WAN optimization is a set of techniques that address various issues experienced in a Wide Area Network (WAN):
Link related issues, caused by physical limitations.
Protocol related issues.
A network link has two characteristics:
The link speed, which defines the speed of the data through the physical layer.
The link delay, which defines how long it takes for a bit to reach the other side of the link.
The link speed is the number of bits per second which can be transferred over a link. In general, the speed of the LAN is much faster than the speed of the WAN.
Around 1995, the speed of a WAN link was measured in multitudes of 64 kbps and a LAN link was still working on a 10 Mbps shared coax cable. It wasn't for another five years in 2000 that an E1 link at 2 Mbps became the standard speed for WAN links and that a 100 Mbps Ethernet cables towards a LAN switch became available for the desktop. Five years later in 2005 the general speed for a WAN link was 10 Mbps and the network switch was connected to the desktop at gigabit Ethernet speed (1000 Mbps).
This LAN/WAN speed difference made it in the past necessary to have services such as file servers, mail servers and web proxy servers, on the local LAN on the remote location (the branch). However, the speed of the WAN link in 2005 is now fast enough to consolidate them into a data center.
Consolidation of these remote servers into data centers made the management of the machines easier: Reduction of the number of machines by consolidating then, better control over the environment the machines are operating in, easier implementation of fail-over scenarios because of the availability of high bandwidth needed for replication, and a simplified management of the services running. However, the bandwidth towards the client was reduced from gigabit speeds back to pre-2000 speeds of 10 Mbps.
The link-speed defines the serialization delay, the time it takes for a packet to be converted from set of bytes in the memory of a host (computer, router, or switch) into a string of bits on the wire.
For example, to forward one packet of 1500 bytes through a router with a WAN interface of 1 Mbps, it will take 12 milliseconds (1500 bytes * 8 bits per byte / 1 000 000 bits per second = 12 ms).
If WAN optimization reduces that packet of 1500 bytes to a packet of 100 bytes, this serialization delay will be reduced to 0.8 milliseconds.
Figure 1.1. Serialization delay
^ | | | _____ H | __________/ | o | /| | | p | / | | | | / | | | d | / | | | e | / | | | l | / | | | a | __________/ | | | y | / | | | | | +----------------------------------------------- network hop distance client WAN router WAN router server
On longer distances, the speed the data travels through the medium comes into play. For example, the speed of light in a fiber cable, which is about 5 microseconds per kilometer (1.5 / 300 000 km/s, where 1.5 is the refraction index for fiber{SOURCE:Wikipedia Latency (engineering)}). For a fiber cable with a length of 1000 kilometers this would add 5 ms for the signal to reach the other side.
The TCP network protocol has had lots of improvements over the last years, but not all TCP stacks support all features. By terminating the TCP session locally on the LAN and setting up a new one which does support all the new features, the WAN optimizer can make these TCP sessions faster over the WAN.
Some protocols are smart, like the connection-oriented TCP protocol, which makes sure that all the data sent by the sending application is presented to the receiver application in such a way that nothing is missing and that it is in the right order.
Application protocols are the layer in which the client and the server talk to each other.
Application protocols can be unintentionally implemented with a special environment in mind. For example results from database queries can be requested either in bulk or one by one. The first implementation works great over a WAN but needs more memory on the client to store all the answers, while the second implementation works bad over a WAN but doesn't need as much memory. Serialization delay is here the cause of the problem for a WAN deployment.