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Understanding how 10/100Base-T Ethernet operates

Since today's local area networks are dominated by 10/100Base-T Ethernet technology, network administrators need to understand the underlying elements of Ethernet communications. Here's a look at Ethernet frames and how they function on a network.

Since the late 1990s, 10/100Base-T Ethernet (also known as IEEE 802.3 and CSMA/CD) has become the de facto standard for local area networks. We’re going to look at the elements involved in using Ethernet in a 10/100Base-T network, including the components of a frame, how Ethernet technology functions, and where this suite of technologies fits into the OSI reference model.

Ethernet frames are the building blocks
The core of the Ethernet system is the Ethernet frame, which is used to deliver data between Ethernet network adapters. The frame (Figure A) consists of a set of bits organized into several fields. These fields include address fields, a variable-size data field that carries from 46 to 1,500 bytes of data, and an error-checking field that checks the integrity of the bits in the frame to make sure that the frame has arrived intact.

Figure A


Here is a closer look at the components of an Ethernet frame:
  • Preamble: These 56 bits having alternating 1 and 0 values are used for synchronization. They give components in the network time to detect the presence of a signal and read the signal before the frame data arrives.
  • Start frame delimiter: This involves 8 bits having the bit configuration 10101011, indicating the start of the frame.
  • Destination and source MAC addresses: These addresses have 48 bits each to identify the frame’s destination and source addresses. The addresses used are the MAC addresses of the network adapters. A destination address may specify either an individual address destined for a single network adapter or a multicast address destined for a group of network adapters, as in the case of a broadcast.
  • Length/Type: These 16 bits indicate the number of bytes in the Data field.
  • Data: These are the 46 to 1,500 bytes that represent the data transferred from the source to the destination.
  • Frame check sequence (FCS): A 4-byte cyclical redundancy check (CRC) value is used for error checking. This value is recalculated at the destination network adapter. If the value is different from what is transmitted, the receiving network adapter assumes that an error has occurred during transmission and discards the frame.

As each Ethernet frame is sent onto the shared medium, all Ethernet network adapters look at the first 48-bit field of the frame, which contains the destination address. The network adapters then compare the destination address of the frame with their address. The network adapter with the same address as the destination address in the frame will read in the entire frame and deliver it to the networking software running on that computer. All other network interfaces will stop reading the frame when they discover that the destination address does not match their own address.

An Ethernet LAN can simultaneously carry several different kinds of software protocol data. A single Ethernet can carry data between computers in the form of TCP/IP protocols, as well as Novell IPX or AppleTalk protocols. The Ethernet is simply a transport system that carries frames of data between computers; it doesn't care what's inside the frames.

The nuts and bolts of the Ethernet
The Ethernet consists of the following elements:

Physical media
The first requirement for the Ethernet to operate is the existence of the actual wires and devices used to carry Ethernet signals between devices. I covered this in my article "Deploying and managing 10/100baseT Ethernet hardware."

Media Access Control rules
Media Access Control rules are embedded in each Ethernet network interface card (network adapter). They allow multiple computers to reasonably decide who gets access to the shared Ethernet medium and when they get that access.

The Media Access Control rules are based on a system called carrier sense multiple access with collision detection (CSMA/CD). Only one network adapter can talk at a time on a shared wire. To send data, a network adapter first listens (carrier sense) to the wire.

Carrier sense
The IEEE 802.3 specification states that before a station can attempt to transmit on the wire, it must wait until it has heard 9.6 microseconds (millionths of a second) of silence. This 9.6-microsecond interframe gap allows the network adapter that last transmitted to cycle its circuitry from transmit mode to receive mode. Without the interframe gap, a network adapter could miss a frame that was destined for it because it had not yet cycled back into receive mode.

This standard is based on 1970s technology, and most network adapters in today's market are capable of switching from transmit to receive in much less time than 9.6 microseconds. Some adapter manufacturers have designed their cards with a smaller interframe gap cycle and advertise higher data transfer rates than their competitors. This is another reason to be consistent with the network adapters you use.

Multiple access
After each frame transmission, all network adapters on the network will wait 9.6 microseconds and compete equally for the next frame transmission opportunity. This ensures that access to the network media is fair and that no single station can lock out the other stations. After the interframe gap, if two network adapters start transmitting at the same instant, they detect each other’s presence (collision detection) and stop transmitting.

Collision detection
Ethernet frames are a series of voltage pulses on a wire and take a specific amount of time to travel from one end of an Ethernet system to the other. The first bits of a transmitted frame do not reach all parts of the network simultaneously. Therefore, it's possible for two network adapters to sense that the network is idle and to start transmitting their frames simultaneously. When this happens, the Ethernet system has a way to sense the collision of signals and to stop the transmission and resend the frames.

The network adapters are notified of this event and instantly reschedule their transmission using a specially designed backoff algorithm. As part of this algorithm, the network adapters involved choose a random time interval to schedule the retransmission of the frame, which keeps the network adapters from constantly colliding during retransmission.

Collisions on an Ethernet network are normal and indicate that the CSMA/CD protocol is functioning properly. As more computers are added to a given Ethernet network, the traffic level will increase and more collisions will occur as part of the normal operations. The design of the system ensures that the majority of collisions on an Ethernet will be resolved in microseconds.

On a network with heavy traffic, there might be multiple collisions for a given frame transmission attempt. This is also normal behavior. If repeated collisions occur for a given transmission attempt, the stations involved begin expanding the set of potential backoff times from which they chose their random retransmission time. Repeated collisions for a given packet transmission attempt indicate a busy network. The expanding backoff process, formally known as the truncated binary exponential backoff, is a feature of the Ethernet MAC that provides an automatic method for network adapters to adjust to traffic conditions on the network.

Only after 16 consecutive collisions for a given transmission attempt will the network adapter finally discard the Ethernet packet. This can happen only if the Ethernet is overloaded for a long period of time or is broken. If a network is experiencing an excessive number of collisions, an Ethernet switch can be used to segment the collision domains.

Delivering the data
Ethernet systems operate as a best effort data delivery system. No guarantee of reliable data delivery is made. The Ethernet is engineered to produce a system that normally delivers data extremely well. However, errors still occur. For instance, electrical noise may occur somewhere in a cabling system corrupting the data in a frame and causing it to be dropped. No LAN system is perfect, which is why higher protocol layers of network software are designed to recover from errors.

It is up to the high-level protocol sending data over the network to make sure that the data is correctly received at the destination. These protocols do this by establishing a reliable data transport service using sequence numbers and acknowledgment mechanisms in the packets that they send over the LAN.

What tips do you have for managing Ethernet networks?
We look forward to getting your input and hearing about your experiences regarding this topic. Join the discussion below or send the editor an e-mail.

 
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