EthernetII ( Ethernet Type 2 )

• Carrier-sense multiple-access with collision detection (CSMA/CD) for controlling access to the network media.


• Use base band broadcasts.


• A method for packing data into data packets called frames.


• Transmit at 10Mbps, 100Mbps, and 1Gbps.

• The physical medium used to carry Ethernet signals between computers.


• A set of medium access control rules embedded in each Ethernet interface that allow multiple computers to fairly arbitrate access to the shared Ethernet channel.


• An Ethernet frame that consists of a standardized set of bits used to carry data over the system.

• Data encapsulation, including frame assembly before transmission, and frame parsing/error detection during and after reception.



• Media access control, including initiation of frame transmission and recovery from transmission failure.

• Logical Link Control (LLC), which provides the interface between the Ethernet MAC and the upper layers in the protocol stack of the end station. The LLC sublayer is defined by IEEE 802.2 standards.



• Bridge entity, which provides LAN-to-LAN interfaces between LANs that use the same protocol (for example, Ethernet to Ethernet) and also between different protocols (for example, Ethernet to Token Ring). Bridge entities are defined by IEEE 802.1 standards.

• MAC Operation

Using the CSMA/CD algorithm for Medium Access Control a node will perform the following steps :

• Checks whether the medium is idle or not.


• If the medium is busy, continues to check until it is idle.


• If the medium is idle, begin transmission and monitor for a collision.


• If no collision has occurred during the Collision Window, transmit the rest of the data.


• If a collision occurs, node transmits a Jamming Signal to all other nodes and backs-off using the exponential back-off algorithm.



• CSMA and Persistence

Carrier Sense Multiple Access is similar to ALOHA, having the additional ability that a node may detect whether the medium is currently in use. If a node detects that the medium is currently idle, it will begin transmission. CSMA does not entirely remove the chance of collisions occurring. Should two or more nodes detect that the medium is idle and then attempt to send data at roughly the same time, a collision will occur.

When using 1-persistent CSMA, upon detecting that the medium is busy, a node will continue to listen until the medium becomes free again, and then transmit. Should a collision occur, the node will wait a random time interval before attempting again. If two nodes are waiting for the medium to become free at the same time, then they will both transmit at the same time once it becomes free, causing a collision. This makes 1-persistent CSMA an inefficient solution for high-traffic networks.

With Nonpersistent CSMA a node will wait for a random time interval instead of transmitting immediately. This reduces the chance of a collision between two nodes waiting for use of the medium at the same time, but may result in inefficient use of a low-traffic network if a node is waiting random time intervals when the medium is free.

An attempt to gain the most efficiency out of both high and low-traffic networks is p-persistant CSMA. With this strategy, upon detecting that medium is busy, a node will wait for the maximum propagation delay of the network before trying again. If the medium is idle, the node will either send the data with probability p, or wait for the same time interval again, with probability 1-p. This process is repeated until the data is sent, or the medium becomes busy, in which case the node will back-off for a random time interval before starting again.


• Collision Detection

Collision Detection (CD) is an alteration to CSMA that gives a node the ability to detect a collision the moment one occurs. The advantage of this is that transmission can then be ceased immediately instead of completing the transmission of the current frame. This technique saves the bandwidth that is wasted on proceeding with the transmission of frames after collisions occur.

Collision Detection can be achieved at a node by comparing the transmitted data, to the data detected on the medium. If the two do not match, a collision has occured, and transmission can be aborted. A second method is to monitor the medium for amplitudes greater than those used in normal transmission. This should only occur when two signals superimpose each other, and cause higher or lower amplitudes than is normally expected.

When a collision is detected by a node it will transmit a Jamming Signal to all other nodes indicating that a collision has occured. The node will then back-off for a random interval of time. This technique ensures that nodes are aware of collisions promptly, and can act accordingly.

