Ethernet – Why RJ45 8-Pin (Cross Connection) is used instead of Straight Connection between 2 computers or Network

As I recall (and I will admit I am too busy ATM to go research this myself), the RJ standards define the mechanical characteristics of the connector (shape, size, contacts, etc). They do not define the manner of crimping the connector to a cable.

Each manufacturer could make their own decisions on the manner of crimping, but there isn't really much you can vary in this process. The result is that there were two major crimp dies and types of connectors that provided at least the majority (or possibly the entirety) of the market.

This is true prior to Cat6. With Cat6 it is more difficult to adhere to the standard when making terminations, as you have very little room to "untwist" pairs and need to maintain proper separation of the pairs.

This led to a whole new range of connector products and tools that may or may not be compatible with older tools. For instance, I have seen Cat6 connectors where you wire the cable to an "insert" that is then inserted and crimped into the connector.

Morale of the story is that you need to do your research and make sure that you are getting connectors that work with your current tools or buy the right tools with the connectors. On top of that you need to know what you are doing or you may produce cables that will only pass a Cat5 certifications at the extra costs of Cat6.

My advice generally, and especially if you are looking to do Cat6 or better, buy your cables pre-manufactured. The failure rate of hand made cables is much higher and can cost more in troubleshooting/downtime in the long run. Especially since many of these are made in organizations that are "trying to save money" and also don't have the tools to properly test the cables they are making (certification testers are not cheap).

The main benefit of hashing rather than a simple compare is that you can do one lookup for any number of enabled multicast addresses. The system that has an exact match for 80 addresses will either have to build logic to fetch 80 addresses into a comparator in turn, or have 80 comparators in parallel. That's a lot of gates, even if it is simple in concept. And that's all extra cost and power.

In contrast, a hash lookup can be easily implemented with a shift register and a few XOR gates. The computation and shifts can be done as the bits arrive on the wire. Even better, the NIC is doing this already, in order to calculate the checksum.

Note also, that Stevens was writing 20-25 years ago; whilst it is still more or less the definitive work on TCP/IP, hardware has come along since then. The cost of adding a few thousand more gates won't make so much difference. Flicking through a few NIC datasheets, most opt for a hybrid approach: e.g. 16 exact addresses and a 4096 bit hashtable.