Module 6: Ethernet Fundamentals

Module 6: Ethernet Fundamentals
James Chen 2018/11/10 YuDa College of Business

YuDa College of Business
Outline Ethernet Fundamentals Introduction to Ethernet IEEE Ethernet naming rules Ethernet and the OSI model Naming Layer 2 framing, Ethernet frame fields Ethernet frame structure Ethernet Operation Media Access Control (MAC) MAC rules and collision detection/backoff Ethernet timing Interframe spacing and backoff Error handling Types of collisions Ethernet errors FCS and beyond Ethernet auto-negotiation Link establishment and full and half duplex 2018/11/10 YuDa College of Business

6.1 Ethernet Fundamentals 2018/11/10 YuDa College of Business

Introduction to Ethernet
The success of Ethernet is due to the following factors: Simplicity and ease of maintenance Ability to incorporate (混合)new technologies Reliability Low cost of installation and upgrade The original idea for Ethernet grew out of the problem of allowing two or more hosts to use the same medium and prevent the signals from interfering with each other. This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii. A system called Alohanet was developed to allow various stations on the Hawaiian Islands structured access to the shared radio frequency band in the atmosphere. This work later formed the basis for the Ethernet access method known as CSMA/CD. The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Company, Intel, and Xerox (DIX). In 1995, IEEE announced a standard for a 100-Mbps Ethernet. In 1999, 1Gbps Ethernet. Now, 10Gbps Ethernet 2018/11/10 YuDa College of Business

IEEE Ethernet naming rules
When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the standard. Naming Rule : Speed : 10, 100, 1G, 10G Signal method : BASE, BROAD Medium : 2, 5, T, TX, FX, CX, SX, LX, ZX 2018/11/10 YuDa College of Business

IEEE Ethernet naming rules (cont.)
2018/11/10 YuDa College of Business

Ethernet and the OSI model
Ethernet operates in two areas of the OSI model the lower half of the data link layer (MAC sublayer ) physical layer. 2018/11/10 YuDa College of Business

Ethernet and the OSI model (cont.)
Repeater A repeater is responsible for forwarding all traffic to all other ports. Traffic received by a repeater is never sent out the originating port. If the signal is degraded through attenuation or noise, the repeater will attempt to reconstruct and regenerate the signal. A collision domain is then a shared resource. Problems originating in one part of the collision domain will usually impact the entire collision domain. Stations separated by repeaters are within the same collision domain. Stations separated by bridges or routers are in different collision domains. 2018/11/10 YuDa College of Business

Ethernet Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various topologies. Ethernet Layer 1 performs a key role in the communication that takes place between devices, but each of its functions has limitations. Layer 2 addresses these limitations. 2018/11/10 YuDa College of Business

Data link sublayers contribute significantly to technology compatibility and computer communication. 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. 2018/11/10 YuDa College of Business

Naming To allow for local delivery of frames on the Ethernet, there must be an addressing system, a way of uniquely identifying computers and interfaces. Ethernet uses MAC addresses that are 48 bits in length and expressed as twelve hexadecimal digits (6 Bytes). MAC addresses are sometimes referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes. 2018/11/10 YuDa College of Business

Naming (cont.) At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended (未來的)for the data link layer in the destination system. The NIC uses the MAC address to assess(評價)whether the message should be passed onto the upper layers of the OSI model. The NIC makes this assessment without using CPU processing time, enabling better communication times on an Ethernet network. On an Ethernet network, when one device sends data it can open a communication pathway to the other device by using the destination MAC address. All devices that are connected to the Ethernet LAN have MAC addressed interfaces including workstations, printers, routers, and switches. 2018/11/10 YuDa College of Business

Layer 2 framing Framing is the Layer 2 encapsulation process. Voltage vs. time graph (large and confusing) Frame format diagram (fields = groupings of bits) Preamble field The pattern of ones and zeroes used for timing synchronization in 10 Mbps Ethernet (Asynchronous). The timing information is redundant but retained for compatibility in faster versions of Ethernet (Synchronous). Start Frame Delimiter field It consists of a one-octet field that marks the end of the timing information, and contains the bit sequence Destination field Destination node MAC address (unicast, multicast (group), broadcast (all nodes)) Source field Source node MAC address (unicast address, virtual entity) Length field / type field length of a frame in bytes (it implies the end) some frames have a type field, which specifies the Layer 3 protocol making the sending request. 2018/11/10 YuDa College of Business