• 10 Mbps - 10Base-T Ethernet (IEEE 802.3)


• 10Base5 and 10Base2


• 10Base5
- Uses Thicknet coaxial cable which requires a transceiver with a vampire tap to connect each computer. There is a drop cable from the transceiver to the Attachment Unit Interface (AIU). The AIU may be a DIX port on the network card. There is a transceiver for each network card on the network. This type of ethernet is subject to the 5-4-3 rule meaning there can be 5 network segments with 4 repeaters, and three of the segments can be connected to computers. It uses bus topology. Maximum segment length is 500 Meters with the maximum overall length at 2500 meters. Minimum length between nodes is 2.5 meters. Maximum nodes per segment is 100.


• 10Base2
- Uses Thinnet coaxial cable. Uses a BNC connector and bus topology requiring a terminator at each end of the cable. The cable used is RG-58A/U or RG-58C/U with an impedance of 50 ohms. RG-58U is not acceptable. Uses the 5-4-3 rule meaning there can be 5 network segments with 4 repeaters, and three of the segments can be connected to computers. The maximum length of one segment is 185 meters. Barrel connectors can be used to link smaller pieces of cable on each segment, but each barrel connector reduces signal quality. Minimum length between nodes is 0.5 meters.



• 10BaseT and Hubs


• 10BaseT
- Uses Unshielded twisted pair (UTP) cable. Uses star topology. Shielded twisted pair (STP) is not part of the 10BaseT specification. Not subject to the 5-4-3 rule. They can use category 3, 4, or 5 cable, but perform best with category 5 cable. Category 3 is the minimum. Require only 2 pairs of wire. Cables in ceilings and walls must be plenum rated. Maximum segment length is 100 meters. Minimum length between nodes is 2.5 meters. Maximum number of connected segments is 1024. Maximum number of nodes per segment is 1 (star topology). Uses RJ-45 connectors.


• 10BaseF
- Uses Fiber Optic cable. Can have up to 1024 network nodes. Maximum segment length is 2000 meters. Uses specialized connectors for fiber optic. Includes three categories:
10BaseFL - Used to link computers in a LAN environment, which is not commonly done due to high cost.
10BaseFP - Used to link computers with passive hubs to get cable distances up to 500 meters.
10BaseFB - Used as a backbone between hubs.



• Switches and Full Duplex Operation

A hub is mainly used to expand the number of nodes that can be attached to a LAN. A switch however is used to partition a LAN into separate networks. Switches are used to segment heavy traffic load on a network.
Since 10BaseT has separate channels for transmission and reception, Full-Duplex Operation is supported. For this to work both the node and the hub must be configured for full-duplex operation. Since full-duplex operation is possible using 10BaseT, Ethernet Switches can carry data in both directions simultaneously, which doubles the normal transmission rate of 10Mb/s to 20Mb/s.


• Bridges

Bridges provide the ability to join more than one LAN together. This could always be performed using Internet routers, but this generally forces the use of a particular protocol since the network layer tends to dictate the rest of the stack. Bridges allow two LANs to be connected together by converting from one MAC protocol into another, and vice-versa. This differs from a switch which does not perform any conversion, and could thereore not join two LANs using different MAC protocols.


• Broadcast and Collision Domains

The Broadcast Domain represents the domain of nodes which be addressed in a (possible switched) LAN. Separate LANs will have separate broadcast domains, i.e. a different IP subnet. The Collision Domain of a network is the domain in which a collision will occur if two nodes attempt to transmit data at the same time. A LAN comprising a single segment, or multiple segments connected by repeaters will function as a single collision domain. A switching hub, router or bridge will not forward a collision signal, and will therefore separate the collision domain.



• 100 Mbps - Fast Ethernet (IEEE 802.3u)

When investigating possible techniques for achieving a data rate of 100 Mb/s, a series of challenges arose which would have to be overcome were this goal to be reached. First of all, the maximum propagation delay of the network had to be minimized. To do this the maximum segment length had to be reduced to around 100 meters. Since multidrop links tend to be harder to engineer than point-to-point links, 100 Mb/s required the use of hubs to create a start topology LAN, instead of the bus topology used in 10 Mb/s Ethernet. The star topology maximizes the use of the maximum cable segment length. Reducing network size also assures that the minimum frame size remains the same. This means that the collision window remains the same since in standard Ethernet the frame size is chosen to ensure that collisions are detected before transmission has completed.

