Radio is always prone to disturbance.收音机总是容易受到干扰。
10-6, is an acceptable bit error rate in radio.10-6，是无线电中可接受的误码率。
This means that at 70 megabitsper second we can have 70 errors per second.这意味着每秒70兆比特，我们每秒可以有70个错误。
In the first video, we have seen that coding is used atthe physical layer to correct some of these errors.在第一个视频中，我们已经看到在物理层使用编码来纠正其中一些错误。
But, how can we handle those that have not been corrected?This is one of the functions of the MAC layer, “Media Access Control”.但是，我们如何处理尚未纠正的问题呢？这是MAC层“媒体访问控制”的功能之一。
Another function of the MAC layer is to let severalservices communicate with the same mobile at the same time.MAC层的另一个功能是让几个服务同时与同一个移动设备通信。
This function is called “Multiplexing”.此功能称为“多路复用”。
In the second part of the video, we will present how itis managed by L. T. E.在视频的第二部分，我们将介绍它是如何由L. T. E.管理的。
L. T. E uses error detection codes – called CRC : Cyclic Redundancy Check.L. T. E使用错误检测代码 - 称为CRC：循环冗余校验。
These codes are calculated by thesender and appended at the end of the message.这些代码由发件人计算并附加在邮件的末尾。
The receiver makes the same calculation.接收器进行相同的计算。
If it gets the same result, it knows that the data are correct.
如果它得到相同的结果，它就知道数据是正确的。
There are 3 possible cases:first, if the message is correct, then thereceiver confirms the reception with a message calledan “acknowledgement” or ACK.有三种可能的情况：首先，如果消息正确，则接收方通过称为“确认”或ACK的消息确认接收。
This message is sent on the control channel we spoke ofin the previous video.此消息在我们在上一个视频中提及的控制频道上发送。
In the second case, the data has been corrupted during transmission.在第二种情况下，数据在传输过程中已损坏。
The receiver informs the transmitter with anon-acknowledgement message, or NACK.
接收器用非确认消息或NACK通知发送器。
The transmitter then resends the message andcontinues to do so until it receives a positive acknowledgement.然后，发送器重新发送消息并继续这样做，直到它收到肯定确认。
In the last case,the transmitter that has sent a message has not received an acknowledgement.在最后一种情况下，发送消息的发送器尚未收到确认。
So, either the message or the acknowledgement got lost.因此，消息或确认都丢失了。
Either way, something went wrong.无论哪种方式，出了点问题。
So, if the receiver doesn’t receiveany acknowledgement after a certain time,it tries to send the data again.因此，如果接收器在一定时间后没有收到任何确认，它会尝试再次发送数据。
If the message is still not received after a given numberof attempts, the MAC layer gives up.如果在给定次数的尝试之后仍未收到消息，则MAC层放弃。
Protocols on a higher level will probably make newattempts later on, but, for the MAClayer, these are considered as new data.更高级别的协议可能会在以后进行新的尝试，但是，对于MAC层，这些被认为是新数据。
This mechanism is called “Send and Wait A. R. Q.”“Send and Wait,” because thetransmitter sends the data and waits for the response.这种机制称为“发送和等待A. R. Q.”“发送和等待”，因为发送器发送数据并等待响应。
And A. R. Q for “Automatic Repeat ReQuest.”This is the mechanism thatretransmits automatically if no acknowledgment is received.并且A. R. Q表示“自动重复请求”。如果没有收到确认，这是自动重新发送的机制。
Processing a message or an acknowledgement may take a certain time.处理消息或确认可能需要一定的时间。
That’s why the norm states that the recipient sendsits acknowledgement 4 sub-frames after receiving the message.
这就是为什么规范声明接收者在收到消息后发送其确认4个子帧的原因。
In the same way, afterreceiving this acknowledgement, the transmitter waits4 sub-frames before either repeating the message orsending a new one.以相同的方式，在接收到该确认之后，发送器在重复消息或发送新消息之前等待4个子帧。
The total cycle lasts8 sub-frames, or, 8 milliseconds.总周期持续8个子帧，或8毫秒。
During this time, the transmitter does not transmit.在此期间，发射器不发射。
This is the principle of "Send and wait.”To optimize resource consumption,L. T. E actually runs 8 Send and Wait processes in parallel.这是“发送和等待”的原则。为了优化资源消耗，L。T. E实际上并行运行8个发送和等待进程。
While the first process is blocked, waitingfor its acknowledgement, another process can transmit data.
