Bài giảng Network+ Certification - Chapter 5, Data-Link Layer Protocols

Chapter 5, Data-Link Layer Protocols |1| Chapter Overview A. Ethernet B. Token Ring C. FDDI D. Wireless Networking Chapter 5, Lesson 1 Ethernet |2| 1. Ethernet Standards A. Ethernet is the most popular LAN protocol operating at the data-link layer. B. There are two sets of Ethernet standards: 1. DIX Ethernet 2. Institute of Electrical and Electronic Engineers (IEEE) 802.3 |3| C. DIX Ethernet: the original Ethernet protocol 1. Developed by Digital, Intel, and Xerox a. Known as thick Ethernet, ThickNet, or 10Base5 2. Privately held standard 3. DIX Ethernet II a. Retains thick Ethernet (10Base5) b. Adds thin Ethernet (10Base2) 4. The frame contains an Ethertype field that identifies the network layer protocol that generated the data in each packet. a. Ethertype is the only element of the DIX Ethernet standard that is still commonly used. |4| D. IEEE 802.3 1. Developed by the IEEE 2. Retains 10Base5 and 10Base2 3. Added 10Base-T to the existing physical standards 4. Not privately held 5. It cannot legally use the name Ethernet. 6. Virtually all implementations called Ethernet today are actually IEEE 802.3. 7. Differences from DIX Ethernet a. IEEE 802.3 has additional physical layer standards. b. Slight frame format differences E. IEEE 802.3u 1. Fast Ethernet F. IEEE 802.3z and IEEE 802.3ab 1. Gigabit Ethernet |5| G. DIX Ethernet/IEEE 802.3 components 2 Outline, Chapter 5 Network+ Certification, Second Edition 1. Physical layer specifications 2. Frame format 3. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Media Access Control (MAC) mechanism 2. Ethernet Types |6| A. Standard 10-Mbps Ethernet |7| B. Fast Ethernet (100 Mbps) |8| C. Gigabit Ethernet (1,000 Mbps) 3. Physical Layer Specifications |9| A. Coaxial Ethernet standards 1. 10Base5 (thick Ethernet) 2. 10Base2 (thin Ethernet) |10| B. Coaxial network characteristics 1. Runs at 10 Mbps 2. Uses the bus topology |11| 3. Uses mixing segments (cable segments that connect more than two computers) 4. 10Base5 a. Also called thick Ethernet or ThickNet b. Uses RG-8 coaxial cable c. 500-meter maximum segment length 5. 10Base2 a. Also called thin Ethernet, ThinNet, or Cheapernet b. Uses RG-58 coaxial cable c. 185-meter maximum segment length |12| C. UTP Ethernet standards 1. 10Base-T 2. 100Base-TX (Fast Ethernet) 3. 100Base-T4 (Fast Ethernet) 4. 1000Base-T (Gigabit Ethernet) |13| D. UTP network characteristics 1. The most popular type of cable used on Ethernet networks today a. Easy to install b. Can be upgraded 2. Runs at various speeds up to 1,000 Mbps 3. Uses the star topology 4. Requires a hub |14| 5. Uses link segments 6. 10Base-T a. Uses Category 3 unshielded twisted pair (UTP) cable b. Uses 2 wire pairs only c. Uses the star topology Outline, Chapter 5 3 Network+ Certification, Second Edition d. Uses link segments up to 100 meters long e. Runs at 10 Mbps 7. 100Base-TX (Fast Ethernet) a. Runs at 100 Mbps b. Requires Category 5 UTP cable (current industry standard) c. Uses two wire pairs only 8. 100Base-T4 (Fast Ethernet) a. Runs at 100 Mbps b. Uses Category 3 UTP cable c. Uses all four wire pairs: one for transmitting, one for receiving, and two bidirectional 9. 1000Base-T (Gigabit Ethernet) a. Runs at 1,000 Mbps b. Requires Category 5 UTP cable, but Category 5E or better is preferred c. Uses all four wire pairs d. Uses a signaling scheme called Pulse Amplitude Modulation-5 (PAM-5) e. 1000Base-T is a Gigabit Ethernet standard that is specially designed to accommodate existing Category 5 cable installations. |15| E. Fiber optic Ethernet 1. Fiber Optic Inter-Repeater Link (FOIRL) a. Included in the DIX Ethernet II standard b. The original fiber optic Ethernet solution 2. IEEE 802.3 fiber optic standards a. 10Base-FL b. 10Base-FB c. 10Base-FP d. The 10-Mbps fiber optic standards were rarely used. (1) FDDI was more common until the Fast Ethernet fiber standard arrived. 3. 100Base-FX a. Uses the same hardware as 10Base-FL b. Runs at 100 Mbps 4. Gigabit Ethernet standards a. Runs at 1,000 Mbps b. Uses various types of singlemode and multimode fiber optic cable 4. Cabling Guidelines A. Ethernet standards include guidelines for cabling the network. 1. Repeaters extend the length that signals can travel over the network. 