The term Ethernet refers to the family of local- area network (LAN). A local-area network (LAN) protocol developed by XeroxCorporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology and supports data transfer rates of 10/100/1000 Mbps. Ethernet defines the lower two layers of the OSI Reference Model.
The Ethernet specification served as the basis for the IEEE 802.3 standard, which specifies the physical and lower software layers. Ethernet uses the CSMA/CD access method to handle simultaneous demands. Ethernet, like other local area networks, falls in a middle ground between long-distance, low-speed networks that carry data for hundreds or thousands of kilometres, and specialized, very high speed interconnections that are generally limited to tens of meters.
FAST ETHERNET Fast Ethernet was designed to compete with LAN protocols such as FDDI (Fiber Distributed Data Interface) or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet. It can transmit data 10 times faster at a rate of 100 Mbps.
The need for higher data rate resulted in the design of the Gigabit Ethernet (1000 Mbps). The IEEE committee calls the standard 802.3z. All configurations of gigabit Ethernet are point to point. Point-to-point, between two computers or one computer – to –switch. It supports two different modes of operation: full duplex mode and half duplex mode. Full duplex is used when computers are connected by a switch. No collision is there and so CSMA/CD is not used.
(a) point to point between two computers(b) (b) point to point between switch and computer
Half duplex is used when computers are connected by a hub. Collision in hub is possible and so CSMA/CD is required. The 802.3z committee considered a radius of 25 meters to be unacceptable and added two new features to increase the radius-Carrier Extension and Frame Bursting. Carrier Extension tells the hardware to add its own padding bits after the normal frame to extend the frame to 512 bytes. Frame Bursting allows a sender to transmit a concatenated sequence of multiple frames in a single transmission. If the total burst is less than 512 bytes, the hardware pads it again.
The heart of the system is a switch containing a high speed back-plane and room for typically 4 to 32 plug- in cards, each containing one to eight connectors. When a station wants to transmit a frame, it outputs a frame to switch. The plug-in card checks to see if the frame is for the other station on the same card. If so, it is copied there otherwise it is sent over high speed back-plane to destination station’s card.
All ports on the same card are wired together to form a local on-card LAN. Collisions on this on-card LAN are detected and handled using CSMA/CD protocol. One transmission per card is possible at any instant. All the cards can transmit in parallel. With this design each card forms its own collision domain. In other design, each input port is buffered, so incoming frames are stored in the card’s on board RAM. It allows all input ports to receive (and transmit) frame at same time
MAC begins the transmission sequence by transferring the LLC information into the MAC frame buffer. The preamble and start-of-frame delimiter are inserted in the PRE and SOF fields. The destination and source addresses are inserted into the address fields. The LLC data bytes are counted, and the number of bytes is inserted into the Length/Type field. The LLC data bytes are inserted into the Data field. If the number of LLC data bytes is less than 46, a pad is added to bring the Data field length up to 46. An FCS value is generated over the DA, SA, Length/Type, and Data fields and is appended to the end of the Data field.
After the frame is assembled, actual frame transmission will depend on MAC. There are two Media Access Control(MAC) protocols defined for Ethernet: Half-Duplex Full-Duplex Half-Duplex is the traditional form of Ethernet that uses the CSMA/CD protocol. Full-Duplex bypasses the CSMA/CD protocol Full-duplex mode allows two stations to simultaneously exchange data over a point to point link that provides independent transmit and receive paths
Refers to the transmission of data in just one direction at a time. Half-Duplex Ethernet is the traditional form of Ethernet that uses the CSMA/CD. The CSMA/CD access rules are summarized by the protocols acronym: carrier sense multiple access collision detect Half duplex Ethernet assumes that all the "normal" rules of Ethernet are in effect on the local network.
The network is monitored for presence of a transmitting station (carrier sense). After sending the jam sequence the transmitting station waits a random period of time (called “backoff”). If an active carrier is not detected then the station immediately begins transmission of the frame. While the transmitting station is sending the frame, it monitors the medium for a collision.
If a collision is detected, the transmitting station stops sending the frame data and sends a 32-bit "jam sequence. If repeated collisions occur, then transmission is repeated - the random delay is increased with each attempt Once a station successfully transmits a frame, it clears the collision counter it uses to increase the backoff time after each repeated collision.
