We saw in the last video that the MAC layer enables
several applications to share the same radio connection.
我们在上一个视频中看到，MAC层使多个应用程序能够共享相同的无线连接。
But what if, these applications don’t have the same
requirements in terms of latency or reliability?
Or, in other words, different requirements
of Quality of Service.
但是，如果这些应用程序在延迟或可靠性方面没有相同的要求呢？或者换句话说，对服务质量的要求不同。
In L. T. E, it is the R. L. C layer’s role
to provide such features.
在L.T.E中，R.L.C层的角色是提供这样的特征。
R. L. C, or Radio Link Control is
just above the MAC layer.
R.L.C或无线电链路控制就在MAC层之上。
Let’s see how it works.
让我们看看它是如何工作的。
What is the definition of the quality of service, or QoS in short?
It is the capability of the network to give applications
a level of service corresponding to their needs, based on
different criteria such as delay,
throughput, or error rate.
什么是服务质量或简称QoS的定义？网络能够根据不同的标准（如延迟，吞吐量或错误率）为应用程序提供与其需求相对应的服务级别。
The thing to understand about QoS is that it is a compromise.
要了解QoS的事情是妥协。
For example, it is possible to
increase the reliability, by re-transmitting lost messages.
例如，可以通过重新发送丢失的消息来提高可靠性。
But, of course, it increases the delay.
但是，当然，它增加了延迟。
The right balance depends on the application.
正确的平衡取决于应用程序。
For a voice communication, we prefer to have a short
delay even if we have some losses.
对于语音通信，即使我们有一些损失，我们也希望有短暂的延迟。
Whereas, for bank transaction, we do not
tolerate error and the delay is not that important.
然而，对于银行交易，我们不容忍错误，延迟并不重要。
In the previous lesson, we saw some limitations of the MAC layer.
在上一课中，我们看到了MAC层的一些局限性。
For example, after a certain number of failures, the MAC
layer gives up retransmitting messages.
例如，在一定数量的故障之后，MAC层放弃重传消息。
We also saw that H. A. R. Q
can introduce de-sequencing in messages.
我们还看到H. A. R. Q可以在消息中引入解序。
Also, the MAC layer does not take care of the message segmentation.
此外，MAC层不处理消息分段。
That is to say, chopping the messages into smaller blocks
that can be inserted into transport blocks.
也就是说，将消息切割成可以插入传输块的较小块。
The R. L. C layer offers services to increase the reliability
of exchanges, to re-order messages, and
to manage segmentation.
R. L. C层提供服务以增加交换的可靠性，重新排序消息和管理分段。
But this is done at the expense of latency.
但这是以延迟为代价的。
This is why these services are optional.
这就是为什么这些服务是可选的。
They can be used for certain data
streams and de-activated for others.
它们可用于某些数据流，也可用于其他数据流。
R. L. C proposes 3 modes.
R. L. C提出了3种模式。
The simplest is transparent mode or TM.
最简单的是透明模式或TM。
In this case, R. L. C does nothing.
在这种情况下，R.L.C什么都不做。
Because there is no segmentation,
this mode is reserved for short messages.
由于没有分段，因此此模式保留用于短消息。
It is mainly used to transport signalling messages of L. T. E.
The second mode is un-acknowledged Mode or UM.
它主要用于传输L.T.E的信令消息。第二种模式是未确认模式或UM。
It takes care of re-sequencing received blocks as
well as segmenting and concatenating messages
depending on the needs of the MAC layer.
它负责根据MAC层的需要对接收到的块进行重新排序以及分段和连接消息。
The impact of this mode on latency is low
but it does not improve reliability.
此模式对延迟的影响很小，但不会提高可靠性。
It is therefore adapted to real time applications such as
voice or video communication.
因此它适用于诸如语音或视频通信的实时应用。
The third mode is Acknowledged Mode or AM.
第三种模式是Acknowledged Mode或AM。
It offers the same services as UM
and adds a system to retransmit lost packets
which increases reliability.
它提供与UM相同的服务，并添加了一个系统来重新传输丢失的数据包,这增加了可靠性。
But, of course, this is done at the cost of latency.
但是，当然，这是以延迟为代价完成的。
It is therefore best adapted to applications such as file
transfer and Web browsing.
因此，它最适用于文件传输和Web浏览等应用程序。
In terms of architecture, several R. L. C
instances can run in parallel for a given link.
