Let's start with the technical stuff:
Bandwidth:
1. Cat5e cable is measured and rated at 100MHz.
2. Cat6 cable is measured and rated at 250MHz.
3. Cat6a cable is measured and rated at 500MHz.
Rated and certified speed:
1. Cat5e - Gigabit Ethernet up to 100 metres
2. Cat6 - Gigabit Ethernet up to 100 metres
3. Cat6a - 10 Gigabit Ethernet up to 100 metres
Explanation:
The reality is, network speed is defined primarily by the electronic transmission equipment. A network switch will negotiate the fastest link it can manage, in increments of 10Mbit/s, 100Mbit/s, 1Gbit/s, or even 10Gbit/s. A switch or NIC card will start off trying for the best speed it is rated for (usually either 100Mbit/s or 1Gbit/s). If the other end and the cable can't handle that speed, it will drop down to the next level. So if you buy a Gigabit switch, and have Gigabit NIC cards in your PCs, then you'll get Gigabit speeds so long as your cable supports that.
The increase from 100MHz bandwidth for Cat5e to 250MHz for Cat6 mostly had to do with emphasis on component quality and installation techniques. Due to the higher testing parameters for Cat6 at 250MHz, the installation has to be more stringent to pass the tests, but it does not have an influence on the rated speed capability which is gigabit for Cat5e and Cat6. For example, a well installed Cat5e network will run gigabit speeds basically as efficiently as a well installed Cat6 network, but there will a be a bit more headroom available on the Cat6 network to allow for adverse conditions, ie, interference, deterioration, installation practices, environmental changes.
Cat6a, on the other hand, allows for 10Gig (10BaseT) communication on copper cable over 100 metres and has major differences and advantages over both Cat5e and Cat6.
Where Cat5e and Cat6 is commonly UTP (Unshielded Twisted Pair) cable, Cat6a is commonly an individually shielded cable which means each of the four pairs in the cable is foiled. This means the cable is extremely well shielded from external and internal cross-talk. Because of the extremely high speeds in 10Gig transmissions, alien cross-talk (where the pairs and actual network cables interfere with each other) is a very big problem and individually shielding the pairs is the best way of preventing alien cross-talk.
Also, Cat6a cable is a 23AWG (0.573mm) cable and Cat5 and Cat6 is a 24AWG (0.511mm) cable. This means the actual copper core of the Cat6a cable is thicker than Cat5 and Cat6, and can carry more power, which is very important for POE (Power over Ethernet) and the advances being made in the high power over Ethernet devices being developed.
Refer to "How to approach 10Gbe cabling in South Africa" for more detailed information around Cat6a cabling.
Conclusion:
Due to the fact that data cabling is a long-term investment (15 years) and will outlive networking hardware, and if the infrastructure is expected to be installed for extended periods of time, it is highly recommended to at least install Cat6a solid/permanent link/horizontal cable. Components can follow at a later stage. If the expectation is that 1Gig will not be surpassed, it is difficult to motivate the extra cost of Cat6 over Cat5e.
Also to be taken into consideration is that when the permanent link or solid cable needs to be upgraded, it is not just the cost of the cable that needs to be considered but the opening and closing of pathways, ie, ceiling tiles, trunking systems, skirting, access to pathways and the cost of downtime and labour. These costs will far outweigh the initial cost of the higher rated cable.
Price difference:
Cat5e to Cat6: +- 30%
Cat5e to Cat6a: +- 250%
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