Bitcoin, the blockchain-powered cryptocurrency has been growing steadily in size and scope since inception in 2009. Not only has the value of the bitcoin currency grown to represent over $10 billion of notional value, but its network, too, has grown exponentially. During this expansion, the system began to experience some growing pains, mainly associated with scaling up to accommodate the larger number and frequency of transactions, while at the same time preserving the virtues of privacy, security, and low transaction costs. The original protocol, written by the anonymous Satoshi Nakamoto has become known as Bitcoin QT or Bitcoin core, and what has resulted is three competing versions of the Bitcoin protocol: Bitcoin Classic, Bitcoin Unlimited and BitPay Core. These three attempts follow the controversial roll-out of Bitcoin XT, which would have increased the block size to 8MB, and was largely rejected by the community. (See also: Can Bitcoin Hard Fork?)
Bitcoin Classic seeks to mitigate the problem of more transactions, which are causing transaction backlogs and increased transaction costs, by increasing the block size - the number of kilobytes in a block of transactions - from 1MB to 2MB. According to Bitcoin Magazine, 2MB was picked purposefully based on data collected by its creators, and from conversations with many Bitcoin miners and mining pools.
Its developers assert that Bitcoin Classic is endorsed by former Bitcoin Core lead and Bitcoin XT developer Gavin Andresenwas well as large mining pools such as AntPooland BW Pool, and wallet/exchanges Coinbase and OKCoin. (See also: The Future of Cryptocurrency)
Bitcoin Unlimited, as its name suggests, embraces the absence of a hard-coded block-size limit. Instead, it allows users to manually set limits on their own nodes; the Bitcoin Unlimited team expects consensus on a limit to emerge naturally at a so-called Schelling or focal point - a solution that people will tend to use in the absence of communication, because it seems natural, special, or relevant to them. Furthermore, Bitcoin Unlimited intends to introduce a level of democracy into development and management of the implementation, allowing the community to vote on changes.
BitPay Core is still in an experimental phase, but its main idea is to have two limits - one a 'hard limit' on block size that will be adjusted on a regular basis, coinciding with difficulty adjustments, and a second 'soft limit' that the miner community will choose to enforce among themselves, similar to the focal points in Unlimited.
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