Curs PC si internet cap 7.4 Layer 2 Devices

7.4 Layer 2 Devices 7.4.1 NICs Instructor Note The purpose of this target indicator is to have the students give a detailed description of a 10Base-T NIC. In Chapter 3, all the devices were introduced. The purpose here is to go deeper, both for understanding and retention. Note that the half-duplex diagram indicates some actual electrical circuits used in sending signals; much of this detail has not been previously introduced. Relate this diagram to the actual pinouts used on the RJ-45 terminations and the actual wires in a Cat 5 cable: pin 1 is TD+ (transmit data), pin 2 is TD (transmit data), pin 3 is RD + (receive data), pin 6 is RD (receive data) and pins 4, 5, 7, and 8 are unused. A network interface card (NIC) plugs into a motherboard and provides ports for network connection. This card can be designed as an Ethernet card, a Token Ring card, or an FDDI card. Network cards communicate with the network through serial connections, and with the computer through parallel connections. They are the physical connections from workstations to the network. Network cards all require an IRQ, an I/O address, and upper memory addresses for DOS and Windows 95/98. When selecting a network card, consider the following three factors: type of network (e.g. Ethernet, Token Ring, FDDI, or other) type of media (e.g. twisted-pair, coaxial, or fiber-optic cable) type of system bus (e.g. PCI and ISA) 7.4 Layer 2 Devices 7.4.2 NIC Layer 2 operations Instructor Note The purpose of this target indicator is to explain how a NIC works in more detail and to make plausible its classification as primarily a Layer 2 and Layer 1device. The NIC has a MAC address burned in. Hence it is where the network hardware data link layer name of the computer resides. The NIC circuitry handles the transition from the upper level encapsulated data to a frame of bits ready to be put on the networking medium. The NICs circuitry is where the MAC algorithm resides. And the NIC handles the placing of bits on the media; in other words, the job of signaling. NICs perform important Layer 2 data link layer functions, such as the following: 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 7.4 Layer 2 Devices 7.4.3 Bridges Instructor Note The purpose of this target indicator is to have the students give a detailed description of a bridge. In Chapter 3, all the devices were introduced. The purpose here is to go deeper, both for understanding and retention. While bridges themselves are becoming less common as stand-alone networking devices, the concept of bridging is extremely important in understanding switching and routing. A bridge connects network segments and must make intelligent decisions about whether to pass signals on to the next segment. A bridge can improve network performance by eliminating unnecessary traffic and minimizing the chances of collisions. The bridge divides traffic into segments and filters traffic based on the station or MAC address.Bridges are not complicated devices. They analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames toward the destination. Bridges are only concerned with passing packets, or not passing packets, based on their destination MAC address. Bridges often pass packets between networks operating under different Layer 2 protocols. View the Figures - to learn the important properties of bridges. 7.4 Layer 2 Devices 7.4.4 Bridge Layer 2 operations Instructor Note The purpose of this target indicator is to explain how a bridge works in more detail and to make plausible its classification as primarily a Layer 2 device. Have the students act out the bridges filtering properties in a kinesthetic activity. For example, use five students one student in the middle playing the role of a bridge, 2 students on the left portraying hosts on one network segment, and 2 students on the right portraying hosts on a different network segment. Label the hosts with MAC addresses A, B, C, D. Have the bridge create a bridging table, which shows each bridge interface and the MAC addresses of the hosts that are accessible through that interface. Have the hosts send frames to each other and have the bridge person explain their filtering and forwarding decisions. Bridging occurs at the data link layer, which controls data flow, handles transmission errors, provides physical addressing, and manages access to the physical medium. Bridges provide these functions by using various link layer protocols that dictate specific flow control, error handling, addressing, and media access algorithms. Examples of popular data link layer protocols include Ethernet, Token Ring, and FDDI.Upper-layer protocol transparency is a primary advantage of bridging. Because bridges operate at the data link layer, they are not required to examine upper-layer information. This means that they can rapidly forward traffic representing any network-layer protocol. It is not uncommon for a bridge to move protocols and other traffic between two network segments.Bridges are not required to examine upper-layer information because they operate at the data link layer or Layer 2 of the OSI model. Bridges filter network traffic by only looking at the MAC address, not protocols. It is not uncommon for a bridge to move protocols and other traffic between two or more network segments. Because bridges only look at MAC addresses, they can rapidly forward traffic representing any network-layer protocol. To filter or selectively deliver network traffic, a bridge builds tables of all MAC addresses located on their directly connected network segments. If data comes along the network media, a bridge compares the destination MAC address carried by the data to MAC addresses contained in its tables. If the bridge determines that the destination MAC address of the data is from the same network segment as the source, it does not forward the data to other segments of the network. - If the bridge determines that the destination MAC address of the