2.1 Ethernet PDF

2.1.1 Network Data Transmission Network data transfer works by modulating the properties of a transmission medium—electric current, infrared light, or radio waves—to encode a signal. One example of modulation is transitioning between low and high...

2.1.1 Network Data Transmission Network data transfer works by modulating the properties of a transmission medium—electric current, infrared light, or radio waves—to encode a signal. One example of modulation is transitioning between low and high voltage states in an electrical circuit. These voltage pulses can encode symbols, which can be mapped to digital bits—ones and zeros. Each media type supports a range of possible frequencies. Higher frequencies allow for more data to be transferred per second. The range of frequencies is referred to as the media bandwidth. The narrow definition of bandwidth is a frequency range measured in cycles per second or Hertz (Hz), but the term is very widely used in data networking to mean the amount of data that can be transferred, measured in multiples of bits per second (bps). Encoding methods mean that, for instance, a signal with 100 MHz frequency bandwidth can transfer much more than 100 Mbps. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 2.1.2 Ethernet Standards Over the years, many protocols, standards, and products have been developed to implement the functions of the Physical and Data Link layers of the OSI model. A standard must define cable and connector specifications and define schemes for modulation and encoding. The Institute of Electrical and Electronics Engineers (IEEE) 802.3 Ethernet standards (ieee802.org/3) are very widely used on both LANs and WANs. Ethernet standards provide assurance that network cabling will meet the bandwidth requirements of applications. These Ethernet media specifications are named using a three-part convention, which is often referred to as xBASE-y. This describes the following: The speed or bit rate in megabits per second (Mbps) or gigabits per second (Gbps). The signal mode (baseband or broadband). All mainstream types of Ethernet use baseband transmissions, so you will only see specifications of the form xBASE-y. A designator for the media type. For example, 10BASE-T denotes an early implementation that works at 10 Mbps (10), uses a baseband signal (BASE), and runs over twisted pair copper cabling (-T). Copper cable is used to transmit electrical signals. The cable between two nodes creates a low voltage electrical circuit between the interfaces on the nodes. There are two main types of copper cable: twisted pair and coaxial (coax). Copper cable suffers from high attenuation, meaning that the signal quickly loses strength over long links. Twisted pair cable is rated to Category (or "Cat") standards that define what bandwidth it should support, up to a given distance. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 2.1.3 Media Access Control and Collision Domains Ethernet is a multiple access area network, which means that the available communications capacity is shared between the nodes that are connected to the same media. Media access control (MAC) refers to the methods a network technology uses to determine when nodes can communicate on shared media and to deal with possible problems, such as two devices attempting to communicate simultaneously. Ethernet uses a contention-based MAC system. Each network node connected to the same media is in the same collision domain. When two nodes transmit at the same time, the signals are said to collide, and neither signal can reach its destination. This means that they must be resent, reducing available bandwidth. The collisions become more frequent as more nodes are added, and consequently the effective data rate is reduced. The Ethernet protocol governing contention and media access is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). A collision is the state when a signal is present on an interface's transmit and receives lines simultaneously. On detecting a collision, the node broadcasts a jam signal. Each node that was attempting to use the media then waits for a random period (backoff) before attempting to transmit again. Description The steps of C S M A or C D media access method are as follows: 1. Data. 2. Check. 3. Transmit data. 4. Collision. 5. Wait. 6. Retransmit data. The CSMA/CD media access method. (Images © 123RF.com.) The collision detection mechanism means that only half-duplex transmission is possible. This means that a node can transmit or receive, but it cannot do both at the same time.  In the 10BASE-T star wiring physical topology, each node is cabled to an Ethernet hub. The hub repeats incoming signals to each connected node. Consequently, every host connected to the same hub is within the same collision domain. However, this 10BASE-T physical topology dates from 1990. You are very unlikely to find it deployed in a modern network. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 2.1.4 100BASE-TX Fast Ethernet Standards The Fast Ethernet standard uses the same CSMA/CD protocol as 10BASE-T but with higher frequency signaling and improved encoding methods, raising the bit rate from 10 Mbps to 100 Mbps. 100BASE-TX refers to Fast Ethernet working over Cat 5 (or better) twisted pair copper cable with a maximum supported link length of 100 meters (328 feet). 