Layer 2 framing (cont.) Data field upper layer data user application data LLC bytes are also included with the data field in the IEEE standard frames. It adds control information to help deliver that IP packet to the destination node. Layer 2 communicates with the upper-level layers through LLC. padding bytes (minimum length for timing purposes) FCS field (Frame Check Sequence number) End of the frame. It contains a number that is calculated by the source node based on the data in the frame. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed, the frame is discarded, and the source is asked to retransmit. The ways to calculate the FCS number: Cyclic Redundancy Check (CRC) – performs calculations on the data. Two-dimensional parity – adds an 8th bit that makes an 8 bit sequence have an odd or even number of binary 1s. Internet checksum – adds the values of all of the data bits to arrive at a sum. 2018/11/10 YuDa College of Business

Layer 2 framing (cont.) 2018/11/10 YuDa College of Business

Ethernet frame structure

Ethernet frame structure (cont.)

Ethernet frame structure (cont.)
The Ethernet II Type field is incorporated into the current frame definition. If the value of Length/Type field is equal to or greater than 1536 or 0x600 (hexadecimal), then the frame is interpreted according to the Ethernet II type code indicated. The maximum transmission unit (MTU) for Ethernet is 1500 octets minimum-sized frame = 64B (preamble is not included) Ethernet requires each frame to be between 64 and 1518 octets 2018/11/10 YuDa College of Business

Ethernet Operation 2018/11/10 YuDa College of Business

Media Access Control (MAC)
Categories of Media Access Control Deterministic(決定論的)(taking turns) In a Token Ring network, individual hosts are arranged in a ring and a special data token travels around the ring to each host in sequence. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring. Token Ring is a collisionless environment as only one host is able to transmit at any given time. Non-deterministic (first come, first served) In a CSMA/CD system, the NIC listens for an absence of a signal on the media and starts transmitting. If two nodes transmit at the same time a collision occurs and none of the nodes are able to transmit. Ethernet – logical bus topology, physical star or extended star, wired as a star Token Ring – logical ring topology, physical star topology, wired as a star FDDI – logical ring topology, physical dual-ring topology, wired as a dual-ring 2018/11/10 YuDa College of Business

MAC rules and collision detection/backoff
In the CSMA/CD access method, networking devices with data to transmit work in a listen-before-transmit mode. This means when a node wants to send data, it must first check to see whether the networking media is busy. If the node determines the network is busy, the node will wait a random amount of time before retrying. If the node determines the networking media is not busy, the node will begin transmitting and listening. The node listens to ensure no other stations are transmitting at the same time. After completing data transmission the device will return to listening mode. Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases. When a collision occurs, each node that is transmitting will continue to transmit for a short time to ensure that all devices see the collision. Once all the devices have detected the collision a backoff algorithm is invoked and transmission is stopped. The nodes stop transmitting for a random period of time, which is different for each device. When the delay period expires, all devices on the network can attempt to gain access to the networking media. When data transmission resumes (恢復)on the network, the devices that were involved in the collision do not have priority to transmit data. 2018/11/10 YuDa College of Business

MAC rules and collision detection/backoff (cont.)

Ethernet timing In half duplex, assuming that a collision does not occur, the sending station will transmit 64 bits of timing synchronization information that is known as the preamble. The sending station will then transmit the following information: Destination and source MAC addressing information Certain other header information (Length / Type) The actual data payload (Data) Checksum (FCS) 10 Mbps and slower versions of Ethernet are asynchronous. Asynchronous means that each receiving station will use the eight octets of timing information to synchronize the receive circuit to the incoming data, and then discard it. If the attached station is operating in full duplex then the station may send and receive simultaneously and collisions should not occur. Full-duplex operation also changes the timing considerations and eliminates the concept of slot time. Full-duplex operation allows for larger network architecture designs since the timing restriction for collision detection is removed. 100 Mbps and higher speed implementations of Ethernet are synchronous. Synchronous means the timing information is not required, however for compatibility reasons the Preamble and SFD are present. 2018/11/10 YuDa College of Business

