2.5 Ethernet Flashcards
Bus Topology
Ring Topology
Star Topology
A star topology uses a hub or switch to connect all network connections to a single physical location. Today it is the most popular type of topology for a LAN. With a star:
All network connections are located in a single place. This makes it easy to troubleshoot and reconfigure.
Nodes can be added to or removed from the network easily.
Cabling problems usually affect only one node.
Carrier Sent Multiple Access Collision Detection (CSMA/CD)
Mesh Topology
A mesh topology exists when there are multiple paths between any two nodes on a network. Mesh topologies are created using point-to-point connections. This increases the network’s fault tolerance because an alternate path can be used when one path fails. Two variations of mesh topologies exist:
Partial Mesh: some redundant paths exist.
Full Mesh: every node has a point-to-point connection with every other node.
Full mesh topologies are usually impractical in a standard LAN, because the number of connections increases dramatically with every new node added to the network. A separate network interface and cable for each host on the network is required. However, a full mesh topology is commonly used to interconnect routers. It provides alternate paths should one path go down or become overloaded.
Mesh networks are also commonly used to create redundant paths between access points in a wireless network. They provide alternate paths back to the wireless controller should one access point go down or become overloaded. With this topology, every access point can communicate directly with every other access point on the wireless network.
Hybrid Topology
A hybrid topology exists when two or more types of network topologies are connected with each other. For example, a network of wireless access points (mesh topology) connected to a network switch (star topology) would be considered a hybrid topology.
Media access
Ethernet uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD) to control access to the transmission medium. Devices use the following process to send data:
- Because all devices have equal access to the transmission media (multiple access), a device with data to send, first listens to the transmission medium to determine if it is free (carrier sense).
- If it is not free, the device waits a random time and listens again to the transmission medium. When it is free, the device transmits its message.
- If two devices transmit at the same time, a collision occurs. The sending devices detect the collision (collision detection) and send a jam signal.
- Both devices wait a random length of time before attempting to resend the original message. The process of waiting before attempting to resend is called a backoff .
Transmission media
Ethernet supports the following cable types:
- Unshielded twisted-pair cables (UTP) with RJ-45 connectors. This is the most common transmission medium used for Ethernet. Each cable consists of eight wires twisted into four pairs. UTP cables are classified by the following types (called categories):Type Speed
Cat5 100 Mbps
Cat5e 1000 Mbps
Cat6 10 Gbps - Fiber optic is most commonly used in high-speed applications, e.g., servers or streaming media. Fiber optic cables have ST, SC, LC, and MT-RJ connectors.
Frame type
The Ethernet frame size is 64 to 1518 bytes This is the same for all Ethernet standards. The most common frame types are:
- Ethernet 802.3 is the original Ethernet frame type.
- Ethernet 802.2 is the frame type that accommodates standards set by the IEEE 802.2 committee related to the logical link control (LLC) sublayer. It is a more current frame type than 802.3.
- Ethernet II is a frame type that provides the ability to use TCP/IP as a transport/network layer protocol.
Physical address
The MAC address (also called the burned-in address) is the Data Link layer physical device address. The MAC address is:
- A 12-digit hexadecimal number (each number ranges from 0-9 or A-F).
- Often written using hyphens (e.g., 00-B0-D0-06-BC-AC), periods, (e.g., 00B0.D006.BCAC), or colons (e.g., 00:B0:D0:06:BC:AC) to separate the address parts.
- Guaranteed unique through design. The first six digits of the MAC address is assigned to each manufacturer. The manufacturer determines the rest of the address, assigning a unique value that identifies the host address. A manufacturer that uses all the addresses in the original assignment can apply for a new MAC address assignment.
Half-duplex
In half-duplex mode:
- Collision detection is turned on.
- The device can only send or receive at any given time.
- Devices connected to a hub must use half-duplex communication.
Up to the rated bandwidth (100 Mbps for 100BaseT, 1000 Mbps for 1000BaseT, etc.)
Full-duplex
In full-duplex mode:
- Collision detection is turned off.
- The device can send and receive at the same time.
- NICs need to be full-duplex capable.
A switch with dedicated switch ports is required.
Double the rated bandwidth (200 Mbps for 100BaseT, 2000 Mbps for 1000BaseT, etc.)
Preamble
The preamble is a set of alternating ones and zeros terminated by two ones (11), which mark it as a frame.
Destination address
The destination address identifies the receiving host’s MAC address.
Source address
The source address identifies the sending host’s MAC address.
