1 Version 3.0 Module 6 Ethernet Fundamentals

2 Version 3.0 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 standard Uses multiple user access on a shared medium (first developed by the University of Hawaii and called “Alohanet”

3 Version 3.0 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

4 Version 3.0 Layer 1 & Layer 2

5 Version 3.0 Ethernet Standards Specifies maximum segment length Specifies maximum number of stations per segment Specifies maximum number of repeaters between segments

6 Version 3.0 StandardMaximum Cable Length in Meters 10Base Base BaseT Base-TX Base-FX400

7 Version 3.0 The MAC Address 48 bits in length 12 hexadecimal digits Burned into ROM on the NIC

8 Version 3.0 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

9 Version 3.0 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 synchron ization 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

11 Version 3.0 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

12 Version 3.0 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

13 Version 3.0 Collision detection flowchart A 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.

14 Version 3.0 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).

15 Version 3.0 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

16 Version 3.0 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

17 Version 3.0 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.

18 Version types of collisions Detected 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)

19 Version 3.0 Module 6 Ethernet Fundamentals