate as a single "logical" link. The Link Aggregation specification was recently developed by the IEEE 802.3ad Working Group. It is in its final stages of approval and is expected to be formally released as an addition to the Ethernet standard in early 2000.
Prior to Link Aggregation it was difficult, if not impossible, to have multiple links between two Ethernet stations. The "spanning tree" algorithm used in Ethernet bridging disables parallel paths to prevent "loops" in the network. An end station could have multiple Ethernet links only if the links were attached to different networks, or to different VLANs within a network.
Link Aggregation resolves this limitation by allowing multiple parallel links between any two Ethernet stations; The links may be between two switches, between a switch and a server, or between a switch and an end user station. The following advantages are provided:
As the Link Aggregation layer distributes frames among the multiple links within the group, it must ensure the frames arrive at the other end in the correct order. To do this, the Link Aggregation algorithm creates sessions, called "conversations", that consist of Ethernet frames with identical sources and destinations. All frames from a conversation are restricted to a single link within the aggregation group. By grouping traffic in this fashion, frames are guaranteed to arrive at the destination in the proper sequence.
The use of Link Aggregation is restricted as follows:
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