Bài giảng Mạng máy tính cơ bản - Chương 6: Nguyên lý cơ bản của Ethernet (Ethernet Fundamentals) - Phan Vĩnh Thuần

Nội dung text: Bài giảng Mạng máy tính cơ bản - Chương 6: Nguyên lý cơ bản của Ethernet (Ethernet Fundamentals) - Phan Vĩnh Thuần

1. Chương 6 NGUYÊN LÝ CƠ BẢN CỦA ETHERNET (ETHERNET FUNDAMENTALS) Phan Vĩnh Thuần Computer Network 1
2. Overview • Ethernet is now the dominant LAN technology in the world. Ethernet is not one technology but a family of LAN technologies and may be best understood by using the OSI reference model. Phan Vĩnh Thuần Computer Network 2
3. • All LANs must deal with the basic issue of how individual stations (nodes) are named, and Ethernet is no exception. Ethernet specifications support different media, bandwidths, and other Layer 1 and 2 variations. However, the basic frame format and addressing scheme is the same for all varieties of Ethernet. Phan Vĩnh Thuần Computer Network 3
4. • For multiple stations to access physical media and other networking devices, various media access control strategies have been invented. Understanding how network devices gain access to the network media is essential for understanding and troubleshooting the operation of the entire network. Phan Vĩnh Thuần Computer Network 4
5. • Students completing this module should be able to: – Describe the basics of Ethernet technology. – Explain naming rules of Ethernet technology. – Define how Ethernet and the OSI model interact. – Describe the Ethernet framing process and frame structure. – List Ethernet frame field names and Phan Vĩnhpurposes Thuần . Computer Network 5
6. – Identify the characteristics of CSMA/CD. – Describe the key aspects of Ethernet timing, interframe spacing and backoff time after a collision. – Define Ethernet errors and collisions. – Explain the concept of auto-negotiation in relation to speed and duplex. Phan Vĩnh Thuần Computer Network 6
7. 6.1. Ethernet Fundamentals 6.1.1. Introduction to Ethernet • Most of the traffic on the Internet originates and ends with Ethernet connections. From its beginning in the 1970s, Ethernet has evolved to meet the increasing demand for high speed LANs. When a new media was produced, such as optical fiber, Ethernet adapted to take advantage of the superior bandwidth and low error rate that fiber offers. Now, the same protocol that transported data at 3 Mbps in 1973 is carrying data at 10 Gbps. Phan Vĩnh Thuần Computer Network 7
8. • The success of Ethernet is due to the following factors: – Simplicity and ease of maintenance – Ability to incorporate new technologies – Reliability – Low cost of installation and upgrade • With the introduction of Gigabit Ethernet, what started as a LAN technology now extends out to distances that make Ethernet a metropolitan-area network (MAN) and wide-area network (WAN) standard Phan Vĩnh Thuần Computer Network 8
9. • 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. Phan Vĩnh Thuần Computer Network 9
10. • 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 (Carrier Sense Multiple Access/Collision Detection) Phan Vĩnh Thuần Computer Network 10
11. • The first LAN in the world was the original version of Ethernet. Robert Metcalfe and his coworkers at Xerox designed it more than thirty years ago. The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Company, Intel, and Xerox (DIX). Metcalfe wanted Ethernet to be a shared standard from which everyone could benefit, so it was released as an open standard. Phan Vĩnh Thuần Computer Network 11
12. • The first products developed using the Ethernet standard were sold during the early 1980s. Ethernet transmitted at up to 10 Mbps over thick coaxial cable up to a distance of two kilometers. This type of coaxial cable was referred to as thicknet • In 1985, the Institute of Electrical and Electronics Engineers (IEEE) standards committee for Local and Metropolitan Networks published standards for LANs. Phan Vĩnh Thuần Computer Network 12
13. • These standards start with the number 802. The standard for Ethernet is 802.3. The IEEE wanted to make sure that its standards were compatible with the International Standards Organization (ISO)/OSI model. To do this, the IEEE 802.3 standard had to address the needs of Layer 1 and the lower portion of Layer 2 of the OSI model. As a result, some small modifications to the original Ethernet standard were made in 802.3. Phan Vĩnh Thuần Computer Network 13
14. • The differences between the two standards were so minor that any Ethernet network interface card (NIC) can transmit and receive both Ethernet and 802.3 frames. Essentially, Ethernet and IEEE 802.3 are the same standards. Phan Vĩnh Thuần Computer Network 14
15. • The 10-Mbps bandwidth of Ethernet was more than enough for the slow personal computers (PCs) of the 1980s. By the early 1990s PCs became much faster, file sizes increased, and data flow bottlenecks were occurring. Most were caused by the low availability of bandwidth. In 1995, IEEE announced a standard for a 100-Mbps Ethernet. This was followed by standards for gigabit per second (Gbps, 1 billion bits per second) Ethernet in 1998 and 1999. Phan Vĩnh Thuần Computer Network 15
