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Media Access Control: Deciding When to Transmit

In Ethernet networks that use half-duplex interfaces, only one interface at a time can transmit, so the interfaces need a way of deciding when it’s OK to transmit. The Ethernet standard refers to the method of deciding who gets to transmit as media access control. There are several ways to achieve media access control. In some networks, one computer is the master, and the other computers transmit only after receiving permission from the master. The USB interface uses this type of media-access control. In a token-passing network, the computers take turns. The token can be as basic as a register bit or sequence of bits that a computer sets to indicate possession.

Only the computer holding the token can transmit. When a computer finishes transmitting, it passes the token to another computer. The token-ring network described in IEEE standard 802.5 is an example of a token-passing network. Ethernet uses a media-access control method called carrier sense multiple access with collision detection, or CSMA/CD. This method allows any interface to attempt to transmit any time the network is idle. If two or more interfaces try to transmit at the same time, both wait a bit, then retry. One way to understand how CSMA/CD works is to examine the words that make up the term. Carrier comes from the world of radio, where audio broadcasts ride on, or are carried by, a higher frequency called the carrier. Ethernet doesn’t have a carrier in this sense. Instead, the carrier is said to be present whenever an interface is transmitting. Carrier sense means that an interface that wants to transmit must monitor the network and sense, or detect, when the network is idle, indicated by the absence of a carrier. Multiple access means that no single interface controls the network traffic. Any interface can attempt to transmit on a network that has been idle for at least the amount of time defined as the interframe gap (IFG). In a 10Mb/s network, the IFG equals 96 bit times, or 9.6 microseconds. The Ethernet controller’s hardware normally handles the sending and receiving of frames, including detecting collisions and deciding when to try again after a collision. The CPU writes the data to send into memory that the controller can access, and the controller stores received data in memory that the CPU can access. The CPU uses interrupts or polling to learn of the success or failure of a transmission and the arrival of received data.

Responding to Collisions

A collision results when two interfaces in the same collision domain try to transmit at the same time. In half-duplex Ethernet networks, the computers connect to repeater hubs that provide attachment points for multiple interfaces. All of the interfaces that connect via repeater hubs share a collision domain, which means that every network transmission goes out to all of the interfaces. Interfaces that connect directly via coaxial cable also share a collision domain. Another option for connecting interfaces is to use switching hubs, popularly called Ethernet switches, or just switches. Like repeater hubs, Ethernet switches provide attachment points for multiple interfaces, but interfaces that connect via switches don’t share a collision domain. Instead, the switches are responsible for managing the storing and forwarding of traffic received from connected interfaces. Chapter 2 has more about repeater hubs and switches and the options for connecting interfaces. On detecting a collision, the transmitting interface doesn’t stop transmitting immediately. Instead, it continues long enough to be sure that the other transmitting interface(s) have time to detect the collision. A transmitting interface that has detected a collision always finishes sending the 64 bits of the Preamble and Start of Frame Delimiter if these haven’t transmitted yet. Following these, the interface sends an additional 32 bits called the jam signal, then stops transmitting. The jam signal can be any arbitrary data except the previous frame’s CRC value.

Delaying before Retransmitting

After an interface stops transmitting due to a collision, the next task is deciding when to try again. If two interfaces wait the same amount of time and then retry, another collision will occur. Instead, the Ethernet standard defines a backoff process where each interface selects a randomly chosen delay time before attempting to retransmit. This reduces the chance that two interfaces will retry at the same time, although multiple retries may be needed at times.

The delays before retrying are multiples of the interface’s slot time, which is specified in units of bit times. For 10-Mb/s and Fast Ethernet, the slot time is 512 bit times, which works out to 51.2 microseconds at 10 Mb/s and 5.12 microseconds at 100 Mb/s. For Gigabit Ethernet, the slot time is 4096 bit times, or 4.096 microseconds. For the first retry, an interface chooses randomly between retrying immediately or waiting one slot time before retrying. If the first retry results in a collision, the interface tries again, randomly selecting a delay of 0, 1, 2, or 3 slot times. Each new attempt, up to ten tries, selects from a larger range of backoff times, as Table 1-2 shows. The formula for determining the number of slot times to choose from is 2x, where x is the number of the retry. In the first retry, the interface selects between 21, or 2, slot times (0 or 1). The second retry selects from 22, or 4, slot times (0, 1, 2, or 3). And so on up to 210, or 1024 slot times (0 to 1023), which the interface uses for the final seven retries if needed. After 16 tries, the interface gives up and reports a failure, typically with an interrupt.

Network Limits to Ensure Collision Detection

To prevent an interface from trying to use a frame that has experienced a collision, a transmitting interface must be able to detect the collision and abandon the frame before transmitting for one slot time. The IEEE 802.3 standard specifies slot times and maximum cable lengths to ensure that a transmitting interface will always be able to detect a collision in time. For 10-Mb/s and Fast Ethernet, one slot time equals the time required to transmit 512 bits, which is the minimum frame size minus the Preamble and Start Frame Delimiter. For Gigabit Ethernet, the minimum frame size is still 512 bits, but the slot time is 4096 bits and a valid transmission must have at least 4096 bits. To extend a short frame to 4096 bits, the transmitting interface has two options. It can follow the frame with carrier extension bits, which are non-data symbols that keep the carrier present for the required time. Or for all frames after the first, the interface may send a burst of additional frames to fill the slot time.