1. Semester 1 v 3.0 1 CCNA 1 Module 6: Ethernet Fundamentals

2. Semester 1 v 3.0 2 Ethernet Ethernet is the most widely used local area network (LAN) technology. Ethernet was designed to fill the middle ground between long- distance, low-speed networks and specialized, computer-room networks carrying data at high speeds for very limited distances. Ethernet network architecture has its origins in the 1960s at the University of Hawaii, where the access method that is used by Ethernet, carrier sense multiple access/collision detection (CSMA/CD), was developed.

3. Semester 1 v 3.0 3 Ethernet Xerox Corporation's Palo Alto Research Center (PARC) developed Ethernet in the 1970s. Ethernet was the technological basis for the IEEE 802.3 specification, which was initially released in 1980 Digital Equipment Corporation, Intel Corporation, and Xerox Corporation jointly developed and released an Ethernet specification, Version 2.0, that was substantially compatible with IEEE 802.3. Ethernet and IEEE 802.3 currently maintain the greatest market share of any LAN protocol. Ethernet and IEEE 802.3 both are CSMA/CD LANs. Ethernet is a connectionless network architecture, it is referred to as a best-effort delivery system.

4. Semester 1 v 3.0 4 IEEE Ethernet Naming Rules Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u.

5. Semester 1 v 3.0 5 Ethernet and OSI Model Ethernet operates in the lower half of the data link layer, known as the MAC sublayer and the physical layer. Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various topologies.

6. Semester 1 v 3.0 6 Ethernet and OSI Model The MAC sublayer is concerned with the physical components that will be used to communicate the information. The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process.

7. Semester 1 v 3.0 7 MAC Address Format MAC addresses are 48 bits in length and are expressed as twelve hexadecimal digits. Hex is a shorthand method for representing the 8-bit bytes that are stored in the computer system. It was chosen to represent identifiers because it can easily represent the 8-bit byte by using only two hexadecimal symbols. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. MAC addresses are sometimes referred to as burned-in addresses (BIAs)

8. Semester 1 v 3.0 8 Framing Once you have a way to name computers, you can move on to framing, which is the next step. Framing is the Layer 2 encapsulation process; a frame is the Layer 2 protocol data unit (PDU). It is an encapsulated packet. When computers are connected to a physical medium, there must be some way they can grab the attention of other computers to broadcast the message "Here comes a frame!" This is done through a signaling sequence of bytes at the beginning of the frame.

9. Semester 1 v 3.0 9 Dealing With Frame Errors All frames contain naming information, such as the name of the source computer (in the form of its MAC address) and the name of the destination computer (in the form of its MAC address). Padding bytes may be added so frames have a minimum length for timing purposes. All frames, and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. One method only discards the bad frames and retransmits them. The frame check sequence (FCS) is a number, derived from the data in the frame, which helps the destination computer determine whether it received the frame correctly, without any noise-corrupted bits, bytes, or fields. (Three primary ways to calculate the Frame Check Sequence number, namely Cyclic Redundancy Check (CRC), two-dimensional parity and internet checksum).

10. Semester 1 v 3.0 10 Ethernet Frame Structure

11. Semester 1 v 3.0 11 Ethernet Frame Fields Ethernet requires that the frame be not less than 46 octets or more than 1518 octets.

12. Semester 1 v 3.0 12 Media Access Control (MAC) Media Access Control (MAC) refers to protocols that determine which computer on a shared-medium environment (collision domain) is allowed to transmit the data. MAC and LLC are both sublayers of Layer 2. There are two broad categories of Media Access Control. The two categories are: –deterministic (taking turns) e.g. Token Ring, FDDI – non-deterministic (first come, first served), e.g. Ethernet Ethernet – logical bus topology (information flow is on a linear bus) and physical star or extended star (wired as a star) Token Ring – logical ring topology (in other words, information flow is controlled in a ring) and a physical star topology (in other words, it is wired as a star) FDDI – logical ring topology (information flow is controlled in a ring) and physical dual-ring topology (wired as a dual-ring)

13. Semester 1 v 3.0 13 MAC rules and collision detection/backoff Ethernet is a shared-media broadcast technology Carrier Sense Multiple Access/Collision Detect (CSMA/CD) is used in Ethernet Before sending data, CSMA/CD stations listen to the network to determine if it is already in use. If it is, then they wait. If the network is not in use, the stations transmit. A collision occurs when two stations listen for network traffic, hear none, and transmit simultaneously. In this case, both transmissions are damaged, and the stations must retransmit at some later time. Backoff algorithms determine when the colliding stations can retransmit. CSMA/CD stations can detect collisions, so they know when they must retransmit.

