ICND1-3-Understanding ethernet-19c.txt

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ICND1-3-Understanding ethernet-19c.txt
2013-07-25 17:23:04

ICND1-3-Understanding ethernet-19c.txt
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  1. What are local-area networks?
    Local-area networks (LANs) are high-speed, low-error data networks that cover a small geographic area. LANs are usually located in a building or campus and do not cover a large distance. They are relatively inexpensive to develop and maintain. LANs connect computers, printers, terminals, and other devices in a single building or a limited area.
  2. What are LAN standards?
    • LAN standards define the physical media and connectors used to connect to the media at the physical layer and the way devices communicate at the data link layer.
    • LAN standards encompass Layers 1 and 2 of the OSI model. Examples of LAN standards are Ethernet and IEEE 802.3.
  3. What functions does the Media Access Control (MAC) sublayer provide?
    • The MAC sublayer specifies how data is placed and transported over the physical wire.
    • It controls access to the physical medium. The LLC sublayer communicates with the network layer, but the MAC sublayer communicates downward directly to the physical layer. Physical addressing (MAC addresses), network topologies, error notification, and delivery of frames are defined at this sublayer.
  4. What does the LLC sublayer do?
    • The Logical Link Control (802.2) sublayer is responsible for identifying different network layer protocols and then encapsulating them to be transferred across the network.
    • Two types of LLC frames exist: service access points (SAP) and Subnetwork Access Protocol (SNAP). An LLC header tells the data link layer what to do with a packet after it is received.
  5. What do the Ethernet and IEEE 802.3 standards define?
    • The Ethernet and IEEE 802.3 standards define a bus-topology LAN that operates at a baseband signaling rate of 10 Mbps, referred to as 10BASE. Within the Ethernet standards are protocol specifications that define the transmission medium and access. The following three protocol specifications exist:
    • - 10BASE2: Known as thin Ethernet, this specification uses thin coaxial cable as its medium and provides access for multiple stations on the same segments.
    • - 10BASE5: Called thick Ethernet, this specification uses a thick coaxial cable as its medium. The maximum segment length of 10BASE5 is over twice that of 10BASE2.
    • - 10BASE-T: This specification provides access for a single station only, so all stations connect to a switch or hub. The physical topology of 10BASE-T is that of a star network. It uses unshielded twisted-pair (UTP) cable Category 3, 4, 5, and 5e as its network medium.
  6. Define the Fast Ethernet standard.
    • The Ethernet standard that defines Fast Ethernet is IEEE 802.3u. This standard raises the speed of the Ethernet standard of 10 Mbps to 100 Mbps with only minimal changes to the existing cable structure. The Fast Ethernet standard defines different protocol specifications depending of the physical medium used. The following are the four different Fast Ethernet specifications:
    • - 100BASE-FX: Uses two strands of multimode fiber-optic cable as its medium and has a maximum segment length of 400 meters.
    • - 100BASE-T: Defines UTP as its medium and has a maximum segment length of 100 meters.
    • - 100BASE-T4: Uses four pairs of Cat 3 to 5 UTP as its medium. It maximum segment length is 100 meters.
    • - 100BASE-TX: Specifies two pairs of UTP or shielded twisted-pair (STP) cable as its medium with a maximum segment distance of 100 meters.
  7. What does BASE mean in 10BASE-T and 100BASE-T?
    BASE in 10BASE-T and 100BASE-T refers to the baseband signaling method. Baseband is a network technology in which only one carrier frequency is used. This means that when a device transmits, it uses the entire bandwidth on the wire and does not share it. Ethernet defined baseband technology.
  8. What is Gigabit Ethernet?
    Gigabit Ethernet is an extension of the IEEE 802.3 Ethernet standard. It increases the speed of the Ethernet protocol to 1000 Mbps, or 1 Gbps. IEEE 802.3z specifies Gigabit over fiber, and IEEE 802.3ab specifies Gigabit over twisted-pair cable.
  9. What is carrier sense multiple access collision detect (CSMA/CD)?
    • CSMA/CD describes the Ethernet access method.
    • In CSMA/CD, many stations can transmit on the same cable, and no station has priority over any other. Before a station transmits, it listens on the wire (carrier sense) to make sure that no other station is transmitting. If no other station is transmitting, the station transmits across the wire. If a collision occurs, the transmitting stations detect the collision and run a backoff algorithm. The backoff algorithm is a random time that each station waits before retransmitting.
  10. What are the three ways LAN traffic is transmitted?
    • LAN traffic is transmitted one of the following three ways:
    • - Unicast: Unicasts are the most common type of LAN traffic. A unicast frame is a frame intended for only one host.
    • - Broadcast: Broadcasts frames intended for everyone. Stations view broadcast frames as public service announcements. All stations receive and process broadcast frames.
    • - Multicast: Multicasts are traffic in which one transmitter tries to reach only a subset, or group, of the entire segment.
  11. How many bits are in an Ethernet address?
    48. Also called a MAC address, an Ethernet address is the Layer 2 address associated with the Ethernet network adapter. Typically burned into the adapter, the MAC address is usually displayed in a hexadecimal format, such as 00-0d-65-ac-50-7f.
  12. What portion of the MAC address is vendor specific?
    • The first half or first 24 bits of the MAC address are vendor specific.
    • A MAC address is 48 bits and is displayed in hexadecimal. The first half of the address identifies the vendor or manufacturer of the card. This is called the Organizational Unique Identifier (OUI). The last half of the address identifies the card address
  13. What portion of the MAC address is vendor assigned?
    The last 24 bits are vendor assigned.
  14. What is UTP cabling?
    • Unshielded twisted-pair (UTP) cabling is a type of twisted-pair cable that relies solely on the cancellation effects produced by the twisted wire pairs to limit electromagnetic interference (EMI) and radio frequency interference (RFI).
    • UTP cable is often installed using an RJ-45 connector, and UTP cabling must follow precise specifications dictating how many twists are required per meter of cable. The advantages of UTP are ease of installation and low cost. A disadvantage of UTP is that it is more prone to EMI than other types of media.
  15. What is the difference between STP and UTP cable?
    Shielded twisted-pair (STP) cable combines the twisting techniques of UTP, but each pair of wires is wrapped in a metallic foil. The four pairs of wires are then wrapped in a metallic braid or foil. STP reduces electrical noise and EMI. STP is installed with an STP data connector but can also use an RJ-45 connector. An advantage of STP is that it is more resistant to outside interference; a disadvantage is that it is more expensive and difficult to install.
  16. What is the maximum cable length for STP?
    100 meters or 328 feet.
  17. What is the maximum cable length for UTP?
    100 meters or 328 feet.
  18. What is a straight-through Ethernet cable, and when would you use it?
    • A straight-through Ethernet cable is wired the same way at both ends. This cable uses pins 1, 2, 3, and 6. The send and receive wires are not crossed.
    • You should use a straight-through Ethernet cable when connecting dissimilar devices (for example, data terminal equipment [DTE] to data communications equipment [DCE]). Examples include connecting PCs (DTE) to switches or hubs (DCE) or a router (DTE) to a switch or a hub (DCE).
  19. What is a crossover Ethernet cable, and when would you use it?
    • A crossover Ethernet cable is a cable that has the send and receive wires crossed at one of the ends. In a Category 5 cable, the 1 and 3 wires are switched and the 2 and 6 wires are switched at one end of the cable.
    • You should use a crossover cable when connecting similar devices (DCE to DCE), such as connecting a router to a router, a switch to a switch or hub, a hub to a hub, or a PC to a PC.