Based on the IEEE 802.3x standard, “Full-Duplex” MAC type bypasses the CSMA/CD protocol Full-duplex mode allows two stations to simultaneously exchange data over a point to point link The aggregate throughput of the link is effectively doubled – A full-Duplex 100 Mb/s station provides 200 Mb/s of bandwidth
Full-Duplex operation is supported by: – 10-Base-T, 10Base-FL, 100Base-TX, 100Base- FX, 100Base- T2, 1000Base-CX, 1000Base-SX, 1000Base-LS, and 1000Base-T. Full-Duplex operation is NOT supported by: – 10Base5, 10Base2, 10Base-FP, 10Base-FB, and 100Base- T4. Full-Duplex operation is restricted to point to point links connecting exactly two stations
Full-duplex MACs must have separate frame buffers and data paths to allow for simultaneous frame transmission and reception. The destination address of the received frame is checked to determine whether the frame is destined for that station If an address match is found the frame length is checked and the received FCS is compared to the FCS that was generated during frame reception. If the frame length is okay and there is an FCS match, the frame type is determined by the contents of the Length/Type field. The frame is then parsed and forwarded to the appropriate upper layer.
ISO data link layer is divided into two IEEE 802 sublayers Media Access Control (MAC) sublayer MAC-client sublayer. IEEE 802.3 physical layer same as ISO physical layer. The MAC-client sublayer may be Logical Link Control (LLC) if the unit is a DTE Bridge entity, if the unit is a DCE Lan to LAN if Ethernet to etnernet Ethernet to token ring
The Ethernet MAC Sublayer has two primary responsibilities: 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
Token ring local area network (LAN) technology is a protocol which resides at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that travels around the ring. Token-possession grants the possessor permission to transmit on the medium. Token ring frames travel completely around the loop. it was eventually standardized with protocol IEEE 802.5. Stations on a token ring LAN are logically organized in a ring topology .
In token ring , Data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access. Token passing mechanism is shared by ARCNET, token bus, and FDDI ,and has theoretical advantages over the stochastic CSMA/CD of Ethernet. A token ring network is wired as a star, with hubs and arms out to each station and the loop going out-and- back through each. Cabling is generally "Type-1" shielded twisted pair, with unique hermaphroditic connectors, commonly referred to as IBM data connectors in formal writing or colloquially as Boy George connectors
In 1985, token ring ran at 4 Mbit/s. In 1989 IBM introduced the first 16 Mbit/s token ring products and the 802.5 standard was extended to support this. In 1981, Apollo Computer introduced their proprietary 12 Mbit/s Apollo token ring (ATR). Proteon introduced their 10 Mbit/s ProNet-10 token ring network in 1984.
A data token ring frame is used by stations to transmit media access control(MAC) management frames or data frames from upper layer protocols and applications. Token Ring support two basic frame types: tokens and data/command frames. Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. Data/command frames vary in size, depending on the size of the Information field
PDU from SD AC SD AC FC DA SA LLC (IEEE CRC ED FS 802.2) up to8 8 8 8 8 48 48 32 8 18200x8 8 bitsbits bits bits bits bits bits bits bits bits bits
Starting Delimiter(SD) consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. J and K are code violations. Access Control(AC) this byte field consists of the following bits from most significant to least significant bit order: P,P,P,T,M,R,R,R. The P bits are priority bits, T is the token bit which when set specifies that this is a token frame, M is the monitor bit which is set by the Active Monitor (AM) station when it sees this frame, and R bits are reserved bits.
Frame Control(FC) a one byte field that contains bits describing the data portion of the frame contents which indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information. + Bits 0–1 Bits 2–7 0 Frame type Control BitsFrame type – 01 indicates LLC frame IEEE 802.2 (data)and ignore control bits; 00 indicates MAC frame andcontrol bits indicate the type of MAC control frame
Destination address (DA) a six byte field used to specify the destination(s) physical address . Source address(SA) Contains physical address of sending station . It is six byte field that is either the local assigned address (LAA) or universally assigned address (UAA) of the sending station adapter. Data a variable length field of 0 or more bytes, the maximum allowable size depending on ring speed containing MAC management data or upper layer information.Maximum length of 4500 bytes. Frame Check Sequence a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver.
Ending Delimiter(ED) The counterpart to the starting delimiter, this field marks the end of the frame and consists of the following bits from most significant to least significant: J,K,1,J,K,1,I,E. I is the intermediate frame bit and E is the error bit. Frame Status(FS)a one byte field used as a primitive acknowledgement scheme on whether the frame was recognized and copied by its intended receiver. A C 0 0 A C 0 0 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit A = 1, Address recognized C = 1, Frame copied
TOKEN FRAME Start Delimiter Access Control End Delimiter 8 bits 8 bits 8 bits Abort Frame SD ED 8 bits 8 bits Used to abort transmission by the sending station
Empty information frames are continuously circulated on the ring. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame. The frame is then examined by each successive workstation. The workstation that identifies itself to be the destination for the message copies it from the frame and changes the token back to 0.
When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.