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Module 6Ethernet

Fundamentals

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Why is Ethernet so Successful?

• In 1973, it could carry data at 3 Mbps

• Now, it can carry data at 10 Gbps

• It is also:– Simple & has low maintenance– Can incorporate new technologies– Is reliable– Installation and upgrade is relatively inexpensive

• Bandwidth can be increased without changing the underlying technology (scalable)

• Still uses 802.3 standard

• Uses multiple user access on a shared medium (first developed by the University of Hawaii and called “Alohanet”

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What’s in a Name?

Ethernet naming rules:

10 Base T

Transmission Rate

Baseband signaling

Copper unshielded twisted pair

Uses the entire bandwidth of the transmission medium

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Layer 1 & Layer 2

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Ethernet Standards

Specifies maximum segment length

Specifies maximum number of stations per segment

Specifies maximum number of repeaters between segments

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Standard Maximum Cable Length in Meters

10Base5 500

10Base2 185

10BaseT 100

100Base-TX 100

100Base-FX 400

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The MAC Address• 48 bits in length• 12 hexadecimal digits• Burned into ROM on the NIC

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Layer 2 Framing

• Generic frames would have:– Start Frame field– Address field– Length/type field– Data field– Frame Check sequence field

• All frames have:– Something to indicate the beginning of a frame (a

beginning sequence of bytes)– A source and destination MAC address in the

address field

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Frame Structure(All speeds of Ethernet have almost the same frame structure)

Or DIX versions

If Type field is

> 0x600 – Ethernet II

Frame Fields

Timing synchronization for slower Ethernet

Timing info finished, frame now begins

MAC – unicast, multicast, broadcast

MAC – from the source

< 0x600, then value is length – LLC provides protocol

> 0x600, then type and contents of the Data field are decoded per the protocol indicated.

Data bytes added to the rest of the frame must not exceed 1518 bytes

Checks for damaged frames

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Media Access Control

• 2 categories– Deterministic (taking turns)

• Ex. Token Ring & FDDI• CSMA/CA• No collisions

– Non-deterministic (first-come, first-served)• Ex. Ethernet• CSMA/CD• Collisions because of shared media

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Collision detection

CSMA/CD functions:

1.Transmits and receives data packets

2. Decodes data packets for valid addresses before passing to upper OSI layers

3. Detects errors in packets or in the network

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Collision detection flowchartA network device knows a collision has taken place because the amplitude of the signal on the media increases.

After a collision, each node again has an equal chance to transmit – no priority given.

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Ethernet Timing• There is a natural delay in the forwarding of frames• If a station does not detect any transmission on the media, it will

begin its transmission – but there is always a delay simply because it takes some time for the signal to travel

• Not a problem with full duplex as there are different receiving and sending channels

• The time it takes to transmit the frame must be long enough that the transmitting station will not finish transmitting before detecting any collision that might possibly occur to its frame.

• This duration is called a slot time and depends on the minimum frame size, the link bandwidth, and the link length.

• If the station detects a collision, then the station continues transmitting until the total time of its transmission is at least one slot time (to ensure that all other transmitting stations detect the collision).

• http://www.wildpackets.com/compendium/EN/EN-Ifgap.html

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Interframe Spacing

• The minimum interval, in bit-times, that a station has to wait before sending another frame

• Allows slower stations to process the frame and “get ready” for the next frame

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Sending the Frame

• If the MAC layer cannot send a frame in 16 attempts, it might be because of:– A physical problem on the network

– Very heavy traffic loads

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Collisions and Errors

• Collisions will happen

• Most happen early in the transmission,

• Higher layers do not know about the collision when it happens before the start of frame delimiter

• If many “runts” are present, then that would indicate many collisions on the network and throughput would be reduced.

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3 types of collisionsDetected late in the transmission – NIC does not know about – upper layer protocols have to handle

Less than 64 bytes, CRC is garbled (will be on the local segment)

Same as local, except filtered over from another segment (the other side of the repeater)

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Module 6Ethernet

Fundamentals