Increasing the Ethernet transmission rate by a factor of ten over 10Base-T was not a simple task, and the effort resulted in the development of three separate physical layer standards for 100 Mbps over UTP cable: 100Base-TX and 100Base-T4 in 1995, and 100Base-T2 in 1997. Each was defined with different encoding requirements and a different set of media-dependent sublayers, even though there is some overlap in the link cabling. Table 7-2 compares the physical layer characteristics of 10Base-T to the various 100Base versions.


• 100BaseT
- Also known as fast ethernet. Uses RJ-45 connectors. Topology is star. Uses CSMA/CD media access. Minimum length between nodes is 2.5 meters. Maximum number of connected segments is 1024. Maximum number of nodes per segment is 1 (star topology). IEEE802.3 specification.


Ethernet Version	Transmit Symbol Rate	Encoding	Cabling	Full-Duplex Operation
10Base-T	10 MBd	Manchester	Two pairs of UTP Category -3 or better	Supported
100Base-TX	125 MBd	4B/5B	Two pairs of UTP Category -5 or Type 1 STP	Supported
100Base-T4	33 MBd	8B/6T	Four pairs of UTP Category -3 or better	Not supported
100Base-T2	25 MBd	PAM5x5	Two pairs of UTP Category -3 or better	Supported




• 100BaseTX
- supports a 100 Mb/s transmission rate over two pairs twisted pair cabling. 100BaseTX uses one pair of wires for receiving data, and the other for transmission. 100BaseTX supports transmission over up to 100 meters of Category 5 (CAT5) Unshielded Twisted Pair (UTP) cabling. CAT5 cable is of better quality than CAT3, and supports transmission up to 100Mhz, whereas CAT3 only offers 16Mhz. 100BaseTX supports full-duplex operation.


• 100BaseT4
- uses four pairs of CAT3 or better twisted pair cabling. This allows 100 Mb/s transmission rates over cheaper CAT3 cabling as opposed to the CAT5 cabling required by 100BaseTX. One of the four pairs of wiring is used for transmission, another is used to receive data, and the other two bi-directional pairs are used for either transmission or to receive data.



• 100BaseFX
- is basically a fiber optic cabling version of 100BaseTX. It supports a 100 Mb/s transmission rate over two fiber optic cables. With a maximum segment length of 412 meters for half-duplex links, and 2000 meters or more for full-duplex links. The fiber optic cable used with 100BaseFX is Multi-Mode Fiber (MMF). The outer cladding is 125 microns in diameter, and the core of the cable is 62.5 microns in diameter. This is also known as 62.5/125. Like 100BaseTX, 100BaseFX supports full-duplex operation. When this mode is used, the maximum segment length increases to 2000 meters or more.


• 100VG-AnyLAN
- Requires category 3 cable with 4 pair. Maximum distance is 100 meters with cat 3 or 4 cable. Can reach 150 meters with cat 5 cable. Can use fiber optic to transmit up to 2000 meters. This ethernet type supports transmission of Token-Ring network packets in addition to ethernet packets. IEEE 802.12 specification. Uses demand-priority media access control. The topology is star. It uses a series of interlinked cascading hubs. Uses RJ-45 connectors.

• 1000 Mbps - Gigabit Ethernet (IEEE 802.3z)

As modern technologies make greater use of multimedia, demand for high bandwidth networks continues to rise. This demand has been met with the introduction of the Gigabit Ethernet. Gigabit Ethernet runs at 1000 Mb/s, but still maintains the CSMA/CD protocol and frame format of Ethernet, making it compatible with older standards.