2. Ethernet limits the number of repeaters to prevent the distances that signals travel from growing too long. 3. Ethernet standards also define the maximum length for a cable segment. 4 Outline, Chapter 5 Network+ Certification, Second Edition 4. Segments that are too long can prevent the Ethernet collision detection mechanism from functioning. |16| B. 5-4-3 rule for using repeaters 1. A standard Ethernet network can have no more than five segments, connected by four repeaters. a. No more than three of the segments can be mixing segments. |17| 2. Coaxial networks can consist of five cable segments, but only three of those segments can be populated with computers. |18| 3. UTP networks consist of all link segments. 4. These guidelines are only rules of thumb. a. The Ethernet standards contain a much more complex and precise method for calculating the maximum allowable segment lengths and hub configurations. b. In practice, you can exceed the maximum limits slightly on a standard Ethernet network and still expect it to function properly. c. On Fast Ethernet and Gigabit Ethernet networks, however, there is far less flexibility built into the guidelines, and the maximums should generally not be exceeded. C. Fast Ethernet guidelines |19| 1. The Fast Ethernet standard defines two types of hubs: a. Class I. Connects different types of cable segments b. Class II. Connects cable segments of the same type |20| 2. Class I hubs a. Maximum of one hub on the network b. Total cable length (for all segments): 200 meters for UTP cable and 272 meters for fiber optic cable 3. Class II hubs a. Maximum of two hubs on the network b. Total cable length (for all segments): 205 meters for UTP cable and 228 meters for fiber optic cable D. Gigabit Ethernet guidelines 1. Only one repeater is permitted. 5. The Ethernet Frame |21| A. Ethernet frame format 1. Preamble (7 bytes). Contains 7 bytes of alternating 0s and 1s, which the communicating systems use to synchronize their clock signals 2. Start Of Frame Delimiter (1 byte). Contains 6 bits of alternating 0s and 1s, followed by two consecutive 1s, which is a signal to the receiver that the transmission of the actual frame is about to begin 3. Destination Address (6 bytes). Contains the 6-byte hexadecimal address of the network interface adapter on the LAN that the packet will be transmitted to 4. Source Address (6 bytes). Contains the 6-byte hexadecimal address of the network interface adapter in the system generating the packet Outline, Chapter 5 5 Network+ Certification, Second Edition 5. Ethertype/Length (2 bytes). a. In the DIX Ethernet frame, this field contains a code identifying the network layer protocol that the data in the packet is intended for. b. In the IEEE 802.3 frame, this field specifies the length of the data field (excluding the pad). 5. Data And Pad (46 to 1500 bytes). Contains the data received from the network layer protocol on the transmitting system, which is sent to the same protocol on the destination system a. Ethernet frames (including the header and footer, except for the Preamble and Start Of Frame Delimiter) must be at least 64 bytes long. b. If the data received from the network layer protocol is less than 46 bytes, the system adds padding bytes to bring it up to its minimum length. 6. Frame Check Sequence (4 bytes). The frame’s footer is a single field that comes after the network layer protocol data and contains a 4-byte checksum value for the entire packet. a. The sending computer computes this value and places it into the field. b. The receiving system performs the same computation and compares it to the field to verify that the packet was transmitted without error. B. Ethernet frame characteristics 1. Regular, Fast, and Gigabit Ethernet all use the same frame. 2. Only data-link layer protocols use a footer as well as a header. C. Ethernet addressing 1. Ethernet uses the 6-byte hardware (or MAC) addresses coded into network interface adapters. 2. A hardware address consists of a 3-byte, organizationally unique identifier (OUI), assigned by the IEEE, plus a unique 3-byte value assigned by the manufacturer. 3. The destination address field in an Ethernet packet always refers to a system on the same LAN that provides access to the destination link. |22| D. Ethertypes 1. DIX Ethernet frames use the Ethertype field. 