16. • All the standards are essentially compatible with the original Ethernet standard. An Ethernet frame could leave an older coax 10-Mbps NIC in a PC, be placed onto a 10- Gbps Ethernet fiber link, and end up at a 100-Mbps NIC. As long as the packet stays on Ethernet networks it is not changed. For this reason Ethernet is considered very scalable. The bandwidth of the network could be increased many times without changing the underlying Ethernet technology. Phan Vĩnh Thuần Computer Network 16
17. • The original Ethernet standard has been amended a number of times in order to manage new transmission media and higher transmission rates. These amendments provide standards for the emerging technologies and maintain compatibility between Ethernet variations. Phan Vĩnh Thuần Computer Network 17
18. 6.1.2. IEEE Ethernet naming rules • Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps. The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet. Phan Vĩnh Thuần Computer Network 18
19. • When Ethernet needs to be expanded to add a new medium or capability, the 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. An abbreviated description (called an identifier) is also assigned to the supplement. Phan Vĩnh Thuần Computer Network 19
20. • The abbreviated description consists of: –A number indicating the number of Mbps transmitted. –The word base, indicating that baseband signaling is used. –One or more letters of the alphabet indicating the type of medium used (F= fiber optical cable, T = copper unshielded twisted pair). Phan Vĩnh Thuần Computer Network 20
21. • Ethernet relies on baseband signaling, which uses the entire bandwidth of the transmission medium. The data signal is transmitted directly over the transmission medium. In broadband signaling, not used by Ethernet, the data signal is never placed directly on the transmission medium. An analog signal (carrier signal) is modulated by the data signal and the modulated carrier signal is transmitted. Radio broadcasts and cable TV use broadband signaling. Phan Vĩnh Thuần Computer Network 21
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23. 6.1.3. Ethernet and the OSI model • Ethernet operates in two areas of the OSI model, the lower half of the data link layer, known as the MAC sublayer and the physical layer. Phan Vĩnh Thuần Computer Network 23
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25. • To move data between one Ethernet station and another, the data often passes through a repeater. All other stations in the same collision domain see traffic that passes through a repeater. A collision domain is then a shared resource. Problems originating in one part of the collision domain will usually impact the entire collision domain. Phan Vĩnh Thuần Computer Network 25
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27. • A repeater is responsible for forwarding all traffic to all other ports. Traffic received by a repeater is never sent out the originating port. Any signal detected by a repeater will be forwarded. If the signal is degraded through attenuation or noise, the repeater will attempt to reconstruct and regenerate the signal. Phan Vĩnh Thuần Computer Network 27
28. • Standards guarantee minimum bandwidth and operability by specifying the maximum number of stations per segment, maximum segment length, maximum number of repeaters between stations, etc. Stations separated by repeaters are within the same collision domain. Stations separated by bridges or routers are in different collision domains. Phan Vĩnh Thuần Computer Network 28
29. • The figure maps a variety of Ethernet technologies to the lower half of OSI Layer 2 and all of Layer 1. 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. 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. Phan Vĩnh Thuần Computer Network 29
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31. • 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. Phan Vĩnh Thuần Computer Network 31
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33. • The figure maps a variety of Ethernet technologies to the lower half of OSI Layer 2 and all of Layer 1. While there are other varieties of Ethernet, the ones shown are the most widely used. Phan Vĩnh Thuần Computer Network 33
34. ➢ Interactive Media Activity Phan Vĩnh Thuần Computer Network 34
35. 6.1.4. Đặt tên (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. Phan Vĩnh Thuần Computer Network 35
36. • The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This portion of the MAC address is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number, or another value administered by the specific equipment manufacturer. 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. Phan Vĩnh Thuần Computer Network 36
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38. • At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended for the data link layer in the destination system. Data from upper layer entities is encapsulated in the data link layer header and trailer. • The NIC uses the MAC address to assess 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. Phan Vĩnh Thuần Computer Network 38
