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Lan Switching

| | Tuesday, April 14, 2009

Fast Computers Need Faster Networks

The PC emerged as the most common desktop computer in the 1980s. LANs emerged as a way to network PCs in a common location. Networking technologies such as Token Ring and Ethernet allowed users to share resources such as printers and exchange files with each other. As originally defined, Ethernet and Token Ring provided network access to multiple devices on the
same network segment or ring. These LAN technologies have inherent limitations as to how many devices can connect to a single segment, as well as the physical distance between computers. Desktop computers got faster, the number of computers grew, operating systems began multitasking (allowing multiple tasks to operate at the same time), and applications became more networkcentric. All these advancements resulted in congestion on LANs. To address these issues, two device types emerged: repeaters and bridges. Repeaters are simple Open Systems Interconnection (OSI) Layer 1 devices that allow networks to extend beyond their defined physical distances (which were limited to about 150 feet without the use of a repeater). Bridges are OSI Layer 2 devices that physically split a segment into two and reduce the amount of traffic on either side of the bridge. This setup allows more devices to connect to the LAN and reduces congestion. LAN switches emerged as a natural extension of bridging, revolutionizing the concept of local-area networking.

Switching Basics: It’s a Bridge

Network devices have one primary purpose: to pass network traffic from one segment to another. (There are exceptions, of course, such as network analyzers, which inspect traffic as it goes by.) With devices that independently make forwarding decisions, traffic can travel from its source to the destination. The higher up the OSI model a device operates, the deeper it looks into a packet to make a forwarding decision. Railroad-switching stations provide a similar example. The switches enable a train to enter the appropriate tracks (path) that take it to its final destination. If the switches are set wrong, a train can end up traveling to the wrong destination or traveling in a circle. Switching technology emerged as the replacement for bridging. Switches provide all the features of traditional bridging and more. Compared to bridges, switches provide superior throughput performance, higher port density, and lower per-port cost. The different types of bridging include the following:

• Transparent bridging primarily occurs in Ethernet networks.
• Source-route bridging occurs in Token Ring networks.
• Translational bridging occurs between different media. For example, a translational bridge might connect a Token Ring network to an Ethernet network.

Bridging and switching occur at the data link layer (Layer 2 in the OSI model), which means that bridges control data flow, provide transmission error handling, and enable access to physical media. Basic bridging is not complicated: A bridge or switch analyzes an incoming frame, determines where to forward the frame based on the packet’s header information (which contains information on the source and destination addresses), and forwards the frame toward
its destination. With transparent bridging, forwarding decisions happen one hop (or network segment) at a time. With source-route bridging, the frame contains a predetermined path to the destination. Bridges and switches divide networks into smaller, self-contained units. Because only a portion of the traffic is forwarded, bridging reduces the overall traffic that devices see on each connected network. The bridge acts as a kind of firewall in that it prevents frame-level errors from propagating from one segment to another. Bridges also accommodate communication among more devices than are supported on a single segment or ring. Bridges and switches essentially extend the effective length of a LAN, permitting more workstations to communicate with each other within a single broadcast domain. The primary difference between switches and bridges is that bridges segment a LAN into a few smaller segments. Switches, through their increased port density and speed, permit segmentation on a much larger scale. Modern-day switches used in corporate networks have hundreds of ports per chassis (unlike the four-port box connected to your cable modem).Additionally, modern-day switches interconnect LAN segments operating at different speeds. Switching describes technologies that are an extension of traditional bridges. Switches connect two or more LAN segments and make forwarding decisions about whether to transmit packets from one segment to another. When a
frame arrives, the switch inspects the destination and source Media Access Control (MAC) addresses in the packet. (This is an example of store-andforward switching.) The switch places an entry in a table indicating that the source MAC address is located off the switch interface on which the packet arrived. The bridge then consults the same table for an entry for the destination MAC address. If it has an entry for the destination MAC address, and the entry indicates that the MAC address is located on a different port from which the packet was received, the switch forwards the frame to the specified port. If the switch table indicates that the destination MAC address is located on the same interface on which the frame was just received, the bridge does not forward the frame. Why send it back onto the network segment from which item came? This decision is where a switch reduces network congestion. Finally, if the destination MAC address is not in the switch’s table, this indicates that the switch has not yet seen a frame destined for this MAC address. In this case, the switch then forwards the frames out all other ports (called flooding) except the one on which the packet was received. At their core, switches are multiport bridges. However, switches have radically matured into intelligent devices, replacing both bridges and hubs. Switches not only reduce traffic through the use of bridge tables, but also offer new functionality that supports high-speed connections, virtual LANs, and even traditional routing.

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