Ethernet timing (cont.)
For all speeds of Ethernet transmission at or below 1000 Mbps, the standard describes how a transmission may be no smaller than the slot time. Slot time for 10 and 100-Mbps Ethernet is 512 bit-times, or 64 octets. Slot time for 1000-Mbps Ethernet is 4096 bit-times, or 512 octets. Slot time is calculated assuming maximum cable lengths on the largest legal network architecture. All hardware propagation delay times are at the legal maximum and the 32-bit jam signal is used when collisions are detected. The actual calculated slot time is just longer than the theoretical amount of time required to travel between the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected. For the system to work the first station must learn about the collision before it finishes sending the smallest legal frame size. To allow 1000-Mbps Ethernet to operate in half duplex the extension field was added when sending small frames purely to keep the transmitter busy long enough for a collision fragment to return. This field is present only on 1000-Mbps, half-duplex links and allows minimum-sized frames to be long enough to meet slot time requirements. Extension bits are discarded by the receiving station. On 10-Mbps Ethernet one bit at the MAC layer requires 100 ns to transmit. At 100 Mbps that same bit requires 10 ns to transmit and at 1000 Mbps only takes 1 ns. 2018/11/10 YuDa College of Business

Ethernet timing (cont.)
As a rough estimate, 20.3 cm (8 inch) 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. 100 m / 20.3 cm per ns = ns 492.6 ns / 100 ns per bit = 5 bits 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 (僅僅)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. 2018/11/10 YuDa College of Business

Interframe spacing and 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. Ethernet devices must allow a minimum idle period between transmission of frames known as the spacing gap. It provides a brief recovery time between frames to allow devices to prepare for reception of the next frame. The minimum spacing gap is 96 bit times, which is 9.6 microseconds for 10 Mb/s Ethernet, 960 nanoseconds for 100 Mb/s Ethernet, and 96 nanoseconds for 1 Gb/s Ethernet. 2018/11/10 YuDa College of Business

Interframe spacing and backoff (cont.)
After a collision occurs and all stations allow the cable to become idle (each waits the full interframe spacing) The stations that collided must wait an additional longer period of time before attempting to retransmit the collided frame. The waiting period is intentionally designed to be random so that two stations do not delay for the same amount of time before retransmitting, which would result in more collisions. If the MAC layer is unable to send the frame after sixteen attempts, it gives up and generates an error to the network layer. Such an occurrence is fairly rare and would happen only under extremely heavy network loads, or when a physical problem exists on the network. The slot time is a key parameter for half-duplex Ethernet network operation. It is defined as 512 bit times for Ethernet networks operating at 10 and 100 Mb/s, and 4096 bit times for gigabit Ethernet. Backoff is the process by which a transmitting interface determines how long to wait following a collision before attempting to retransmit the frame. 2018/11/10 YuDa College of Business

Error handling The most common error condition on an Ethernet is the collision. Collisions are the mechanism for resolving contention for network access. Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal (32 bits), all stations have a chance to detect the collision. Jam signal is simply a repeating one, zero, one, zero pattern, the same as Preamble. When viewed by a protocol analyzer this pattern appears as either a repeating hexadecimal 5 or A sequence. 2018/11/10 YuDa College of Business

Error handling (cont.) 2018/11/10 YuDa College of Business

Types of collisions To create a local collision on coax cable (10BASE2 and 10BASE5), the signal travels down the cable until it encounters (遇到)a signal from the other station. The waveforms then overlap, canceling some parts of the signal out and reinforcing or doubling other parts. The doubling of the signal pushes the voltage level of the signal beyond the allowed maximum. This over-voltage condition is then sensed by all of the stations on the local cable segment as a collision. In the beginning the waveform in Figure represents normal Manchester encoded data. A few cycles into the sample the amplitude of the wave doubles. That is the beginning of the collision, where the two waveforms are overlapping. Just prior to the end of the sample the amplitude returns to normal. This happens when the first station to detect the collision quits transmitting, and the jam signal from the second colliding station is still observed. 2018/11/10 YuDa College of Business