14. Semester 1 v 3.0 14 Carrier Sense Multiple Access Collision Detect (CSMA/CD)

15. Semester 1 v 3.0 15 Carrier Sense Multiple Access Collision Detect (CSMA/CD)

16. Semester 1 v 3.0 16 Ethernet Timing Slot time is the amount of time a device waits after a collision before transmission. Slot time for 10 and 100 Mbps is 512 bit-times. Slot time for 1000- Mbps is 4096 bit-times As a rough estimate, 20.3 cm (8 in) per nanosecond is often used for calculating propagation delay down a UTP cable. For 100 meters of UTP, this means that it takes just under 5 bit-times for a 10BASE-T signal to travel the length the cable

17. Semester 1 v 3.0 17 Ethernet Timing For CSMA/CD Ethernet to operate, the sending station must become aware of a collision before it has completed transmission of a minimum-sized frame. At 100 Mbps the system timing is barely able to accommodate 100 meter cables. At 1000 Mbps special adjustments are required as nearly an entire minimum-sized frame would be transmitted before the first bit reached the end of the first 100 meters of UTP cable. For this reason half duplex is not permitted in 10-Gigabit Ethernet In Full duplex, the station may send and receive simultaneously and collisions should not occur

18. Semester 1 v 3.0 18 Interframe spacing & Backoff The minimum spacing between two non-colliding frames is also called the interframe spacing. This is measured from the last bit of the FCS field of the first frame to the first bit of the preamble of the second frame. All stations on a 10-Mbps Ethernet are required to wait a minimum of 96 bit-times (9.6 microseconds) before any station may legally transmit the next frame. On faster versions of Ethernet the spacing remains the same, 96 bit-times, but the time required for that interval grows correspondingly shorter. This interval is referred as the spacing gap. After a collision occurs and all stations allow the cable to become idle (each waits the full interframe spacing), then the stations that collided must wait an additional and potentially progressively longer period of time before attempting to retransmit the collided frame. The waiting period is intentionally designed to be random.The waiting is measured is measured in increments of the parameter of slot time. If the MAC layer is unable to send the frame after sixteen attempts, it gives up and generates an error to the network layer.

19. Semester 1 v 3.0 19 Error Handling Collisions occurring before the Start of Frame Delimiter (SFD) are usually not reported to the higher layers, as if the collision did not occur. As soon as a collision is detected, the sending stations transmit a 32-bit “jam” signal that will enforce the collision. This is done so that any data being transmitted is thoroughly corrupted and all stations have a chance to detect the collision The corrupted, partially transmitted messages are often referred to as collision fragments or runts Normal collisions are less than 64 octets in length and that fails both min. length test and FCS checksum test.

20. Semester 1 v 3.0 20 Types of Collisions Local Collisions –In coaxial cable, the signal travels down the cable until it encounters a signal from the other station (I.e. symptom of over-voltage) –In UTP cable, cable fault such as excessive crosstalk can cause a station to perceive its own transmission (I.e. simultaneous RX/TX activity) Remote Collision –The characteristics of a remote collision are a frame that is less than the minimum length, has an invalid FCS checksum but does not have the symptom of local collision Collisions occurring after the first 64 octets are called “late collisions. NIC will not automatically retransmit a frame that was collided late.

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22. Semester 1 v 3.0 22 Ethernet Errors The following are the sources of Ethernet error: Collision or runt – Simultaneous transmission occurring before slot time has elapsed Late collision – Simultaneous transmission occurring after slot time has elapsed Jabber, long frame and range errors – Excessively or illegally long transmission Short frame, collision fragment or runt – Illegally short transmission FCS error – Corrupted transmission Alignment error – Insufficient or excessive number of bits transmitted Range error – Actual and reported number of octets in frame do not match Ghost or jabber – Unusually long Preamble or Jam event While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error.

23. Semester 1 v 3.0 23 Ethernet Auto-negotiation Method of automatically configuring a given interface to match the speed and capabilities 10, 100 and 100Mbps Ethernet. Auto-Negotiation is accomplished by transmitting a burst of 10BASE-T Link Pulses from each of two link partners

24. Semester 1 v 3.0 24 Link Establishment and Full/Half Duplex