Frame forwarding methods on Switches

There are 2 methods that all switch use for forwarding the frames (switching data) between the network ports:

1. Store and forward switching
2. Cut-through switching

These are explained as following below.

1. Store and forward switching :
In this switching technique, when the switch receives the frame, it stores the frame data in buffers until the full frame has been received. During the this process, the switch analyses the current frame for information about its destination. This process also involves the another process of an error check using the Cyclic Redundancy Check (CRC) which is operated by the switch.

This switching technique is required for Quality of Service (QoS) analysis on converged networks where the classification of the frame for traffic prioritization is necessary. For example, VoIP data streams need to have high priority over other kind of traffic.

CRC checks the number of bits (1s) in the frame to determine whether the received frame has an error or not. After confirming that is there no error in the frame, the frame is forwarded out the appropriate port, toward its destination.

When an error is found in a frame, the switch discards the frame. Bandwidth consumed by the corrupt data is reduced by discarding the error containing frame.

Figure – Store and forward switching

2. Cut-through switching :
In this switching technique, the switch acts upon the data as soon as it is received, even if the full frame is not received (transmission is not complete). The switch buffers is enough of the frame to read the destination MAC address so that it can find the port where it supposed to send the data. The switch takes the destination MAC address from switching table, determines the outgoing interface port, and forwards the frame onto its destination through the designated switch port. This switching technique does not involve any error check process by the switch.

Figure – Cut-through switching

There are 2 types of Cut-through switching:

1. Fast-forward switching –
   This switching technique provides the lowest level of latency (it is measured from the first bit received to the first bit transmitted) because it immediately forwards a packet after reading the destination address.

   Fast-forward switching starts forwarding as soon as it received the first byte of the packet, there may be a chance when packets are relayed with errors. This occurs rarely, and during this situation destination network adapter discards the faulty packet upon receipt. This switching is the typical cut-through method of switching.

2. Fragment-free switching –
   In this switching technique there is a trade-off between the high latency – high integrity of store and forward switching and the low latency – reduced integrity of fast-forward switching. The switch stores and perform a small error check on the first 64 bytes of the frame before forwarding.

   This switching technique includes the concept of store and forward switching and fast-forward switching. This switching technique only stores the first 64 bytes of the frame because the most network errors and collisions occur during the first 64 bytes and tries to enhance fast-forward switching by doing a small error check to ensure that a collision has not occurred before forwarding the frame.