39. • 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. The source device attaches a header with the MAC address of the intended destination and sends data onto the network. As this data propagates along the network media the NIC in each device on the network checks to see if the MAC address matches the physical destination address carried by the data frame. Phan Vĩnh Thuần Computer Network 39
40. • If there is no match, the NIC discards the data frame. When the data reaches the destination node, the NIC makes a copy and passes the frame up the OSI layers. On an Ethernet network, all nodes must examine the MAC header even if the communicating nodes are side by side. • All devices that are connected to the Ethernet LAN have MAC addressed interfaces including workstations, printers, routers, and switches. Phan Vĩnh Thuần Computer Network 40
41. 6.1.5. Tạo frame ở lớp 2 (Layer 2 framing) • Encoded bit streams (data) on physical media represent a tremendous technological accomplishment, but they, alone, are not enough to make communication happen. Framing helps obtain essential information that could not, otherwise, be obtained with coded bit streams alone. Phan Vĩnh Thuần Computer Network 41
42. • Examples of such information are: – Which computers are communicating with one another – When communication between individual computers begins and when it terminates – Provides a method for detection of errors that occurred during the communication – Whose turn it is to "talk" in a computer "conversation" • Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit. Phan Vĩnh Thuần Computer Network 42
43. • There are many different types of frames described by various standards. A single generic frame has sections called fields, and each field is composed of bytes. The names of the fields are as follows: – Start frame field – Address field – Length / type field – Data field – Frame check sequence field Phan Vĩnh Thuần Computer Network 43
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45. • When computers are connected to a physical medium, there must be a way they can grab the attention of other computers to broadcast the message, "Here comes a frame!" Various technologies have different ways of doing this process, but all frames, regardless of technology, have a beginning signaling sequence of bytes. • All frames contain naming information, such as the name of the source node (MAC address) and the name of the destination node (MAC address). Phan Vĩnh Thuần Computer Network 45
46. • Most frames have some specialized fields. In some technologies, a length field specifies the exact length of a frame in bytes. Some frames have a type field, which specifies the Layer 3 protocol making the sending request. Phan Vĩnh Thuần Computer Network 46
47. • The reason for sending frames is to get upper layer data, ultimately the user application data, from the source to the destination. The data package has two parts, the user application data and the encapsulated bytes to be sent to the destination computer. Padding bytes may be added so frames have a minimum length for timing purposes. Phan Vĩnh Thuần Computer Network 47
48. • Logical link control (LLC) bytes are also included with the data field in the IEEE standard frames. The LLC sub-layer takes the network protocol data, an IP packet, and adds control information to help deliver that IP packet to the destination node. Layer 2 communicates with the upper-level layers through LLC. Phan Vĩnh Thuần Computer Network 48
49. • All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The Frame Check Sequence (FCS) field contains a number that is calculated by the source node based on the data in the frame. This FCS is then added to the end of the frame that is being sent. Phan Vĩnh Thuần Computer Network 49
50. • 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. Phan Vĩnh Thuần Computer Network 50
51. • There are three primary ways to calculate the Frame Check Sequence 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. Phan Vĩnh Thuần Computer Network 51
52. • The node that transmits data must get the attention of other devices, in order to start a frame, and to end the frame. The length field implies the end, and the frame is considered ended after the FCS. Sometimes there is a formal byte sequence referred to as an end-frame delimiter Phan Vĩnh Thuần Computer Network 52
53. 6.1.6. Cấu trúc frame Ethernet (Ethernet frame structure) • At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps. However, at the physical layer almost all versions of Ethernet are substantially different from one another with each speed having a distinct set of architecture design rules. Phan Vĩnh Thuần Computer Network 53
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55. • In the version of Ethernet that was developed by DIX prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start Frame Delimiter (SFD) were combined into a single field, though the binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version, as both uses of the field were common throughout industry. Phan Vĩnh Thuần Computer Network 55
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57. • The Ethernet II Type field is incorporated into the current 802.3 frame definition. The receiving node must determine which higher-layer protocol is present in an incoming frame by examining the Length/Type field. If the two-octet value is equal to or greater than 0x600 (hexadecimal), then the frame is interpreted according to the Ethernet II type code indicated. Phan Vĩnh Thuần Computer Network 57
