2.0 THE BASICS OF NETWORKING

2.4 Media Access Method

A media access method defines how computing devices access the network cable and sends data. Fast Ethernet, Ethernet, Token Ring and LocalTalk media access methods are used primarily for connecting desktop machines (computers, printers, etc.) to the network, whereas ATM is used primarily for high-speed backbones, high-speed network access (e.g. file servers) and very high-speed workgroup applications.

2.4.1 Ethernet

Ethernet has emerged over the last several years as the most popular media access method to the desktop. Since network equipment manufacturers everywhere have embraced this non-proprietary industry standard, Ethernet network components from multiple vendors will work together and communicate effortlessly. You will find the Ethernet media access method in both small and large network environments. Set-up in star and/or bus configurations, Ethernet networks transmit data over UTP, thin coaxial, thick coaxial and fiber-optic cables at rates of 10Mbps.

On an Ethernet network, each computer listens to the cable before sending a data packet. If the cable is clear, the computer will transmit. If it is not clear, the computer waits and tries again. If two or more computers transmit simultaneously, a collision occurs. (A collision is when the signals from two devices run into each other.) Each device will then attempt to resend its data when the line is silent. This process is referred to as the CSMA/CD (Carrier Sense Multiple Access, with Collision Detection) media access control mechanism. With the CSMA/CD control mechanism, it is normal to have collisions, as long as the number remains low compared to the number of signals that transmit successfully.

2.4.2 Fast Ethernet

Fast Ethernet refers to 100Mbps Ethernet. Fast Ethernet is 10 times faster than Ethernet. Fast Ethernet evolved from 10Base-T Ethernet; Fast Ethernet uses the same CSMA/CD media-access method, and at the MAC layer, retains many of the same characteristics as 10Base-T. Because 100Base-T Fast Ethernet utilizes the same cabling as 10Mbps Ethernet, the transition to Fast Ethernet does not require major changes to the organization's infrastructure.

"Link Integrity" and "Auto-partition" are part of the 10Base-T specification. This means that all network equipment claiming compliance with 10Base-T must support Link Integrity and Auto-partitioning. Link Integrity is concerned with the condition of the cable between the network adapter and the hub. If the cable is broken, the hub will automatically disconnect that port. Auto-partitioning occurs when an Ethernet hub port experiences more than 31 collisions in a row. When this happens, the hub will turn off that port, essentially isolating the problem.

2.4.3 Token Ring

The Token Ring media access method was developed by IBM Corporation in the mid 1980s and subsequently defined by the IEEE in Project 802. Since Token Ring has been IBM's preferred method for networking, it is found primarily in large IBM mini and mainframe installations. Due to the increasing popularity of Fast Ethernet and Ethernet, the number of Token Ring networks has significantly decreased. Token Ring networks use a star-wired ring topology over shielded and unshielded twisted-pair wiring. A hub (sometimes referred to as a MAU) is at the center of the ring. Two versions of Token Ring are available: 4Mbps and 16Mbps. Token Ring networks use a token passing media access control mechanism to circulate packets around the ring. An electronic token travels from station to station in a single, logical direction. If the token is free, a station can attach data to the token, change the token's status to busy, and then send the token on to the next station. Each consecutive station then checks the destination address of the data to see if it should process the data. It then passes the token on. When the station that originated the token receives it back, it removes the data from the token and changes the token status back to free.

2.4.4 LocalTalk

LocalTalk is a proprietary media access method built into Apple Macintosh computers and LaserWriter printers. LocalTalk networks are best suited for small networks of Macs (e.g. an independently networked departments). With LocalTalk, computers are set up in a bus configuration using both shielded and unshielded twisted-pair wiring. Data transmits at only 230Kbps or about 1/40 the rate of Ethernet. For this reason, many companies, which are using Macs computers, are upgrading their LocalTalk Macintosh installations to Ethernet and Fast Ethernet in order to handle better large file transfers. LocalTalk uses the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) media access control mechanism for transmitting data. CSMA/CA is similar to CSMA/CD in that a computer listens to the cable before transmitting data. If the computer does not detect a signal, it will send out its own signal saying, "I'm about to transmit, so stay off the line."

2.4.5 FDDI

The FDDI (Fiber Distributed Data Interface) media access method transfers data at very high speeds (100Mbps) over fiber-optic cable. Like Token Ring, this access method employs a token passing media access control mechanism to transmit data. FDDI, however, uses a dual counter-rotating ring topology, meaning there are two rings of cable with two tokens circulating in opposite directions. This set-up creates a relatively fault-tolerant network and is commonly used in networks working with mission-critical data. Many institutions are installing fiber-optic backbone cables to carry Ethernet signals today and FDDI signals in the future.

2.4.6 ATM

An ATM (Asynchronous Transfer Mode) network is set up in a star configuration using fiber-optic (and, in some newer incarnations, twisted-pair) cables. Data transmits at 155Mbps and higher. A switch at the center of the star establishes a dedicated circuit between the sending and receiving stations. ATM is ideal for video, voice and teleconferencing applications. The rapid adoption of Fast Ethernet has slowed the move to ATM. In addition, ATM requires new network adapters and, in most cases, fiber-optic cable connections. Furthermore, its implementations are limited and proprietary. Until ATM becomes more affordable and standardized, it is more cost-effective to purchase a Fast Ethernet switch or Ethernet to increase a network's bandwidth.

2.4.7 Gigabit

Gigabit Ethernet is similar to 10Base-T Ethernet and 100Base-TX Fast Ethernet in many ways. Gigabit can utilize both fiber cable and twisted-pair depending on usage (e.g., server links, backbones, etc.) and distance requirements. The major advantage of Gigabit Ethernet is its backward compatibility to 10Base-T and 100Base-TX technology, along with a 10 to 100 times improvement in bandwidth. Because Gigabit retains the CSMA/CD media access control mechanism of other flavors of Ethernet, this high-speed medium is best suited for server pipes, backbone links, and connection to dedicated power users. Gigabit is quickly capturing much of the high-speed connections once reserved for ATM. Gigabit is perfectly suited for deployment in an image and data-intensive environment, whereas ATM is best utilised for multimedia, mixed packet usage, such as voice and video.