data is not from the same network segment as the source, it forwards the data to the appropriate segment. - By doing this, bridges can significantly reduce the amount of traffic between network segments by eliminating unnecessary traffic. View the Figures - to see how bridges handle local network traffic. In contrast, view Figures - to see how bridges handle non-local network traffic.Bridges are internetworking devices that can be used to reduce large collision domains. Collision domains are areas where packets are likely to interfere with each other. They do this by dividing the network into smaller segments and reducing the amount of traffic that must be passed between the segments. Bridges operate at Layer 2 or the data link layer of the OSI model, because they are only concerned with MAC addresses. As data is passed along the network on its way to a destination, it is picked up and examined by every device on the network including bridges. Bridges work best where traffic is low from one segment of a network to other segments. When traffic between network segments becomes heavy, bridges can become a bottleneck and slow down communication. There is another potential problem with using a bridge. Bridges always spread and multiply a special kind of data packet. These data packets occur when a device on a network wants to reach another device on the network, but does not know the destination address of the device. When this occurs, frequently the source sends out a broadcast to all devices on a network. Since every device on the network has to pay attention to such broadcasts, bridges always forward them. If too many broadcasts are sent out over the network a broadcast storm can result. A broadcast storm can cause network time-outs, traffic slowdowns, and the network to operate at less than acceptable performance. 7.4 Layer 2 Devices 7.4.5 Switches Instructor Note The purpose of this target indicator is to have the students give a detailed description of a switch. In Chapter 3, all the devices were introduced. The purpose here is to go deeper, both for understanding and retention. Switching is a technology that alleviates congestion, in Ethernet LANs, by reducing traffic and increasing bandwidth. Switches, also referred to as LAN switches, often replace shared hubs and work with existing cable infrastructures to ensure they are installed with minimal disruption of existing networks. Today, in data communications, all switching and routing equipment perform two basic operations:switching data frames -- This is a store-and-forward operation in which a frame arrives on an input media and is transmitted to an output media. maintenance of switching operations -- Switches build and maintain switching tables and search for loops. Routers build and maintain both routing tables and service tables. Like bridges, switches connect LAN segments, use a table of MAC addresses to determine the segment on which a datagram needs to be transmitted, and reduce traffic. Switches operate at much higher speeds than bridges, and can support new functionality, such as virtual LANs. An Ethernet switch has many benefits, such as allowing many users to communicate in parallel through the use of virtual circuits and dedicated network segments in a collision-free environment. This maximizes the bandwidth available on the shared medium. Another benefit is that moving to a switched LAN environment is very cost effective because existing hardware and cabling can be reused. Finally, network administrators have great flexibility in managing the network through the power of the switch and the software to configure the LAN. 7.4 Layer 2 Devices 7.4.6 Switch Layer 2 operations Instructor Note Have the students act out the switches filtering and forwarding properties in a kinesthetic activity. For example, use five students one student in the middle playing the role of a switch, 2 students on the left portraying hosts on one subnet, and 2 students on the right portraying host on a different subnet. Label the hosts with MAC addresses A, B, C, D. Have the switch create a switching table, which shows each switch interface and the MAC addresses of the hosts that are accessible through that interface. Have the hosts send frames to each other and have the switch person explain their filtering and forwarding decisions. LAN switches are considered multi-port bridges with no collision domain, because of microsegmentation. Data is exchanged at high speeds by switching the packet to its destination. By reading the destination MAC address Layer 2 information, switches can achieve high-speed data transfers, much like a bridge does. The packet is sent to the port of the receiving station prior to the entire packet entering the switch. This leads to low latency levels and a high rate of speed for packet forwarding. Ethernet switching increases the bandwidth available on a network. It does this by creating dedicated network segments, or point-to-point connections, and connecting these segments in a virtual network within the switch. This virtual network circuit exists only when two nodes need to communicate. This is called a virtual circuit because it exists only when needed, and is established within the switch. Even though the LAN switch reduces the size of collision domains, all hosts connected to the switch are still in the same broadcast domain, therefore a broadcast from one node will still be seen by all other nodes connected through the LAN switch. Switches are data link layer devices that, like bridges, enable multiple physical LAN segments to be interconnected into single larger network. Similar to bridges, switches forward and flood traffic based on MAC addresses. Because switching is performed in hardware instead of in software, it is significantly faster. You can think of each switch port as a micro-bridge; this process is called microsegmentation. Thus each switch port acts as a separate bridge and gives the full bandwidth of the medium to each host.