58. 6.1.7. Các field trong Ethernet frame (Ethernet frame fields) • Some of the fields permitted or required in an 802.3 Ethernet Frame are: – Preamble – Start Frame Delimiter – Destination Address – Source Address – Length/Type – Data and Pad – FCS Phan Vĩnh Thuần Computer Network 58
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60. • The Preamble is an alternating pattern of ones and zeroes used for timing synchronization in the asynchronous 10 Mbps and slower implementations of Ethernet. Faster versions of Ethernet are synchronous, and this timing information is redundant but retained for compatibility Phan Vĩnh Thuần Computer Network 60
61. • A Start Frame Delimiter consists of a one- octet field that marks the end of the timing information, and contains the bit sequence 10101011. • The Destination Address field contains the MAC destination address. The destination address can be unicast, multicast (group), or broadcast (all nodes). Phan Vĩnh Thuần Computer Network 61
62. • The Source Address field contains the MAC source address. The source address is generally the unicast address of the transmitting Ethernet node. There are, however, an increasing number of virtual protocols in use that use and sometimes share a specific source MAC address to identify the virtual entity. Phan Vĩnh Thuần Computer Network 62
63. • The Length/Type field supports two different uses. If the value is less than 1536 decimal, 0x600 (hexadecimal), then the value indicates length. The length interpretation is used where the LLC Layer provides the protocol identification. The type value specifies the upper-layer protocol to receive the data after Ethernet processing is completed. Phan Vĩnh Thuần Computer Network 63
64. • The length indicates the number of bytes of data that follows this field. If the value is equal to or greater than 1536 decimal (0600 hexadecimal), the value indicates that the type and contents of the Data field are decoded per the protocol indicated. • The Data and Pad field may be of any length that does not cause the frame to exceed the maximum frame size. Phan Vĩnh Thuần Computer Network 64
65. • The maximum transmission unit (MTU) for Ethernet is 1500 octets, so the data should not exceed that size. The content of this field is unspecified. An unspecified pad is inserted immediately after the user data when there is not enough user data for the frame to meet the minimum frame length. Ethernet requires that the frame be not less than 46 octets or more than 1518 octets. Phan Vĩnh Thuần Computer Network 65
66. • A FCS contains a four byte CRC value that is created by the sending device and is recalculated by the receiving device to check for damaged frames. Since the corruption of a single bit anywhere from the beginning of the Destination Address through the end of the FCS field will cause the checksum to be different. Phan Vĩnh Thuần Computer Network 66
67. 6.2. Họat động của Ethernet (Ethernet Operation) 6.2.1. Điều khiển truy nhập môi trường MAC (Media Access Control) • MAC refers to protocols that determine which computer on a shared-medium environment, or collision domain, is allowed to transmit the data. MAC, with LLC, comprises the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. There are two broad categories of Media Access Control, deterministic (taking turns) and non-deterministic (first come, first served). Phan Vĩnh Thuần Computer Network 67
68. • Examples of deterministic protocols include Token Ring and FDDI. 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. Phan Vĩnh Thuần Computer Network 68
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70. • Non-deterministic MAC protocols use a first- come, first-served approach. CSMA/CD is a simple 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. • Three common Layer 2 technologies are Token Ring, FDDI, and Ethernet. All three specify Layer 2 issues, LLC, naming, framing, and MAC, as well as Layer 1 signaling components and media issues. Phan Vĩnh Thuần Computer Network 70
71. • The specific technologies for each are as follows: – 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 Phan Vĩnh Thuần Computer Network 71 dual-ring topology (wired as a dual-ring)
72. 6.2.2. Các quy tắc MAC và sự phát hiện/ sự vãn hồi đụng độ (MAC rules and collision detection/backoff) • Ethernet is a shared-media broadcast technology. The access method CSMA/CD used in Ethernet performs three functions: – Transmitting and receiving data packets – Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model – Detecting errors within data packets or on the network Phan Vĩnh Thuần Computer Network 72
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74. • 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. Phan Vĩnh Thuần Computer Network 74
75. • 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 Phan Vĩnh Thuần Computer Network 75
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77. • 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. ➢Interactive Media Activity Phan Vĩnh Thuần Computer Network 77
