IEEE STANDARDSIEEE STANDARDS

In 1985, the Computer Society of the IEEE started a In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable project, called Project 802, to set standards to enable intercommunication among equipment from a variety of intercommunication among equipment from a variety of manufacturers. Project 802 is a way of specifying manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of functions of the physical layer and the data link layer of major LAN protocols.major LAN protocols.The original Ethernet was created in 1976 at Xerox’s The original Ethernet was created in 1976 at Xerox’s Palo Alto Research Center (PARC). Since then, it has Palo Alto Research Center (PARC). Since then, it has gone through four generations. We briefly discuss the gone through four generations. We briefly discuss the Standard (or traditional) EthernetStandard (or traditional) Ethernet in this section. in this section.

13.2

IEEE standard for LANs

The IEEE subdivided the data link layer into two sub layers: logical link controlMedium access control. IEEE has also created several physical layer standards for Different LAN protocols.

Data link layer

Logical Link Control:Flow control , error control and part of the framing duties are collected into one Sub layer called LLC. Framing is handled both the LLC sub layer and MAC Sub layer.LLC provides one single data link control protocol for all IEEE LANs, but MAC Provides different protocols for different LANs.Need for LLC:The purpose of the LLC is to provide flow and error control for upper layer Protocol.MAC:It defines CSMA/CD as the media access method for Ethernet LANs and token Passing method for Token Ring and Token Bus LANS. Framing function is also handled by the MAC layer.

13.4

HDLC frame compared with LLC and MAC frames

Ethernet evolution through four generations

802.3 MAC frame

Preamble: The first field of the 802.3 frame contains 7 bytes (56 bits) of alternatingOs and Is that alerts the receiving system to the coming frame .Start frame delimiter (SFD): The second field (l byte: 10101011) signals thebeginning of the frame. The SFD warns the station or stations that this is the lastchance for synchronization. The last 2 bits is 11 and alerts the receiver that the nextfield is the destination address.Destination address (DA):The DA field is 6 bytes and contains the physicaladdress of the destination station or stations to receive the packet.Source address (SA): The SA field is also 6 bytes and contains the physicaladdress of the sender of the packet.Length or type: This field is defined as a type field or length field. The originalEthernet used this field as the type field to define the upper-layer protocol using theMAC frame. The IEEE standard used it as the length field to define the number ofbytes in the data field. Both uses are common today.Data:This field carries data encapsulated from the upper-layer protocols. It is aminimum of 46 and a maximum of 1500 bytes.CRC:The last field contains error detection information

Minimum and maximum lengths

An Ethernet frame needs to have a minimum length of 512 bits or 64 bytes.Part of this length is the header and the trailer. If we count 18 bytes of headerTrailer (6 bytes of source address, 6 bytes of destination address , 2 bytesOf length and type , and 4 bytes of CRC), then the minimum length of dataFrom the upper layer is 64 – 18 = 46 bytes. If the upper layer packet isLess than 46 bytes, padding is added to make up the difference.The maximum length of a frame is 1518 bytes. If we subtract the 18 bytesOf header and trailer, the maximum length of the payload is 1500 bytes.The maximum length restriction has two historical reasons. First, memory Was very expensive when Ethernet was designed: a maximum lengthRestriction helped to reduce the size of the buffer. Second the maximum length restriction prevents one station from monopolizing the shared Medium, blocking other stations that have data to send.

Frame length:Minimum: 64 bytes (512 bits)

Maximum: 1518 bytes (12,144 bits)

Note

Example of an Ethernet address in hexadecimal notation

Unicast and multicast addresses

The least significant bit of the first byte defines the type of address.

If the bit is 0, the address is unicast;otherwise, it is multicast.

Note

The broadcast destination address is a special case of the multicast address in which all bits are 1s.