• 1000BaseX
refers to the collective family comprising 1000BaseLX, 1000BaseSX, and 1000BaseCX which are part of the Gigabit Ethernet standard. Each of these standards are based on specifications from the ANSI X3.230-1994 Standard for Fiber Channel. 1000BaseT, an additional standard which is currently being defined, will not use a Fiber Channel and is not part of the 1000BaseX family. All these standards operate at 1000 Mb/s second, or 2000 Mb/s if used in full-duplex mode.
1000BaseX standard defines a Gigabit Media Independent Interface (GMII) that attaches the MAC physical layer functions of a Gigabit Ethernet device.


• 1000BaseLX
standard uses long wavelength lasers for data transmission over fiber optic cable. These lasers operate within a wavelength of 1270-1355 nanometers. Both single mode and multi-mode optical fibers are supported. The "L" in the name comes the long wavelength lasers that are used.


• 1000BaseSX
uses short wavelength lasers that operate within a wavelength range of 770-860 nanometers. The motivation for 1000BaseSX is that short wavelength lasers are less expensive than long wavelength lasers.


• 1000BaseCX
uses shielded balanced copper jumper cables also known as Twin ax. The "C" in the name comes from the copper in the jumper cables. 1000BaseCX only supports a maximum segment length of 25 meters which makes the use of 1000BaseCX restricted to very small areas.


• 1000BaseT
standard was released in June 1999, defined by IEEE 802.3ab. It supports Gigabit Ethernet over 100 meters of CAT5 balanced copper cabling. It achieves a data rate of 1000 Mb/s is by transmitting data at a rate of 250 Mb/s over four CAT5 wire pairs. Full-duplex operation is possible by the use of Hybrids and Cancellers by allowing symbols to be sent and received on the same wire pairs simultaneously.

• 10-Gigabit - 10 Gbps Ethernet (IEEE 802.3ae)

Despite the high level of bandwidth provided by Gigabit Ethernet, business continues to make increasing demands on the speed of network systems available. On June 17, 2002 the IEEE 802.3ae specification for 10 Gigabit Ethernet was approved as an IEEE standard by the IEEE Standards Association (IEEE-SA) Standards Board. 10 Gigabit Ethernet includes a long haul (40+ km) optical transceiver or interface for single mode fiber that can be used with either the LAN physical layer or wide area network for building Metropolitan Area Networks (MANs). It also features an option for WANs to allows 10 Gigabit Ethernet to be transported across existing SONET (synchronous optical network) OC-192c or SDH (synchronous digital hierarchy) VC-4-64c infrastructures.

• Preamble (PRE)

7 bytes. PRE is an alternating pattern of '1's and '0's that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.


• Start-of-frame delimiter (SFD)

1 byte. SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.


• Destination address (DA)

6 bytes. DA field identifies which station(s) should receive the frame. The left-most bit in the DA field indicates whether the address is an individual address (indicated by a 0) or a group address (indicated by a 1). The second bit from the left indicates whether the DA is globally administered (indicated by a 0) or locally administered (indicated by a 1). The remaining 46 bits are a uniquely assigned value that identifies a single station, a defined group of stations, or all stations on the network.


• Source addresses (SA)

6 bytes. The SA field identifies the sending station. The SA is always an individual address and the left-most bit in the SA field is always 0.


• Length/Type

2 bytes. This field indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format. If the Length/Type field value is less than or equal to 1500, the number of LLC bytes in the Data field is equal to the Length/Type field value. If the Length/Type field value is greater than 1536, the frame is an optional type frame, and the Length/Type field value identifies the particular type of frame being sent or received.


• Data

Data field holds the actual data which the frame is transmitting. This data can be of a maximum size of 1500 bytes. If the contents of this field fall below 46 bytes then the Pad field must be used to bring the frame up to the minimum required size of 64 bytes.


• Frame check sequence (FCS)

4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.


• MAC Frame with Gigabit Ethernet Carrier Extension (IEEE 803.3z)

1000Base-X has a minimum frame size of 416bytes, and 1000Base-T has a minimum frame size of 520bytes. The Extension is a non-data variable extension field to frames that are shorter than the minimum length.

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