78. • When the delay period expires, all devices on the network can attempt to gain access to the networking media. When data transmission resumes on the network, the devices that were involved in the collision do not have priority to transmit data. Phan Vĩnh Thuần Computer Network 78
79. 6.2.3. Định thời Ethernet (Ethernet timing) • The basic rules and specifications for proper operation of Ethernet are not particularly complicated, though some of the faster physical layer implementations are becoming so. Despite the basic simplicity, when a problem occurs in Ethernet it is often quite difficult to isolate the source Phan Vĩnh Thuần Computer Network 79
80. • Because of the common bus architecture of Ethernet, also described as a distributed single point of failure, the scope of the problem usually encompasses all devices within the domain. In situations where repeaters are used, this can include devices up to four segments away Phan Vĩnh Thuần Computer Network 80
81. • Any station on an Ethernet network wishing to transmit a message first “listens” to ensure that no other station is currently transmitting. If the cable is quiet, the station will begin transmitting immediately. The electrical signal takes time to travel down the cable (delay), and each subsequent repeater introduces a small amount of latency in forwarding the frame from one port to the next. Because of the delay and latency, it is possible for more than one station to begin transmitting at or near the same time. This results in a collision. Phan Vĩnh Thuần Computer Network 81
82. • 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. Phan Vĩnh Thuần Computer Network 82
83. • 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 – The actual data payload – Checksum (FCS) used to ensure that the message was not corrupted along the way Phan Vĩnh Thuần Computer Network 83
84. • The 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. 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. Phan Vĩnh Thuần Computer Network 84
85. • 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 (Tính hiệu bồi) is used when collisions are detected Phan Vĩnh Thuần Computer Network 85
86. • 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. Phan Vĩnh Thuần Computer Network 86
87. • 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. Phan Vĩnh Thuần Computer Network 87
88. • On 10-Mbps Ethernet one bit at the MAC layer requires 100 nanoseconds (ns) to transmit. At 100 Mbps that same bit requires 10 ns to transmit and at 1000 Mbps only takes 1 ns. 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 Phan Vĩnh Thuần Computer Network 88
89. • 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. Phan Vĩnh Thuần Computer Network 89
90. Phan Vĩnh Thuần➢Interactive ComputerMedia Network Activity 90
91. 6.2.4. Khỏang cách frame và sự giản hồi (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. Phan Vĩnh Thuần Computer Network 91
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93. • After a frame has been sent, 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 to as the spacing gap. The gap is intended to allow slow stations time to process the previous frame and prepare for the next frame. Phan Vĩnh Thuần Computer Network 93
94. • A repeater is expected to regenerate the full 64 bits of timing information, which is the preamble and SFD, at the start of any frame. This is despite the potential loss of some of the beginning preamble bits because of slow synchronization. • 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. Phan Vĩnh Thuần Computer Network 94
95. • 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. Phan Vĩnh Thuần Computer Network 95
96. 6.2.5. Kiểm sóat lỗi (Error handling) • The most common error condition on an Ethernet is the collision. Collisions are the mechanism for resolving contention for network access. A few collisions provide a smooth, simple, low overhead way for network nodes to arbitrate contention for the network resource. When network contention becomes too great, collisions can become a significant impediment to useful network operation. Phan Vĩnh Thuần Computer Network 96
97. • Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal. This is consumption delay and affects all network nodes possibly causing significant reduction in network throughput. Phan Vĩnh Thuần Computer Network 97
98. • The considerable majority of collisions occur very early in the frame, often before the SFD. Collisions occurring before the 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. Phan Vĩnh Thuần Computer Network 98
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100. • In the Figure, two stations listen to ensure that the cable is idle, then transmit. Station 1 was able to transmit a significant percentage of the frame before the signal even reached the last cable segment. Station 2 had not received the first bit of the transmission prior to beginning its own transmission and was only able to send several bits before the NIC sensed the collision. Station 2 immediately truncated the current transmission, substituted the 32-bit jam signal and ceased all transmissions. Phan Vĩnh Thuần Computer Network 100
