Continue the series of Abstract CCNA study guide book .
Ethernet Networking
Ethernet is a contention media access method that allows all hosts on a network to share the same bandwidth of a link.
Ethernet uses both Data Link and Physical layer specifications, and this section of the chapter will give you both the Data Link layer and Physical layer information you need to effectively implement, troubleshoot,and maintain an Ethernet network.
Ethernet networking uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium. CSMA/CD was created to overcome the problem of those collisions that occur when packets are transmitted simultaneously from different nodes.
Only bridges and routers can effectively prevent a transmission from propagating throughout the entire network!
When a host wants to transmit over the network, it first checks for the presence of a digital signal on the wire. If all is clear (no other host is transmitting), the host will then proceed with its transmission. But it doesn’t stop there. The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting. If the host detects another signal on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop sending data (think busy signal). The nodes respond to that jam signal by waiting a while before attempting to transmit again.
Backoff algorithms determine when the colliding stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting to transmit will then timeout.
When a collision occurs on an Ethernet LAN, the following happens:
- A jam signal informs all devices that a collision occurred.
- The collision invokes a random backoff algorithm.
- Each device on the Ethernet segment stops transmitting for a short time until the timers expire.
- All hosts have equal priority to transmit after the timers have expired.
The following are the effects of having a CSMA/CD network sustaining heavy collisions:
- Delay
- Low throughput
- Congestion
Half- and Full-Duplex Ethernet
Half-duplex Ethernet is defined in the original 802.3 Ethernet; Cisco says it uses only one wire pair with a digital signal running in both directions on the wire. It also uses the CSMA/CD protocol to help prevent collisions and to permit retransmitting if a collision does occur.
If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions.
Half-duplex Ethernet—typically 10BaseT—is only about 30 to 40 percent efficient as Cisco sees it because a large 10BaseT network will usually only give you 3 to 4Mbps, at most.
full-duplex Ethernet uses two pairs of wires instead of one wire pair like half duplex.
And full duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device. And because the transmitted data is sent on a different set of wires than the received data, no collisions will occur.
Full-duplex Ethernet is supposed to offer 100 percent efficiency in both directions—for example, you can get 20Mbps with a 10Mbps Ethernet running full duplex or 200Mbps for Fast Ethernet.
Full-duplex Ethernet can be used in three situations:
- With a connection from a switch to a host
- With a connection from a switch to a switch
- With a connection from a host to a host using a crossover cable
Full-duplex Ethernet requires a point-to-point connection when only two nodes are present. You can run full-duplex with just about any device except a hub.
when a full-duplex Ethernet port is powered on, it first connects to the remote end and then negotiates with the other end of the Fast Ethernet link. This is called an auto-detect mechanism.
This mechanism first decides on the exchange capability, which means it checks to see if it can run at 10 or 100Mbps.
It then checks to see if it can run full duplex, and if it can’t, it will run half duplex.
Lastly, remember these important points:
- There are no collisions in full-duplex mode.
- A dedicated switch port is required for each full-duplex node.
- The host network card and the switch port must be capable of operating in full-duplex mode.
Ethernet at the Data Link Layer
Ethernet at the Data Link layer is responsible for hardware addressing or MAC addressing.
Ethernet is also responsible for framing packets received from the Network layer and preparing them for transmission on the local network
Ethernet Addressing
Ethernet uses the Media Access Control (MAC) address burned into each and every Ethernet network interface card (NIC). The MAC address is a 48-bit (6-byte) address written in a hexadecimal format.
Next Figure shows the 48-bit MAC addresses and how the bits are divided.
The organizationally unique identifier (OUI) is assigned by the IEEE to an organization.
It’s composed of 24 bits, or 3 bytes. The organization, in turn, assigns a globally administered address (24 bits, or 3 bytes) that is unique to each and every adapter it manufactures.
Look at the figure. The high-order bit is the Individual/Group (I/G) bit. When it has a value of 0, then the address is the MAC address of a device and when it is a 1, we can assume that the address represents either a broadcast or multicast address.
The next bit is the global/local bit, or just G/L bit, when set to 0, this bit represents a globally administered address (as by the IEEE). When the bit is a 1, it represents a locally governed and administered address.
The low-order 24 bits of an Ethernet address represent a locally administered or manufacturer assigned
code. This portion commonly starts with 24 0s for the first card made and continues in order until there are 24 1s for the last (16,777,216th) card made.
Ethernet Frames
Frames are used at the Data Link layer to encapsulate packets handed down from the Network layer for transmission on a type of media access.
The 802.3 frames and Ethernet frame are shown in Figure 1.20.
Next FIGURE 802.3 and Ethernet frame formats
Following are the details of the different fields in the 802.3 and Ethernet frame types:
Preamble An alternating 1,0 pattern provides a 5MHz clock at the start of each packet.
Start Frame Delimiter (SFD)/Synch The preamble is seven octets and the SFD is one octet (synch). The SFD is 10101011, detect the beginning of the data.
Destination Address (DA) The MAC address this is used by receiving stations to determine whether an incoming packet is addressed to a particular node. The destination address can be an individual address or a broadcast or multicast MAC address. Remember that a broadcast is all 1s (or Fs in hex) and
is sent to all devices but a multicast is sent only to a similar subset of nodes on a network.
Source Address (SA):The MAC address used to identify the transmitting device.
Length or Type: 802.3 uses a Length field, but the Ethernet frame uses a Type field to identify the Network layer protocol. 802.3 cannot identify the upper-layer protocol and must be used with a proprietary LAN—IPX, for example.
Data: a packet sent down to the Data Link layer from the Network layer (size varies from 64 to 1,500 bytes).
Frame Check Sequence (FCS): FCS is a field at the end of the frame that’s used to store the CRC.
Ethernet at the Physical Layer
Here are the original IEEE 802.3 standards:
10Base2 10Mbps, baseband technology, up to 185 meters in length. Known as thinnet and can support up to 30 workstations on a single segment. Uses a physical and logical bus with AUI connectors. The 10 means 10Mbps, Base means baseband technology , and the 2 means almost 200 meters. 10Base2 Ethernet cards use BNC and T-connectors to connect to a network.
10Base5 10Mbps, baseband technology, up to 500 meters in length. Known as thicknet. Uses a physical and logical bus with AUI connectors. Up to 2,500 meters with repeaters and 1,024 users for all segments.
10BaseT 10Mbps using category 3 UTP wiring. each device must connect into a hub or switch, and you can have only one host per segment or wire. Uses an RJ45 connector (8-pin modular connector) with a physical star topology and a logical bus.
Here are the expanded IEEE Ethernet 802.3 standards:
100BaseTX (IEEE 802.3u):category 5, 6, or 7 UTP two-pair wiring. One user per segment; up to 100 meters long. It uses an RJ45 connector with a physical star topology and a logical bus.
100BaseFX (IEEE 802.3u): Uses fiber cabling 62.5/125-micron multimode fiber. Point-to-point topology; up to 412 meters long. It uses an ST or SC connector.
1000BaseCX (IEEE 802.3z): Copper twisted-pair called twinax (a balanced coaxial pair) that can only run up to 25 meters.
1000BaseT (IEEE 802.3ab): Category 5, four-pair UTP wiring up to 100 meters long.
1000BaseSX (IEEE 802.3z): MMF using 62.5- and 50-micron core; uses an 850 nano-meter laser and can go up to 220 meters with 62.5-micron, 550 meters with 50-micron.
1000BaseLX (IEEE 802.3z) Single-mode fiber that uses a 9-micron core and 1300 nanometer laser and can go from 3 kilometers up to 10 kilometers.
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