| A | B |
|---|
| MAC | Media Access Control |
| LLC | Logical Link Control |
| Framing | Layer 2 organization of bits |
| 802.3 | IEEE designation for Ethernet |
| 802.5 | IEEE designation for Token Ring |
| IEEE designations for Layer 2 | LLC & MAC |
| PDU | Protocol Data Unit |
| MAC address | On NIC card |
| IEEE 802.2 specification | LLC - Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP). |
| 4 main concepts of Layer 2 | Layer 2 communicates with the upper-level layers through Logical Link Control (LLC). |
| Base 16 numbering system | hexadecimal (hex) |
| Number of bits in MAC address | 48 |
| OUI | Organizational Unique Identifier - first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor |
| BIAs | burned-in addresses - burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes. |
| sixteen symbols of HEX | 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. |
| remainder method | repeatedly divide the decimal number by the base number (in this case 16). We then convert the remainder each time into a hex number |
| Broadcast Networks | All stations see all frames. Each station must examine every frame to determine whether that station is a destination. |
| important part of both encapsulation and decapsulation | the addition of source and destination MAC addresses. Information cannot be properly sent or delivered on a network without these addresses. |
| flat address spaces | They have no structure |
| Framing | which computers are communicating with one another. Framing is the Layer 2 encapsulation process; a frame is the Layer 2 protocol data unit. |
| frame format diagram | based on voltage versus time graphs. You read them from left to right, just like an oscilloscope graph. The frame format diagram shows different groupings of bits (fields) that perform other functions. |
| Frame fields | frame start field |
| Frame Check Sequence | a number that is calculated by the source computer and is based on the data in the frame. When the destination computer receives the frame, it recalculates the FCS number and compares it 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. |
| three primary ways to calculate the Frame Check Sequence number | cyclic redundancy check(CRC)- performs polynomial calculations on the data |
| deterministic | taking turns |
| non-deterministic | first come, first served |
| first-come, first-served (FCFS) | eventually became a modern MAC protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). |
| Three common Layer 2 technologies | 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) and FDDI - logical ring topology (information flow is controlled in a ring) and physical dual-ring topology (wired as a dual-ring) |
| Tokens | Tokens are 3 bytes in length and consist of a start delimiter, an access control byte, and an end delimiter. |
| start delimiter | alerts each station to the arrival of a token, or data/command frame |
| Access Control Byte | contains the priority and reservation field, and a token and monitor bit. The token bit distinguishes a token from a data/command frame, and a monitor bit determines whether a frame is continuously circling the ring. The end delimiter signals the end of the token or data/command frame. |
| frame control byte | indicates whether the frame contains data or control information |
| frame check sequence (FCS) field | The source station fills this field with a calculated value dependent on the frame contents. The destination station recalculates the value to determine whether the frame has been damaged in transit |
| Token Passing | Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit data. If a node that receives a token has no information to send, it passes the token to the next end station |
| Priority System | permits certain user-designated, high-priority stations to use the network more frequently |
| Management Mechanisms | One mechanism is to select one station in the Token Ring network to be the active monitor. This station acts as a centralized source of timing information for other ring stations and performs a variety of ring maintenance functions. The active monitor station can potentially be any station on the network. |
| Beaconing | a Token Ring formula - detects and tries to repair network faults. When a station detects a serious problem with the network (e.g. a cable break) it sends a beacon frame. The beacon frame defines a failure domain. A failure domain includes the station that is reporting the failure, its nearest active upstream neighbor (NAUN), and everything in between |
| Fiber Distributed Data Interface (FDDI) | FDDI is frequently used as a backbone technology, and to connect high-speed computers in a LAN. |
| FDDI has four specifications | 1. Media Access Control (MAC); 2. Physical Layer Protocol (PHY); 3. Physical Layer Medium (PMD); 4. Station Management (SMT) |
| FDDI frame fields | preamble; start delimiter; frame control; destination address; source address; data; frame check sequence (FCS); end delimiter; frame status |
| 2 types of FDDI traffic | Synchronous & Asynchronous |
| FDDI encoding scheme | 4B/5B. Every 4 bits of data are sent as a 5 bit code. The signal sources in FDDI transceivers are LEDs or lasers. |
| Single-mode fiber | often used for inter-building connectivity |
| multi-mode fiber | often used for intra-building connectivity |
| Ethernet Frame Format | preamble; start-of-frame; destination and source addresses; type (Ethernet); length (IEEE 802.3); data (Ethernet); data (IEEE 802.3); frame check sequence (FCS) |
| TIA/EIA-568-A | specifies that the physical layout, or topology that is to be used for horizontal cabling, must be a star topology |
| TIA/EIA-568-A max. cable length | maximum length of horizontal cabling for unshielded twisted pair cable, is 90 m; patch cords at the telecommunications outlet/connector is 3 m, and the maximum length for patch cords/jumpers at the horizontal cross-connect is 6 m. |
| Layer 2 data link layer functions | logical link control - communicates with upper layers in the computer; naming - provides a unique MAC address identifier; framing - part of the encapsulation process, packaging the bits for transport; Media Access Control (MAC) - provides structured access to shared access media; signaling - creates signals and interface with the media by using built-in transceivers |
| Bridges | connects network segments and must make intelligent decisions about whether to pass signals on to the next segment. |
| Upper-layer protocol transparency | a primary advantage of bridging. Bridges are not required to examine upper-layer information because they operate at the data link layer or Layer 2 of the OSI model |
| Switches | often replace shared hubs and work with existing cable infrastructures to ensure they are installed with minimal disruption of existing networks |
| microsegmentation | think of each switch port as a micro-bridge and gives the full bandwidth of the medium to each host. |
| latency | due to the decision making that is required of the bridge, or bridges, when transmitting data to the correct segment |