2. Identify the protocol that generated the data in the frame 3. IEEE 802.3 frames contain a length field in place of the Ethertype field. E. IEEE protocol identification 1. IEEE 802.3 frames use the Logical Link Control (LLC) and Subnetwork Access Protocol (SNAP) subheaders. 2. The IEEE documents split the data-link layer into two sublayers: a. The MAC layer contains the elements defined in IEEE 802.3. b. The Logical Link Control (LLC) sublayer is defined in IEEE 802.2. 3. The LLC sublayer a. IEEE 802.2 defines an additional subheader, carried in the frame’s Data field. b. The LLC subheader is 3 or 4 bytes long. 6 Outline, Chapter 5 Network+ Certification, Second Edition c. The LLC subheader contains service access points (SAPs) that specify locations in memory where the source and destination systems store the packet data. d. To identify the protocol that generated the data in the frame, the LLC adds a 5-byte Subnetwork Access Protocol (SNAP) subheader that contains the same type of code as an Ethertype. 6. CSMA/CD |23| A. What is Media Access Control (MAC)? 1. The mechanism that enables computers on a shared network medium to transmit their data in an orderly fashion a. If two computers in a shared network medium transmit packets simultaneously, a collision occurs. b. MAC prevents collisions from occurring. B. All Ethernet systems use Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as their MAC mechanism. |24| C. CSMA/CD phases 1. Carrier sense. A computer listens to the network before transmitting, to see if it is in use. a. If the network is busy, the computer waits and tries again. 2. Multiple access. All of the computers on the network contend for access to the same network medium. 3. Collision detection. While transmitting, a computer checks to see if a collision has occurred. a. If a collision occurs, the computers involved in the collision wait and then retransmit the packets. b. The wait is called a backoff period. c. The length of the wait is calculated using the truncated binary exponential backoff algorithm. d. The collision detection process varies depending on the type of network medium. For example: (1) A computer on a UTP network assumes that a collision has occurred when it detects a signal on its transmit and receive wire pairs at the same time. (2) On a coaxial network, the computers check for a voltage spike. |25| D. Collisions 1. Also called signal quality errors 2. Are normal and expected on Ethernet networks 3. The frequency of collisions increases as network traffic increases, thereby decreasing the efficiency of the network. 4. Late collisions occur after the transmission is completed and are a sign of a serious problem. Outline, Chapter 5 7 Network+ Certification, Second Edition Chapter 5, Lesson 2 Token Ring 1. Token Ring Standards A. Originally designed by IBM B. Standardized as IEEE 802.5 2. Token Ring Characteristics A. Less commonly used than Ethernet B. Requires hardware that is more expensive than Ethernet hardware C. Topology 1. Uses the logical ring topology 2. Uses the physical star topology |26| 3. Physical Layer Specifications A. Include two cable types: 1. IBM Type 1. Proprietary STP 2. IBM Type 3. Standard Category 5 UPT |27| B. IBM Type 1 cable characteristics 1. Proprietary cables developed by IBM as the original medium for Token Ring 2. Shielded twisted pair (STP) 3. Uses a proprietary connector called the IBM data connector (IDC) or the Universal Data Connector (UDC) 4. Cables connecting a computer to a multistation access unit (MAU) are called lobe cables. a. Lobe cables can be up to 300 meters long. 5. Cables connecting MAUs together are called patch cables. 