Types of collisions (cont.)
Collisions are only recognized on UTP when the station is operating in half duplex. If the station is not engaged in transmitting it cannot detect a local collision. Conversely(相反地), a cable fault such as excessive crosstalk can cause a station to perceive(察覺)its own transmission as a local collision. A remote collision usually results from collisions occurring on the far side of a repeated connection. A repeater will not forward an over-voltage state, and cannot cause a station to have both the TX and RX pairs active at the same time. Collisions occurring after the first 64 octets are called “late collisions". The most significant difference between late collisions and collisions occurring before the first 64 octets is that the Ethernet NIC will retransmit a normally collided frame automatically, but will not automatically retransmit a frame that was collided late. As far as the NIC is concerned everything went out fine, and the upper layers of the protocol stack must determine that the frame was lost. Other than retransmission, a station detecting a late collision handles it in exactly the same way as a normal collision. 2018/11/10 YuDa College of Business

Ethernet errors The following are the sources of Ethernet error: Collision or runt – Simultaneous transmission occurring before slot time has elapsed (過去) Late collision – Simultaneous transmission occurring after slot time has elapsed Jabber(吱喳地叫), long frame and range errors – Excessively or illegally long transmission Short frame, collision fragment or runt – Illegally short transmission FCS error – Corrupted (篡改)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 Jabber and Long Frames Jabber : at least 20,000 to 50,000 bit times in duration Long Frames : excess of the maximum frame size (1518 octets) both significantly larger whether or not the frame was tagged whether or not the frame had a valid FCS checksum 2018/11/10 YuDa College of Business

Ethernet errors (cont.)
Short frames less than the minimum frame size (64 octets) with valid(有效的)FCS checksums Some protocol analyzers and network monitors call these frames “runts". The term runt is generally an imprecise slang(俚語)term that means something less than a legal frame size. It may refer to short frames with a valid FCS checksum although it usually refers (歸因於)to collision fragments. 2018/11/10 YuDa College of Business

FCS and beyond A received frame that has a bad FCS. The frame is then discarded. High numbers of FCS errors from a single station a faulty NIC a faulty software drivers a bad cable connecting FCS errors are associated with many stations bad cabling a faulty hub noise in the cable system 2018/11/10 YuDa College of Business

FCS and beyond (cont.) A message that does not end on an octet boundary is known as an alignment error. bad software drivers Collision A frame with a valid value in the Length field but did not match the actual number of octets counted in the data field of the received frame is known as a range error. The length field value is less than the minimum legal unpadded size of the data field. Out of Range, is reported when the value in the Length field indicates a data size that is too large to be legal. Fluke Networks has coined the term ghost to mean energy (noise) detected on the cable that appears to be a frame, but is lacking a valid SFD. The frame must be at least 72 octets long, including the preamble. Ground loops and other wiring problems are usually the cause of ghosting. Most network monitoring tools do not recognize the existence of ghosts. The tools rely entirely on what the chipset tells them. 2018/11/10 YuDa College of Business

Ethernet auto-negotiation
Auto-Negotiation - This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. If anything interrupts communications and the link is lost, the two link partners first attempt to link again at the last negotiated speed. If that fails, or if it has been too long since the link was lost, the Auto-Negotiation process starts over. Example : 10BASE-T required each station to transmit a link pulse about every 16 milliseconds, whenever the station was not engaged in transmitting a message. Auto-Negotiation adopted this signal and renamed it a Normal Link Pulse (NLP). When a series of NLPs are sent in a group for the purpose of Auto-Negotiation, the group is called a Fast Link Pulse (FLP) burst. 2018/11/10 YuDa College of Business

Link establishment and full and half duplex
Duplex mismatch one end is forced to full duplex the other is forced to half duplex result in collisions and errors 10-Gigabit Ethernet does not support half duplex The list is priority ranked, with the most desirable link configuration at the top. Fiber-optic Ethernet implementations are not included in this priority resolution list because the interface configuration is fixed. 2018/11/10 YuDa College of Business

END 2018/11/10 YuDa College of Business

Module 6: Ethernet Fundamentals

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