The S9 is the latest 2016 product offering from Bitmain. With 14 TH for $2100 this is one of the best machines on the market for the home bitcoin miner. With the bitcoin reward halving coming in mid 2016 the S9 is sure to remain one of the more competitive machines in the cost curve. 

S9 Specifications:

1. Hash Rate: 14.0TH/s ±5%

2. Power Consumption: 1375W + 7% (at the wall, with APW3 ,93% efficiency, 25C ambient temp)

3. Power Efficiency: 0.098 J/GH + 7%(at the wall, with APW3 93% efficiency, 25°C ambient temp)

4. Rated Voltage: 11.60 ~13.00V

5. Chip quantity per unit: 189x BM1387

6. Dimensions: 350mm(L)*135mm(W)*158mm(H)

7. Cooling: 2x 12038 fan

8. Operating Temperature: 0 °C to 40 °C

9. Network Connection: Ethernet

10. Default Frequency: 650M

If you are deciding to join a Bitcoin mining pool there are quite a few considerations to take into account – mainly their method of distributing the block reward and the fees they charge for managing the pool. Pools also try to stop cheating by miners – i.e. for them to swap between pools.

The main consideration is the fees, which vary according to which model of payment distribution the mining pool is operating and determines which party is assuming the risk – the miners or the mining pool operator. If the mining pool operator is assuming the risk, then the fees are higher, and if the miners assume the risk then fees are lower.

The model where the mining pool operator assumes all the risk is when they guarantee a payment per each proof of work – or potential hash solution – that their miners offer. For example if the total network is 100GH, the mining pool operating this Pay Per Share (PPS) method has a hash rate of 10GH, and the block reward is 25 Bitcoins, then the expected return is 2.5 Bitcoins per block.

The pool will give money to their miners even if their pool hasn’t successfully mined the block, meaning the risk of lumpy payments is assumed by the operator, and hence why the fees are at the higher end of the range at 10%. Miners will then only receive an expected return of 2.25 Bitcoins per block distributed proportionally by how much hashing power they have contributed towards the block.

When the miners assume the risk the fees are generally lower as they take on the risk that they might not solve a block for an extended period of time and receive no payment of Bitcoins.

There are varying methods of this with the aim of keeping the pool hashing power stable.

- Proportional – the simplest method whereby for each block, the reward is split between the hashing power contributed proportionally by the miners of the block.
- Pay Per Last N Shares – PPLNS – looks at the last N shares instead of just the last block. This smooth’s the returns for mining rig operators if they haven’t been connected for one reason or another. If they contributed to the majority of Bitcoin blocks 1-6, when a reward was found by their pool in block 7, for which they had become disconnected through no fault of their own, then they are still eligible for payouts depending on the time of N.

There are other inventions and variations that have been implemented. For example the DGM method (Double Geometric Method), where the operator receives some payments over short rounds and distributes them over longer rounds. There are also some other ways where the more recent proofs of work are allocated a higher weighting in terms of the proportion they are eligible for.

Some pools have extra fees on top of PPS (Pay Per Share) schemes – but in generally fees range from 0% for Proportional and PPLNS pool management schemes to 10% for PPS schemes. There also pools that offer the ability to merge mine other SHA-256 coins as well as Scrypt pools that allow you to merge mine other popular crypto currencies such as Dogecoin and litecoin.

Selfish is an attack on the integrity of the Bitcoin network. This is where one miner, or mining pool, does not publish and distribute a valid solution to the rest of the network.

The selfish miner then continues to mine the next block and so on maintaining its lead. When the rest of the network is about to catch up with the selfish miner, he, or they, then release here portion of solved blocks into the network.

The result is that their chain and proof of work is longer and more difficult so the rest of the network adopts their block solutions and they claim the block rewards.

The selfish mining attack is a method for mining pools to increase their returns by not playing fair. Although this can be seen in some crypto currencies where pool shares are not so fairly distributed it is more difficult to carry off with Bitcoin.

It is also advantageous for a mining pool to increase in size to be able to perform selfish mining. At over 51% of network power the returns earned by a selfish mining pool are justified as probabilistically they can maintain their advantage in terms of hashing the next block. As the mining pool controls the majority of the network hashing power it can also censor or nullify certain transactions essentially holding the network to ransom.

There have been BIPS, Bitcoin Improvement Proposals, to lower the probability of a selfish mining attack such as randomly assigning miners to various branches when a fork occurs – or alternatively providing a threshold limit to which a mining pool can reach. This is the same as Governments trying to halt natural monopolies to allow competition.

A further solution is to discriminate against a block depending on the timestamp it was released – so if a miner releases a long list of blocks in one shot – then the rest of the network would weight their validity against the timestamp they were hashed and the timestamp they were reported to the network.