Note

Access Method: CSMA/CDStandard Ethernet uses I-persistent CSMA/CD

Slot time and collisionThe choice of a 512 bit slot time was not accidental. It was chosen to allow the proper functioning of CSMA/CD.we assume that the sender sends a minimum- size packet of 512 bits. Before the sender can send the entire packet out, the signal travelsThrough the network and reaches the end of the network. If there is anotherSignal at the end of the network, a collision occurs. The sender has the Opportunity to abort the sending of the frame and to send a jam sequence toInform other stations of the collision. The round – trip time plus the time requiredTo send the jam sequence should be less than the time needed for the senderTo send the minimum frame, 512 bits. The sender needs to be aware of the collision before it is too late, that is, before it has sent the entire frame.

Slot time and maximum network length

There is a relationship between the slot time and maximum length of the Network ( collision domain). It is dependent on the propagation speed of theSignal in the particular medium. In most transmission media, the signal propagates at 2 X 108 m/s. For traditional Ethernet, MaxLength = propagation speed X slot time/2 = 2 X 108 X 51.2 X 106 /2 = 5120 mWe need to consider the delay times in repeaters and interfaces, and the time Required to send the jam sequence. These reduce the maximum – length of a traditional Ethernet network to 2500 m, just 48 % of the theoretical calculation.

Categories of traditional Ethernet

•<data rate><Signaling method><Max segment length or cable type>

IEEE 802.3 Cable TypesName Cable Max. Max Cable

Segment Length

Nodes /segment

10Base5 thick coax 500 meters 100

10Base2 thin coax 185 meters 30

10BaseT twisted pair 100 meters 1

10BaseF Fiber Optic 2Km 1

Toplogy

Bus

Bus

Star

Star

Networks: Ethernet 20

Ethernet Evolution10BASE5 {1983}

• 10 Mbps• 500 meter segment length• Signal-regenerating repeaters• Thick Coax

– Advantages: Low attenuation, excellent noise immunity, superior mechanical strength

– Disadvantages: Bulky, transceiver(responsible for transmitting, receiving , and detecting collisions) boxes too expensive

* Wiring represented a significant part of total installed cost.

10Base5 implementation

Networks: Ethernet 22

10BASE2 Cheapernet {1985}• 10 Mbps• 185 meter segment length• Signal-regenerating repeaters• Transceiver was integrated onto the adapter• Thin Coax (coax thinner and lighter)

– Advantages: Easier to install, reduced hardware cost, BNC connectors widely deployed lower installation costs.

– Disadvantages: Attenuation not as good, could not support as many stations due to signal reflection caused by BNC Tee Connector.

Ethernet Evolution

10Base2 implementation

Networks: Ethernet 24

(a)

(b)

transceivers

Figure 6.55

Thick Ethernet Cable

Thin Ethernet Cable

Networks: Ethernet 25

10BASET {1990} **Most popular• 10 Mbps• 100 meter segment length• Signal-regenerating repeaters• Transceiver integrated onto adapter• Two pairs of UTP• Hub-and-spoke topology {Hub in the closet}

– Advantages: could be done without pulling new wires. Each hub amplifies and restores incoming signal.

Ethernet Evolution

10Base-T implementation

10Base-F implementation

CHANGES IN THE STANDARDCHANGES IN THE STANDARD

The 10-Mbps Standard Ethernet has gone through The 10-Mbps Standard Ethernet has gone through several changes before moving to the higher data several changes before moving to the higher data rates. These changes actually opened the road to the rates. These changes actually opened the road to the evolution of the Ethernet to become compatible with evolution of the Ethernet to become compatible with other high-data-rate LANs. other high-data-rate LANs.

Bridged EthernetSwitched EthernetFull-Duplex Ethernet

A network with and without a bridge

Collision domains in an unbridged network and a bridged network

Switched Ethernet

Full-duplex switched Ethernet

FAST ETHERNETFAST ETHERNET

Fast Ethernet was designed to compete with LAN Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. rate of 100 Mbps.

Fast Ethernet implementations

GIGABIT ETHERNETGIGABIT ETHERNET

The need for an even higher data rate resulted in the The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the standard 802.3z.The IEEE committee calls the standard 802.3z.

Gigabit Ethernet implementations