What Is a Wireless Router Fragmentation Threshold?

Fragmentation increases the overhead on network traffic.

Fragmentation is the consequence of a network being unable or unwilling to handle a large amount of data traveling together. Each network has its own fragmentation threshold setting, which is called the Maximum Transmission Unit (MTU). Although the network administrator or home Wi-Fi user can set this factor at any level, it is better to use generally accepted conventions.

Data Packets

Data travels across networks in packets. Transport protocols break up data for transfer into sections and put each section into the body of a packet. It then adds on its own header to the packet. A packet header contains information on how to reassemble the data and also the source and destination addresses for the two applications and computers involved in the transfer. The number of headers on the front of a packet varies, according to the application that originated the data. However, the packet will always have a transport protocol header and an IP header.

IP Headers

The IP header is defined by the Internet Protocol. It contains the source and destination IP addresses of the packet, identifying the two computers in the transaction. It also contains a number of fields relating to fragmentation. A fragmentation flag enables the sender to forbid fragmentation; however, if this is set and the packet is bigger than the receiving network's MTU, the packet will be dropped. Other fragmentation fields include the start point of the data in the current packet, relative to its position in the original, larger packet. These fields leave their origin empty and are only filled in if a router on the packet's path fragments it.

Fragmentation

If a router receives a packet that is larger than its MTU, it splits it up into smaller packets, filling in the fragmentation fields to enable the receiver to reassemble the original order of the data. Fragmentation may occur at any point on the journey from sender to receiver. The packet will remain in its fragmented state until it arrives at its destination, even if other intermediate routers are able to handle the larger size of the original packet. Headers are an overhead on data transfers and so greater efficiency is achieved if packets are the largest possible size. However, fragmentation greatly increases overhead, so the sending network has to judge its packet size to be as large as possible without provoking fragmentation somewhere on the journey.

Wireless MTU

Wireless network have a higher standard MTU than cabled networks. Data packets get converted into the payload of a frame when they travel across local networks and it is the maximum size of that payload that determines the maximum size of a data packet, not the router. However, the router implements the network's fragmentation threshold policy. The MTU for wireless networks is 2,312 bytes. However, any Internet access will involved connecting to, or crossing cabled networks, where the MTU is 1,500 bytes, so all packets larger than this size will be fragmented. Furthermore, many ADSL Internet service providers use the Point-to-Point over Ethernet (PPPoE) protocol for connections to their clients, and these connections cannot accept packets larger than 1,492 bytes.

References

• "TCP/IP Illustrated,Volume 1"; IP: Internet Protocol; W. Richard Stevens
• Wireless Center: Structure of the IEEE 802.11 MAC Frames
• Cisco Systems: PPPoE Baseline Architecture for the Cisco 6400 UAC
• Microsoft Support: HOW TO: Change the Default Maximum Transmission Unit (MTU) Size Settings for PPP Connections or for VPN Connections