Local Area Networks (LANs)
Local Area Networks (LANs) are networks that connect devices within a limited geographic area, such as a home, office building, or campus. LANs typically use high-speed connections and are used for sharing resources, such as files, printers, and internet access, among connected devices. Switches play a crucial role in the operation of LANs. Here's an overview of the role and operation of switches in LANs
Role of Switches in LANs
Packet Switching: Switches are devices that operate at the data link layer (Layer 2) of the OSI model. They use packet switching to forward data packets between devices on the same LAN.
Packet Forwarding: Switches forward data packets from the source device to the destination device based on the Media Access Control (MAC) addresses of the devices. Unlike hubs, which broadcast data to all devices on the network, switches only send data packets to the intended recipient, reducing network congestion and improving efficiency.
Segmentation: Switches segment LANs into multiple collision domains, isolating traffic and preventing collisions. Each port on a switch represents a separate collision domain, allowing devices connected to different ports to communicate simultaneously without interference.
Broadcast Control: Switches filter and control broadcast traffic within LANs. Broadcast packets, such as Address Resolution Protocol (ARP) requests and Dynamic Host Configuration Protocol (DHCP) requests, are only forwarded to the ports where the intended recipients are located, reducing unnecessary network traffic.
Enhanced Performance: Switches offer higher performance compared to hubs and older networking devices. They provide full-duplex communication, allowing devices to send and receive data simultaneously, and support faster transmission speeds, such as Gigabit Ethernet and 10 Gigabit Ethernet.
VLAN Support: Many switches support Virtual LANs (VLANs), which enable the segmentation of a single physical LAN into multiple logical LANs. VLANs improve network security, scalability, and manageability by grouping devices based on logical criteria rather than physical location.
Learning MAC Addresses: Switches build and maintain a MAC address table, also known as a forwarding table or content-addressable memory (CAM) table, which maps MAC addresses to switch ports. When a data packet arrives at a switch, the switch examines the source MAC address and associates it with the port on which the packet arrived.
Forwarding Decisions: When a switch receives a data packet destined for a specific MAC address, it consults its MAC address table to determine the outgoing port for the destination device. If the MAC address is already in the table, the switch forwards the packet directly to the appropriate port. If the MAC address is not in the table, the switch floods the packet to all ports except the incoming port to learn the location of the device.
Address Learning: Switches dynamically learn MAC addresses by observing the source MAC addresses of incoming data packets. When a switch receives a packet from a device, it records the source MAC address and the port on which the packet arrived in its MAC address table. Over time, the switch builds a comprehensive MAC address table that it uses to forward data packets efficiently.
Packet Forwarding: Once a switch determines the outgoing port for a data packet based on its MAC address table, it forwards the packet directly to the destination device. Switches operate at wire speed, meaning they can forward data packets at the maximum speed supported by the network interface without introducing significant latency or delays.
Data Generation:
- The packet flow process begins when a device, like a computer or smartphone, generates data. This could be anything from sending an email to accessing a website or sharing files.
Packetization:
- The generated data is divided into smaller units called packets. Each packet consists of a header containing control information and a payload containing a portion of the original data.
- Packetization helps in efficient transmission and allows the network to handle data in manageable chunks.
Addressing:
- Source and destination addresses are assigned to each packet. In a LAN, devices are typically identified by Media Access Control (MAC) addresses.
- The source address identifies the device sending the packet, while the destination address specifies the intended recipient within the same network.
Switching:
- When a device on the LAN sends a packet, it first checks its ARP (Address Resolution Protocol) cache to find the MAC address corresponding to the IP address of the destination device. If not found, an ARP request is sent to obtain the MAC address.
- The switch receives the packet and examines its destination MAC address. It looks up the MAC address in its MAC address table to determine which port the destination device is connected to.
- If the destination device is within the same LAN, the switch forwards the packet directly to the port connected to the destination device.
Transmission:
- The switch forwards the packet to the port where the destination device is connected. The packet is transmitted over the physical medium, such as Ethernet cables, to reach the destination device.
Reception and Processing:
- The destination device's network interface receives the packet. It checks the destination MAC address in the packet header to determine if the packet is intended for it.
- If the destination MAC address matches the device's MAC address or broadcast address, the packet is accepted and processed by the device's network stack.
Payload Delivery
- Once the packet is accepted by the destination device, the payload (data) is extracted from the packet and delivered to the appropriate application or service running on the device.
Acknowledgment
- In some cases, the destination device may send an acknowledgment back to the source device to confirm successful receipt of the packet. However, this step is optional and depends on the communication protocol being used.