History of LAN Switching

Desktop computers got faster, the number of computers grew, operating systems became multitasking (allowing multiple tasks to operate at the same time), and applications became more network-centric. All these advancements resulted in the congestion of LANs.

To address these issues, two device types emerged: repeaters and bridges. Repeaters are simple Open System Interconnection (OSI) Layer 1 devices that allow networks to extend beyond their defined physical distances. 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
The term switching originally described devices that made packet-forwarding decisions in wide area networks (WANs). Today, LAN switching describes technologies that are an extension of traditional bridges.

Bridges connect two or more LAN segments and make forwarding decisions on whether to transmit packets from one segment to another. When a packet arrives, the bridge inspects the destination and source Media Access Control (MAC) addresses in the packet. (This is an example of store-and-forward switching.) It places an entry in a table indicating that the source MAC address is located off the bridge interface in 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 the MAC address is located on a different port from which the packet was received, the bridge forwards the packet onto the specified port.

If the bridge table indicates the destination MAC address is located off the interface for which it was received, then the bridge drops the packet. Why send it back onto the segment from which it came? This decision is where a bridge reduces congestion.

Finally, if the destination MAC address is not in the bridge’s table, then that indicates the bridge has not seen a packet destined to this MAC address. The bridge then forwards the packets out all other ports (called flooding) except the one in 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 support high-speed connections, virtual LANs (to be explained shortly), and even traditional routing.

Switching Ethernets
Before the introduction of switches, the use of hubs and bridges improved Ethernet network performance. Multiple devices connected to a single Ethernet by connecting to Ethernet hubs. Hubs are Layer 1 devices that essentially provide two things: nondisruptive addition and removal of devices to an Ethernet and repeater functionality.

Ethernet, by specification, allows for multiple devices to connect to a single Ethernet. A device cannot transmit a packet if it is receiving traffic. This ability, called half-duplex, is similar to the idea that you can’t hear what someone is saying if you are talking. Two devices transmitting at the same time causes a collision. When a collision is detected, both devices back off and retransmit at random intervals.

Each Ethernet port on a switch, like a bridge, is a separate Ethernet, with its separate collision domain and its own bandwidth. Collisions on one switch port are not seen on other switch ports.

As switch Ethernet ports became less expensive, switches replaced hubs in the wiring closet. Initially, when switches were first introduced, network administrators plugged hubs (containing multiple hosts) into switch ports. But eventually, it became cost-effective to plug the hosts directly into a switch port. This arrangement gives each host its own dedicated Ethernet and removes the possibility of collisions. Because a dedicated switch connection has only two hosts (the switch and the host), you can configure an Ethernet switch port as full duplex, which means a device can both receive incoming traffic and transmit traffic simultaneously.

End stations have considerable more bandwidth when they use switches. Ethernet has the capability to run at multiple speeds: 10 Mbps, 100 Mbps, 1 Gbps, and 10 Gbps, and therefore switches can provide connectivity at these speeds.

However, network applications and the web create considerably more network traffic, re-introducing new forms of congestion problems. Switches can use quality of service (QoS) and other mechanisms to help the congestion issue.