The first Ethernet, Ethernet DIX, was named after the companies that proposed it: Digital, Intel, and Xerox. During this time, the Institute of Electrical and Electronics Engineers (IEEE) had been working on Ethernet standardization, which became known as Project 802. Upon its success, the Ethernet plan evolved into the IEEE 802.3 standard. Based on carrier sensing, as originally developed by Robert Metcalfe, David Boggs, and their team of engineers, Ethernet became a major player in communication mediums, competing head-to- head with IBM’s proposed Token Ring, or IEEE 802.5.
Carrier Transmissions
When a station on an Ethernet network is ready to transmit, it must first listen for transmissions on the channel. If another station is transmitting, it is said to be ‘‘producing activity.” This activity, or transmission, is called a carrier. In a nutshell, this is how Ethernet became known as the carrier- sensing communication medium. With multiple stations, all sensing carriers, on an Ethernet network, this mechanism was called Carrier Sense with Multiple Access, or CSMA.
If a carrier is detected, the station will wait for at least 9.6 microseconds, after the last frame passes, before transmitting its own frame. When two stations transmit simultaneously, a fused signal bombardment, otherwise known as a collision, occurs. Ethernet stations detect collisions to minimize problems. This technology was added to CSMA to become Carrier Sense with Multiple Access and Collision Detection or CSMA/CD.
Figure 3.1 Ethernet topology breakdown.
Stations that participated in the collision immediately abort their transmissions. The first station to detect the collision sends out an alert to all stations. At this point, all stations execute a random collision timer to force a delay before attempting to transmit their frames. This timing delay mechanism is termed the back-off algorithm. And, if multiple collisions are detected, the random delay timer is doubled.
After 10 consecutive collisions and multiple double random delay times, network performance will not improve significantly. This is a good example of an Ethernet flooding method.
Ethernet Design, Cabling, Adapters
Ethernet comes in various flavors. The actual physical arrangement of nodes in a structure is termed the network topology. Ethernet topology examples include bus, star, and point-to-point (see Figure 3.1).
Ethernet options also come in many variations, some of which are shown in Figure 3.2 and defined in the following list:
Figure 3.2 An Ethernet specification chart by type, for comparison.
Figure 3.3 Ethernet and 10Base5 network.
• Ethernet, 10Base5. Ethernet with thick coaxial (coax) wire uses cable type RG08. Connectivity from the NIC travels through a transceiver cable to an external transceiver and finally through the thick coax cable (see Figure 3.3). Due to signal degradation, a segment is limited to fewer than 500 meters, with a maximum of 100 stations per segment of 1,024 stations total.
• 10Base2. Thin-wire Ethernet, or thinnet, uses cable type RG-58. With 10Base2, the transceiver functionality is processed in the NIC. BNC T connectors link the cable to the NIC (see Figure 3.4). As with every media type, due to signal degradation, a thinnet segment is limited to fewer than 185 meters, with a maximum of 30 stations per segment of 1,024 stations total.
• 10BaseT. Unshielded twisted pair (UTP) wire uses cable type RJ-45 for 10BaseT specifications. Twisted pair Ethernet broke away from the electric shielding of coaxial cable, using conventional unshielded copper wire. Using the star topology, each station is connected via RJ-45 with UTP wire to a unique port in a hub or switch (see Figure 3.5). The hub simulates the signals on the Ethernet cable. Due to signal degradation,
Figure 3.4 10Base2 network diagram.
Figure 3.5 10BaseT example diagram.
• the cable between a station and a hub is limited to fewer than 100 meters.
• Fast Ethernet, 100BaseT. To accommodate bandwidth- intensive applications and network expansion, the Fast Ethernet Alliance promoted 100 Mbps technology. This alliance consists of 3Com Corporation, DAVID Systems, Digital Equipment Corporation, Grand Junction Networks, Inc., Intel Corporation, National Semiconductor, SUN Microsystems, and Synoptics Communications.
To understand the difference in transmission speed between 10BaseT and 100BaseT, let’s look at the formula:
Station-to-Hub Diameter (meters) = 25,000/Transmission Rate (Mbps). Given: 10 Mbps 10BaseT Ethernet network:
Diameter (meters) = 25,000/10 (Mbps) Diameter = 2,500 meters
Given: 100 Mbps 100BaseT Fast Ethernet network: Diameter (meters) = 25,000 / 100 (Mbps)
Diameter = 250 meters
From these equations, we can deduce that 100 Mbps Fast Ethernet requires a station-to-hub diameter, in meters, that is one-tenth that of 10 Mbps Ethernet. This speed versus distance ratio in Fast
Ethernet include 100 Mbps station NICs, Fast Ethernet hub or switch, and Category 5 UTP (data grade) wire.
Hardware Addresses, Frame Formats
Having touched upon Ethernet design and cabling, we can address the underlying Ethernet addressing and formatting. We know that every station in an Ethernet network has a unique 48-bit address bound to each NIC (described in Chapter 1). These addresses not only specify a unique, single station, but also provide for transmission on an Ethernet network to three types of addresses: Unicast Address. Transmission destination to a single station.
Multicast Address. Transmission destination to a subset or group of stations. Broadcast Address. Transmission destination to all stations.
It doesn’t necessarily matter whether the transmission destination is unicast, multicast, or broadcast, because each frame will subsequently pass by every interface.
The Ethernet frame is variable length, which is to say that no frame will be smaller than 64 octets or larger than 1,518 octets. Each frame consists of a preamble, a destination address, a source address, the frame type, frame data, and cyclic redundancy check (CRC) fields (see Figure 3.6). These fields are defined as follows:
Preamble. Aids in the synchronization between sender and receiver(s). Destination Address. The address of the receiving station.
Source Address. The address of the sending station.
Frame Type. Specifies the type of data in the frame to determine which protocol software module should be used for processing.
Frame Data. Indicates the data carried in the frame based on the type latent in the Frame Type field.
Cyclic Redundancy Check (CRC).
Helps detect transmission errors. The sending station computes a frame value before transmission. Upon frame retrieval, the receiving station must compute the same value based on a complete, successful transmission.