6. Supports up to 260 workstations 7. Drawbacks a. Cable is thick and inflexible. b. Difficult to install c. Requires a port initialization “key” 8. Rarely used today C. IBM Type 3 cable characteristics 1. Standard Category 5 UTP cable with RJ-45 connectors 2. Easy to install 3. Does not require an initialization key 4. Spans only 150 meters 5. Supports up to 72 workstations |28| 4. Token Passing A. Is the MAC mechanism for Token Ring and other networks B. Uses a separate frame type called a token C. The token circulates continuously around the ring. 8 Outline, Chapter 5 Network+ Certification, Second Edition 1. Gives each system on the network an equal opportunity to transmit data D. Only the computer holding the token can transmit data. E. There are no collisions on a Token Ring network. F. Token passing is better suited to high-traffic networks, because traffic does not degrade performance. G. After a computer transmits a data frame, the frame circulates around the ring and is eventually removed from the ring by the computer that transmitted it. |29| 5. Token Ring Frames A. Token Ring uses four types of frames: data frame, token frame, command frame, and abort delimiter frame. B. The data frame 1. Used to carry network layer protocol information across the network 2. Operates at the LLC sublayer |30| 3. Data frame format a. Start Delimiter (1 byte). Contains a bit pattern that signals the beginning of the frame to the receiving system b. Access Control (1 byte). Contains bits that can be used to prioritize Token Ring transmissions, enabling certain systems to have priority access to the token frame and the network c. Frame Control (1 byte). Contains bits that specify whether the frame is a data or a command frame d. Destination Address (6 bytes). Contains the 6-byte hexadecimal address of the network interface adapter on the local network to which the packet will be transmitted e. Source Address (6 bytes). Contains the 6-byte hexadecimal address of the network interface adapter in the system generating the packet f. Information (up to 4500 bytes). Contains the data generated by the network layer protocol, including a standard LLC header, as defined in IEEE 802.2 g. Frame Check Sequence (4 bytes). Contains a 4-byte checksum value for the packet (excluding the Start Delimiter, End Delimiter, and Frame Status fields) that the receiving system uses to verify that the packet was transmitted without error h. End Delimiter (1 byte). This field contains a bit pattern that signals the end of the frame, including a bit that specifies if there are further packets in the sequence yet to be transmitted and a bit that indicates that the packet has failed the error check. i. Frame Status (1 byte). Contains bits that indicate whether the destination system has received the frame and copied it into its buffers C. The token frame 1. Used to control access to the network medium |31| 2. Token frame format a. Start Delimiter (1 byte) Outline, Chapter 5 9 Network+ Certification, Second Edition b. Access Control (1 byte) c. End Delimiter (1 byte) D. The command frame 1. Also called the MAC frame 2. Operates at the MAC sublayer 3. Performs control functions only; carries no data 4. Uses the same basic format as the data frame E. The abort delimiter frame 1. Used to flush all data from the ring when a problem occurs 2. Abort delimiter frame format a. Start Delimiter (1 byte) b. End Delimiter (1 byte) Chapter 5, Lesson 3 FDDI |32| 1. FDDI Characteristics A. Pronounced “fiddy” B. Standardized by the American National Standards Institute (ANSI) C. First commercial protocol to run at 100 Mbps over fiber optic cable D. Used primarily on backbone networks E. A copper-based version (CDDI) intended to run to the desktop never achieved great acceptance. F. Uses the token passing MAC mechanism 2. The FDDI Physical Layer A. Supports both singlemode and multimode cable 1. Includes 62.5/125 multimode, which supports segments up to 100 kilometers |33| B. Double ring topology 1. The network is physically cabled as a ring. 2. Two separate rings with traffic running in opposite directions a. The second ring provides fault tolerance. b. When there is a cable fault in one ring, the transmitting computer switches to the other. 3. A computer connected to both rings is called a dual attachment station (DAS). 4. A double ring network in which both rings are in use due to a fault is called a wrapped ring. 5. Best suited to a backbone network C. Logical ring topology 1. Uses a hub called a dual attachment concentrator (DAC) 2. The DAC implements a logical (single) ring internally, much like a Token Ring MAU. 10 Outline, Chapter 5 Network+ Certification, Second Edition 3. A computer connected to a DAC is called a single attachment station (SAS). 