下面关于Manchester编码的叙述中，错误的是()。
- A.Manchester编码是一种双相码
- B.Manchester编码提供了比特同步信息
- C.Manchester编码的效率为50％
- D.Manchester编码应用在高速以太网中
正确答案及解析
正确答案
解析
本题考查数据编码的基础知识。
Manchester编码是一种双相码，即码元取正负两个不同的电平，或者说由正负两个不同的码元表示一个比特，这样编码的效率为50％，但是由于每个比特中间都有电平跳变，因而提供了丰富的同步信息。这种编码用在数据速率不太高的以太网中。
差分Manchester编码也是一种双相码，但是区分“0”和“1”的方法不同。Manchester编码正变负表示“0”，负变正表示“1”，而差分Manchester编码是“0”比特前沿有跳变，“1”比特前沿没有跳变。这种编码用在令牌环网中。
在曼彻斯特和差分曼彻斯特编码中，每比特中间都有一次电平跳变，因此波特率是数据速率的两倍。对于100Mb／s的高速网络，如果采用这类编码方法，就需要200M的波特率，其硬件成本是100M波特率硬件成本的5～10倍。
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