100BASE-TX can be implemented with a hub, but the standard was created at a time that switches started to replace hubs as the connection point for end systems. The contention-based access method used by a hub does not scale to large numbers of end systems within the same collision domain. Where a hub works only at the Physical layer, a switch uses information about source and destination addresses carried in layer 2frames to establish a temporary circuit between two nodes. Unlike a hub, each switch port is a separate collision domain. By eliminating the effect of contention, switches allow for full-duplex transmissions, where a node can transmit and receive simultaneously, and each node can use the full 100 Mbps bandwidth of the cable link to the switch port. To support compatibility with hosts still equipped with 10 Mbps Ethernet interfaces, Fast Ethernet introduced an autonegotiation protocol to allow a host to choose the highest supported connection parameters (10 or 100 Mbps and half- or full-duplex). 10BASE-T Ethernet specifies that a node should transmit regular electrical pulses when it is not transmitting data to confirm the viability of the link. Fast Ethernet codes a 16-bit data packet into this signal, advertising its service capabilities. This is called a Fast Link Pulse. A node that does not support autonegotiation can be detected by one that does and sent ordinary link integrity test signals, or Normal Link Pulses. Fast Ethernet would not be deployed on new networks, but you may need to maintain it in legacy installations. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 2.1.5 Gigabit Ethernet Standards Gigabit Ethernet builds on the standards defined for Ethernet and Fast Ethernet to implement rates of 1,000 Mbps (1 Gbps). When installed using Cat 5e or better copper wire, Gigabit Ethernet is specified as 1000BASE-T. Gigabit Ethernet does not support hubs; it is implemented only using switches. The maximum distance of 100 meters (328 feet) applies to cabling between the node and a switch port, or between two switch ports. Gigabit Ethernet is the mainstream choice for new installations of access networks; that is, cabling to connect client workstations to a local network. The main decision would be whether to use copper or fiber optic cable. Fiber gives better upgrade potential in the future, while copper cable is cheaper to install and far more hosts are installed with network cards that support copper than support fiber. 10 Gigabit Ethernet (10 GbE) multiplies the nominal speed of Gigabit Ethernet by a factor of 10. Because of the higher frequencies required, 10 GbE can only run at reduced distances over unshielded copper cable. Longer runs require higher categories of copper cable with some type of shielding, or the use of fiber optic cable. There are also specifications for 40 Gbps operation. Specification Cable Maximum Distance 10GBASE-T UTP (Cat 6) 55 m (180 feet) F/UTP (Cat 6A) 100 m (328 feet) S/FTP (Cat 7) 100 m (328 feet) 40GBASE-T S/FTP (Cat 8) 30 m (100 feet) 10/40 GbE Ethernet is not deployed in many access networks, as the cost of 10/40 GbE compatible network adapters and switch transceiver modules is high. It might be used where a company's business requires very high-bandwidth data transfers, such as TV and film production. It is also widely used as backbone cabling, where it supports high-bandwidth links between switches and routers, or between appliances in a datacenter. Copyright © The Computing Technology Industry Association, Inc. All rights reserved. 2.1.6 Fiber Ethernet Standards Fiber optic cable uses infrared light signals. The light signals are also not susceptible to interference or noise from other sources and less effected by attenuation. Consequently, fiber optic cable supports higher bandwidth over longer links than copper cable. Fiber optic cabling is divided into single mode (SMF) and multimode (MMF) types, and MMF is categorized by optical mode designations (OM1, OM2, OM3, and OM4). Ethernet standards over fiber set out the use of different types of cable for 100 Mbps, 1 Gbps, 10 Gbps, and 40/100 Gbps operation. There are variants for long wavelength optics, required for long-distance transmission, and short wavelength optics. Some of the main standards for speeds up to 10 Gbps are listed in the table. Maximum Specification Optics Cable Connectors Distance 100BASE-FX 1300 nm MMF (OM1) 4 km (2.48 miles) ST, SC, MT-RJ 100BASE-SX 850 nm MMF (OM1) 300 m (984 feet) ST, SC, LC MMF (OM2) 1000BASE-SX 850 nm MMF (OM1) 275 m (902 feet) ST, SC, LC, MT-RJ MMF (OM2) 550 m (1804 feet) MMF (OM3) 1000BASE-LX 1,300 nm MMF 550 m (1,804 feet) SC, LC (OM1/OM2/OM3) 5 km (3.1 miles) 1,310 nm SMF (OS1/OS2) 10GBASE-SR 850 nm MMF (OM1) 33 m (108 feet) SC, LC MMF (OM2) 82 m (269 feet) MMF (OM3) 300 m (984 feet) MMF (OM4) 400 m (1,312 feet) 10GBASE-LR 1,310 nm SMF (OS1/OS2) 10 km (6.2 miles) SC, LC Fiber is often used for backbone cabling in office networks and for workstations with high-bandwidth requirements, such as video editing. The principal applications of 10 GbE (and better) are the following: Increasing bandwidth for server interconnections and network backbones, especially in datacenters and for storage area networks (SANs). Replacing existing switched public data networks based on proprietary technologies with simpler Ethernet switches (Metro Ethernet). Copyright © The Computing Technology Industry Association, Inc. All rights reserved.