101. • During the collision and jam event that Station 2 was experiencing, the collision fragments were working their way back through the repeated collision domain toward Station 1. Station 2 completed transmission of the 32-bit jam signal and became silent before the collision propagated back to Station 1 which was still unaware of the collision and continued to transmit. Phan Vĩnh Thuần Computer Network 101
102. • When the collision fragments finally reached Station 1, it also truncated the current transmission and substituted a 32- bit jam signal in place of the remainder of the frame it was transmitting. Upon sending the 32-bit jam signal Station 1 ceased all transmissions. • A jam signal may be composed of any binary data so long as it does not form a proper checksum for the portion of the frame already transmitted. Phan Vĩnh Thuần Computer Network 102
103. • The most commonly observed data pattern for a 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 0x5 or 0xA sequence. The corrupted, partially transmitted messages are often referred to as collision fragments or runts. Normal collisions are less than 64 octets in length and therefore fail both the minimum length test and the FCS checksum test. Phan Vĩnh Thuần Computer Network 103
104. 6.2.6. Các lọai đụng độ (Types of Collisions ) • Collisions typically take place when two or more Ethernet stations transmit simultaneously within a collision domain. A single collision is a collision that was detected while trying to transmit a frame, but on the next attempt the frame was transmitted successfully. Phan Vĩnh Thuần Computer Network 104
105. • Multiple collisions indicate that the same frame collided repeatedly before being successfully transmitted. The results of collisions, collision fragments, are partial or corrupted frames that are less than 64 octets and have an invalid FCS. Three types of collisions are: – Local – Remote – Late Phan Vĩnh Thuần Computer Network 105
106. Phan Vĩnh Thuần Computer Network 106
107. • To create a local collision on coax cable (10BASE2 and 10BASE5), the signal travels down the cable until it encounters 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. Phan Vĩnh Thuần Computer Network 107
108. Phan Vĩnh Thuần Computer Network 108
109. • On UTP cable, such as 10BASE-T, 100BASE-TX and 1000BASE-T, a collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair. Since the two signals are on different pairs there is no characteristic change in the signal. Collisions are only recognized on UTP when the station is operating in half duplex. A cable fault such as excessive crosstalk can cause a station to perceive its own transmission as a local collision. Phan Vĩnh Thuần Computer Network 109
110. • The characteristics of a remote collision are a frame that is less than the minimum length, has an invalid FCS checksum, but does not exhibit the local collision symptom of over-voltage or simultaneous RX/TX activity. This sort of collision usually results from collisions occurring on the far side of a repeated connection. Phan Vĩnh Thuần Computer Network 110
111. • 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. ➢Interactive Media Activity Phan Vĩnh Thuần Computer Network 111
112. 6.2.7. Các lỗi Ethernet (Ethernet errors) • Knowledge of typical errors is invaluable for understanding both the operation and troubleshooting of Ethernet networks. Phan Vĩnh Thuần Computer Network 112
113. • 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 Phan Vĩnh Thuần Computer Network 113
114. – 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 Phan Vĩnh Thuần Computer Network 114
115. • While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error. The presence of errors on a network always suggests that further investigation is warranted. The severity of the problem indicates the troubleshooting urgency related to the detected errors. Phan Vĩnh Thuần Computer Network 115
116. • Jabber is defined in several places in the 802.3 standard as being a transmission of at least 20,000 to 50,000 bit times in duration. However, most diagnostic tools report jabber whenever a detected transmission exceeds the maximum legal frame size, which is considerably smaller than 20,000 to 50,000 bit times. Most references to jabber are more properly called long frames. Phan Vĩnh Thuần Computer Network 116
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118. • A long frame is one that is longer than the maximum legal size, and takes into consideration whether or not the frame was tagged. It does not consider whether or not the frame had a valid FCS checksum. This error usually means that jabber was detected on the network. Phan Vĩnh Thuần Computer Network 118
119. • A short frame is a frame smaller than the minimum legal size of 64 octets, with a good frame check sequence. Some protocol analyzers and network monitors call these frames “runts". In general the presence of short frames is not a guarantee that the network is failing. Phan Vĩnh Thuần Computer Network 119
120. Phan Vĩnh Thuần Computer Network 120
121. • The term runt is generally an imprecise slang 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 to collision fragments ➢Interactive Media Activity Phan Vĩnh Thuần Computer Network 121
