• Ethernet and IEEE 802.3---LAN specifications that operate at 10 Mbps over coaxial cable.
• 100-Mbps Ethernet---A single LAN specification, also known as Fast Ethernet, that operates at 100 Mbps over twisted-pair cable.
• 1000-Mbps Ethernet---A single LAN specification, also known as Gigabit Ethernet, that operates at 1000 Mbps (1 Gbps) over fiber and twisted-pair cables.

   

This chapter provides a high-level overview of each technology variant.

Under the Ethernet CSMA/CD media-access process, any station on a CSMA/CD LAN can access the network at any time. Before sending data, CSMA/CD stations listen for traffic on the network. A station wanting to send data waits until it detects no traffic before it transmits.

The IEEE Higher Speed Ethernet Study Group was formed to assess the feasibility of running Ethernet at speeds of 100 Mbps. The Study Group established several objectives for this new higher-speed Ethernet but disagreed on the access method. At issue was whether this new faster Ethernet would support CSMA/CD to access the network medium or some other access method.

The study group divided into two camps over this access-method disagreement: the Fast Ethernet Alliance and the 100VG-AnyLAN Forum. Each group produced a specification for running Ethernet (and Token Ring for the latter specification) at higher speeds: 100BaseT and 100VG-AnyLAN, respectively.

 

• 100BaseX
• 4T+

   

To achieve the increased throughput of 100BaseT, the size of the collision domain had to shrink. This is because the propagation speed of the medium has not changed, so a station transmitting 10 times faster must have a maximum distance that is 10 times less. As a result, any station knows within the first 64 bytes whether a collision has occurred with any other station.

Autonegotiaton supports a number of capabilities, including speed matching for devices that support both 10-and 100-Mbps operation, full-duplex mode of operation for devices that support such communications, and an automatic signaling configuration for 100BaseT4 and 100BaseTX stations.

 

• 4-pair Category 3UTP
• 2-pair Category 4 or 5 UTP
• STP
• Fiber optic

   

The key differences between the use of long-wave and short-wave laser technologies are cost and distance. Lasers over fiber-optic cable take advantage of variations in attenuation in a cable. At different wavelengths, "dips" in attenuation will be found over the cable. Short-wave and long-wave lasers take advantage of those dips and illuminate the cable at different wavelengths. Short-wave lasers are readily available because variations of these lasers are used in compact disc technology. Long-wave lasers take advantage of attenuation dips at longer wavelengths in the cable. The net result is that short-wave lasers will cost less, but transverse a shorter distance. In contrast, long-wave lasers will be more expensive but will transverse longer distances.

Single-mode fiber has traditionally been used in networking cable plants to achieve long distances. In Ethernet, for example, single-mode cable ranges reach up to 10 kilometers. Single-mode fiber, using a 9-micron core and 1300-nanometer laser, demonstrate the highest-distance technology. The small core and lower-energy laser elongate the wavelength of the laser and allow it to transverse greater distances. This enables single-mode fiber to reach the greatest distances of all media with the least reduction in noise.

The application for this type of cabling will be short-haul data-center interconnections and inter-or intrarack connections. Because of the distance limitation of 25 meters, this cable will not work for interconnecting data centers to riser closets.

Encoding data transmitted at high speeds provides some advantages:

 

• Encoding limits the effective transmission characteristics, such as ratio of 1s to 0s, on the error rate.
• Bit-level clock recovery of the receiver can be greatly improved by using data encoding.
• Encoding increases the possibility that the receiving station can detect and correct transmission or reception errors.
• Encoding can help distinguish data bits from control bits.

   

All these features have been incorporated into the Fibre Channel FC1 specification.

In Gigabit Ethernet, the FC1 layer will take decoded data from the FC2 layer, 8 bits at a time from the reconciliation sublayer (RS), which "bridges" the Fibre Channel physical interface to the IEEE 802.3 Ethernet upper layers. Encoding takes place via an 8-bit to 10-bit character mapping. Decoded data comprises 8 bits with a control variable. This information is, in turn, encoded into a 10-bit transmission character.

In contrast, Gigabit Ethernet switches without GBICs either cannot support other lasers or need to be ordered customized to the laser types required. Note that the IEEE 802.3z committee provides the only GBIC specification. The 802.3ab committee may provide for GBICs as well.

Acceleration of Ethernet to Gigabit speeds has created some challenges in terms of the implementation of CSMA/CD. At speeds greater than 100 Mbps, smaller packet sizes are smaller than the length of the slot-time in bits. (Slot-time is defined as the unit of time for Ethernet MAC to handle collisions.) To remedy the slot-time problem, carrier extension has been added to the Ethernet specification. Carrier extension adds bits to the frame until the frame meets the minimum slot-time required. In this way, the smaller packet sizes can coincide with the minimum slot-time and allow seamless operation with current Ethernet CSMA/CD.

Another change to the Ethernet specification is the addition of frame bursting. Frame bursting is an optional feature in which, in a CSMA/CD environment, an end station can transmit a burst of frames over the wire without having to relinquish control. Other stations on the wire defer to the burst transmission as long as there is no idle time on the wire. The transmitting station that is bursting onto the wire fills the interframe interval with extension bits such that the wire never appears free to any other end station.

It is important to point out that the issues surrounding half-duplex Gigabit Ethernet, such as frame size inefficiency (which in turn drives the need for carrier extension) as well as the signal round-trip time at Gigabit speeds, indicate that, in reality, half-duplex is not effective for Gigabit Ethernet.

Full-duplex transmission will be utilized in Gigabit Ethernet to increase aggregate bandwidth from 1 Gbps to 2 Gbps for point-to-point links as well as to increase the distances possible for the particular media. Additionally, Gigabit EtherChannel "bundles" will allow creation of 8 Gbps connecting between switches. The use of full-duplex Ethernet eliminates collisions on the wire; therefore, CSMA/CD need not be utilized as a flow control or access medium. However, a full-duplex flow control method has been put forward in the standards committee with flow control as on optional clause. That standard is referred to as IEEE 802.3x; it formalizes full-duplex technology and is expected to be supported in future Gigabit Ethernet products. Because of the volume of full-duplex 100-Mbps network interface cards (NICs), it is unlikely that this standard will realistically apply to Fast Ethernet.