4. Best suited for desktop connections |34| D. Dual ring of trees 1. Enterprise network constructed by connecting multiple DACs to a double ring backbone 2. Servers requiring fault tolerance are connected to the double ring. 3. Workstations are connected to a DAC. 3. The FDDI Frames |35| A. The data frame is the most common type. 1. Used to transmit network layer data across the network 2. The data frame format a. Preamble (PA, 8 bytes). Contains series of alternating 0s and 1s; used for clock synchronization b. Starting Delimiter (SD, 1 byte). Indicates the beginning of the frame c. Frame Control (FC, 1 byte). Indicates the type of data found in the Data field d. Destination Address (DA, 6 bytes). Specifies the hardware address of the computers that will receive the frame e. Source Address (SA, 6 bytes). Specifies the hardware address of the system sending the frame f. Data (variable). Contains network layer protocol data, or an SMT header and data, or MAC data, depending on the function of the frame g. Frame Check Sequence (FCS, 4 bytes). Contains a cyclical redundancy check (CRC) value, used for error detection h. Ending Delimiter (ED, 4 bits). Indicates the end of the frame i. End of Frame Sequence (FS, 12 bits). Contains three indicators that may be modified by intermediate systems when they retransmit the packet B. The token frame 1. The token frame format a. Preamble (PA, 8 bytes). Contains series of alternating 0s and 1s; used for clock synchronization b. Starting Delimiter (SD, 1 byte). Indicates the beginning of the frame c. Frame Control (FC, 1 byte). Indicates the type of data found in the Data field d. Ending Delimiter (ED, 4 bits). Indicates the end of the frame C. The station management frame 1. Responsible for ring maintenance and network diagnostics Outline, Chapter 5 11 Network+ Certification, Second Edition Chapter 5, Lesson 4 Wireless Networking 1. History of Wireless Networking A. Wireless products have been available for several years. 1. Early wireless products were slow and unreliable. 2. Limitations were tolerated in situations where wireless communication was required. B. Today, wireless networks are much faster and more reliable. 1. However, they are not likely ever to be as fast or reliable as a cabled LAN. |36| 2. IEEE 802.11 Standards A. IEEE 802.11 is the dominant wireless LAN standard today. 1. IEEE 802.11a. Speeds of 1 to 2 Mbps 2. IEEE 802.11b. Speeds of 5.5 to 11 Mbps 3. The IEEE 802.11 Physical Layer |37| A. Supports two topologies: ad hoc and infrastructure B. Ad hoc topology (also called independent topology) 1. Wireless computers communicate with each other only, on a peer-to- peer basis. 2. Intended for home or small office networks C. Infrastructure topology 1. Wireless computers communicate with an access point connected to a cabled network. a. The access point enables the wireless devices to communicate with the cabled network. b. The access point can be a dedicated device or a computer with both wireless and wired network interface adapters. c. One access point can typically support 10 to 20 wireless computers at a maximum range of 100–200 feet. 2. The wireless range can be extended by using an extension point (a wireless signal repeater that functions as a relay station). D. Wireless LANs can use three types of signals: 1. Direct Sequence Spread Spectrum (DSSS) a. Most commonly used type of signal 2. Frequency Hopping Spread Spectrum (FHSS) 3. Infrared 4. The IEEE 802.11 MAC Layer A. IEEE 802.11 splits the data-link layer into the LLC and MAC sublayers. B. LLC is defined by IEEE 802.2. C. The MAC mechanism is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). |38| D. CSMA/CA is a variation of CSMA/CD. 12 Outline, Chapter 5 Network+ Certification, Second Edition 1. The standard Ethernet collision detection is impractical because it would require full-duplex communications. 2. On a CSMA/CA network, systems transmit acknowledgment messages to indicate that no collision has occurred. a. If a computer does not receive an acknowledgment to its transmissions, it retransmits the packet automatically. |39| Chapter Summary A. Ethernet 1. Supports multiple cable types and speeds 2. Uses the CSMA/CD MAC mechanism to detect collisions B. Token Ring 1. Uses a logical ring topology 2. Uses the token passing MAC mechanism C. FDDI 1. Uses a double ring or star topology 2. Uses the token passing MAC mechanism D. Wireless networking 1. Runs at speeds up to 11 Mbps 2. Uses the CSMA/CA MAC mechanism