122. 6.2.8. FCS và lỗi (FCS and beyond) • A received frame that has a bad Frame Check Sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least one bit. In an FCS error frame the header information is probably correct, but the checksum calculated by the receiving station does not match the checksum appended to the end of the frame by the sending station. The frame is then discarded. Phan Vĩnh Thuần Computer Network 122
123. • High numbers of FCS errors from a single station usually indicates a faulty NIC and/or faulty or corrupted software drivers, or a bad cable connecting that station to the network. If FCS errors are associated with many stations, they are generally traceable to bad cabling, a faulty version of the NIC driver, a faulty hub port, or induced noise in the cable system. Phan Vĩnh Thuần Computer Network 123
124. • A message that does not end on an octet boundary is known as an alignment error. Instead of the correct number of binary bits forming complete octet groupings, there are additional bits left over (less than eight). Such a frame is truncated to the nearest octet boundary, and if the FCS checksum fails, then an alignment error is reported. This is often caused by bad software drivers, or a collision, and is frequently accompanied by a failure of the FCS checksum. Phan Vĩnh Thuần Computer Network 124
125. • 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. This error also appears when the length field value is less than the minimum legal unpadded size of the data field. A similar error, Out of Range, is reported when the value in the Length field indicates a data size that is too large to be legal. Phan Vĩnh Thuần Computer Network 125
126. • 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. To qualify as a ghost, the frame must be at least 72 octets long, including the preamble. Because of the peculiar nature of ghosts, it is important to note that test results are largely dependent upon where on the segment the measurement is made. • Ground loops and other wiring problems are usually the cause of ghosting. Most network monitoring tools do not recognize the existence of ghosts. Phan Vĩnh Thuần Computer Network 126
127. Phan Vĩnh Thuần➢InteractiveComputer Media Network Activity 127
128. 6.2.9. Sự đàm phán tự động của Ethernet (Ethernet auto-negotiation) • As Ethernet grew from 10 to 100 and 1000 Mbps, one requirement was to make each technology interoperable, even to the point that 10, 100, and 1000 interfaces could be directly connected. A process called Auto- Negotiation of speeds at half or full duplex was developed. Phan Vĩnh Thuần Computer Network 128
129. • Specifically, at the time that Fast Ethernet was introduced, the standard included a method of automatically configuring a given interface to match the speed and capabilities of the link partner. This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. It has the additional advantage of only involving the lowest part of the physical layer. Phan Vĩnh Thuần Computer Network 129
130. • 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. Each FLP burst is sent at the same timing interval as an NLP, and is intended to allow older 10BASE-T devices to operate normally in the event they should receive an FLP burst Phan Vĩnh Thuần Computer Network 130
131. • Auto-Negotiation is accomplished by transmitting a burst of 10BASE-T Link Pulses from each of the two link partners. The burst communicates the capabilities of the transmitting station to its link partner. After both stations have interpreted what the other partner is offering, both switch to the highest performance common configuration and establish a link at that speed. 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. The link may be lost due to external influences, such as a cable fault, or due to one of the partners issuing a Phanreset Vĩnh Thuần. Computer Network 131
132. 6.2.10. Thiết lập chế độ truyền thông Full duplex và half duplex (Link establishment and full and half duplex) • There are two duplex modes, half and full. For shared media, the half-duplex mode is mandatory. All coaxial implementations are half duplex in nature and cannot operate in full duplex. UTP and fiber implementations may be operated in half duplex. 10-Gbps implementations are specified for full duplex only. Phan Vĩnh Thuần Computer Network 132
133. • In half duplex only one station may transmit at a time. For the coaxial implementations a second station transmitting will cause the signals to overlap and become corrupted. Since UTP and fiber generally transmit on separate pairs the signals have no opportunity to overlap and become corrupted. Ethernet has established arbitration rules for resolving conflicts arising from instances when more than one station attempts to transmit at the same time. Both stations in a point-to-point full- duplex link are permitted to transmit at any time, regardless of whether the other station is Phantransmitting Vĩnh Thuần . Computer Network 133
134. • Auto-Negotiation avoids most situations where one station in a point-to-point link is transmitting under half-duplex rules and the other under full-duplex rules. Phan Vĩnh Thuần Computer Network 134