当第一个进程被阻塞，等待其确认时，另一个进程可以传输数据。
And so on, until we have 8 processes in parallel.依此类推，直到我们并行处理8个进程。
This is called “PARALLEL Send and Wait.”The problem with parallel Send and Wait is that it can cause de-sequencing.这称为“PARALLEL发送和等待”。并行发送和等待的问题是它可能导致解码。
In this example, blocks 1,3 and 4were received correctly, but block 2 was lost.在此示例中，正确接收块1,3和4，但块2丢失。
Block 2 will be re-sent 8 sub-frames later.块2将在稍后重新发送8个子帧。
So, the sequence delivered by the MAC layer to the higherlevel will be block 1, block 3, block 4, and finally block 2.因此，MAC层传递到更高级别的序列将是块1，块3，块4，最后是块2。
The MAC layer does not dealwith the re-sequencing.MAC层不处理重新排序。
We will see in the next video that that is the role of theR. L. C layer which is right above the MAC layer.我们将在下一个视频中看到，这是R.L.C层的角色，它位于MAC层的正上方。
If you look at L. T. E.E documentation you will probably see the term HA. R. Qinstead of A. R. Q.
如果你看看L. T. E.E文档您可能会看到术语HA。 R. Q.而不是A. R. Q.
It is not too important but let’s take a quicklook at this slight difference.这不是太重要，但让我们快速看看这个微小的差异。
The idea is that to increaseerror correction performances, the receiver storesall erroneous messages.想法是为了增加纠错性能，接收器存储所有错误消息。
This accumulation increases redundancyand thereby the possibility of getting the correct data.这种累积增加了冗余，从而增加了获得正确数据的可能性。
HA. R. Q stands forHybrid A. R. Q. “Hybrid” because the MAClayer has to collaborate with the physical layer to letit know that the data it processes are actuallyrepetitions of the same transport block.哈。 R. Q代表Hybrid A. R. Q.“Hybrid”，因为MAC层必须与物理层协作，让它知道它处理的数据实际上是同一传输块的重复。
Now, let’s speak about another function of the MAC layer.
现在，我们来谈谈MAC层的另一个功能。
You use your smart-phone for voice conversations as wellas receiving emails or browsing web pages.您可以使用智能手机进行语音对话以及接收电子邮件或浏览网页。
And you may want these at the same time.你可能同时想要这些。
But there is a single communication channel between thebase station and your mobile.但是基站和移动设备之间只有一个通信通道。
So, all this traffic has to be mixed over this pipe.因此，所有这些流量必须通过此管道混合。
This is what we call “MULTIPLEXING”.这就是我们所说的“MULTIPLEXING”。多路复用
Before getting further, let me introduce some vocabulary.在进一步讨论之前，让我介绍一些词汇。
In the classical layeredmodel of networks, data at the topof a layer are called “SDU” or“Service Data Units” and the data at the bottomare called “PDU” or “Protocol Data Units.”In our case, the MAC layer receives the MAC-SDUsand produces MAC-PDUs.在经典的网络分层模型中，层顶部的数据称为“SDU”或“服务数据单元”，底部的数据称为“PDU”或“协议数据单元”。在我们的例子中，MAC 层接收MAC-SDU并产生MAC-PDU。
Now, let’s talk about multiplexing.现在，我们来谈谈多路复用。
Data handled by the MAC layer are provided by the upper layer,R. L. C, they're provided through message queues.由MAC层处理的数据由上层提供。 R. L. C，它们是通过消息队列提供的。
These queues are called logical channels.这些队列称为逻辑通道。
We will see in the next video that there are several ofthem, corresponding to different levels of service.我们将在下一个视频中看到它们中有几个，对应于不同的服务级别。
Here, for example, is a channel dedicated tosignaling (L. T. E internal control), another to voiceand a last one for general data.这里，例如，是专用于信令（L.T.E内部控制）的信道，另一个是语音信道，最后一个信道用于一般数据。
The output of the MAC layer, the MAC_PDUsare the transport blocks that will be transmittedby the physical layer.MAC层的输出MAC_PDU是将由物理层发送的传输块。
To form the transport blocks, the MAC layer assemblesthe SDUs from the different logical channels.为了形成传输块，MAC层组装来自不同逻辑信道的SDU。
This is referred to as multiplexing because there areseveral incoming streams and only one outgoing.
这被称为多路复用，因为有几个输入流，只有一个输出。
To help the receiver break down, or de-multiplex,the MAC layer adds headers that indicate whereeach message starts and towhich logical channel they belong to.为了帮助接收器分解或解复用，MAC层添加标头，指示每个消息的开始位置以及它们属于哪个逻辑信道。
Padding can also be addedto ensure that the size of the MAC_PDU isexactly the same size as the transport block to be transmitted.
还可以添加填充以确保MAC_PDU的大小与要传输的传输块的大小完全相同。
In short: as well as allocating resources, as seen in the previous video,the MAC layer ensures the reliability oftransmission using HA. R. Q.
简而言之：除了分配资源之外，如前面的视频所示，MAC层确保了使用HA. R. Q.进行传输的可靠性。
It ensures retransmission in case of error.它确保在发生错误时重新传输。
The MAC layer can also merge different data fromdifferent logical channels into the same transport block.MAC层还可以将来自不同逻辑信道的不同数据合并到同一传输块中。
转载于:https://www.cnblogs.com/sec875/articles/9959385.html