在体系结构方面，几个R.L.C实例可以并行运行给定链接。
For example, a Voice communication over IP
can use an R. L. C instance in UM
while files are being transferred with another instance in AM.
例如，基于IP的语音通信可以使用UM中的R.L.C实例，而文件与AM中的另一个实例一起传输。
As discussed previously, the MAC layer picks up
packets on both instances depending on the parametrized
priorities and throughput targets.
如前所述，MAC层根据参数化优先级和吞吐量目标在两个实例上获取数据包。
How does the un-acknowledged mode work?
When it receives a packet from the upper layer,
R. L. C saves it in a buffer
while waiting for the MAC layer to request a MAC-SDU.
未确认模式如何工作？当它从上层接收数据包时，R。 L. C将其保存在缓冲区中，同时等待MAC层请求MAC-SDU。
When the MAC layer requests a MAC-SDU,
it specifies the size of the MAC-SDU it is expecting.
当MAC层请求MAC-SDU时，它指定它期望的MAC-SDU的大小。
R. L. C then assembles the stored packets to make
an R. L. C-PDU of the requested size.
然后R.L.C组装所存储的分组以产生所请求大小的R.L.C-PDU。
To do that, it can concatenate packets,
or split them to reach
the specified size.
为此，它可以连接数据包，或将它们拆分以达到指定的大小。
R. L. C also adds headers to tell the receiver where to
find the different fragments and which
R. L. C instance they belong to.
R. L. C还添加标题以告诉接收者在哪里找到不同的片段以及它们属于哪个R.L.C实例。
These headers also contain sequence numbers
that will enable putting blocks in the correct order if
they are de-sequenced by H. A. R. Q.
Let’s stop for a second and look at two things.
这些标题还包含序列号，如果它们被H.A.R.Q重新排序，它将使块按正确的顺序放置。让我们停下来看看两件事。
Firstly, the MAC layer can regroup in the
same transport block
SDUs from different R. L. C instances.
首先，MAC层可以在来自不同R.L.C实例的相同传输块SDU中重新组合。
Secondly, when trying to
retrieve the correct sequence of messages,
R. L. C in UM can remain blocked
if an intermediate segment never arrives.
其次，当试图检索正确的消息序列时，R。 如果中间段永远不会到达，则UM中的R.L.C可以保持阻塞。
To avoid this, after a certain time,
R. L. C will eventually consider this packet as lost and
will go on to the next.
为了避免这种情况，经过一段时间后，R。 L. C最终会认为这个数据包丢失了，并将继续下一个。
Acknowledged Mode works in the same way as UM.
已确认模式的工作方式与UM相同。
But it also manages the repetition of lost messages.
但它也管理丢失消息的重复。
To do that, R. L. C memorizes
the MAC-SDUs it delivers to the MAC layer, so that it
will be able to resend them if necessary.
为此，R. L. C记忆它传递给MAC层的MAC-SDU，以便在必要时能够重新发送它们。
It regularly asks the recipient
to send the status of the messages it has received.
它会定期要求收件人发送收到的邮件的状态。
The receiver answers to this request by sending
an R. L. C signaling message tha t is transferred on the data channel.
接收器通过发送在数据信道上传送的R.L.C信令消息来应答该请求。
Let’s note in passing that this is the first time we see
this case in the radio interface.
让我们顺便说一下，这是我们第一次在无线电接口中看到这种情况。
Until now, all control exchanges were made on
dedicated control channels.
到目前为止，所有控制交换都是在专用控制通道上进行的。
But now, the answer from the recipient must be separated
from the data flow and delivered to the R. L. C controller.
但是现在，收件人的答案必须与数据流分开并传递给R. L. C控制器。
The controller can then erase from its memory the messages
that are being correctly received and resend ones that have not
be delivered to the recipient.
然后，控制器可以从其存储器中擦除正确接收的消息，并重新发送尚未传递给接收者的消息。
Thus, for Acknowledged Mode, it must be the same body
that manages the transmission and the reception part
while in UM, these processes are independent.
因此，对于已确认模式，它必须是管理传输和接收部分的相同主体，而在UM中，这些过程是独立的。
To summaries, the R. L. C layer
is located above the MAC layer.
总之，R.L.C层位于MAC层之上。
It takes care of :
Re-sequencing of blocks held up by H. A. R. Q.
It also manage the concatenation and segmentation of
the data to fit the size requested by the MAC layer.
它负责：重新排序由H. A. R. Q.提供的块。
它还管理数据的连接和分段以适应MAC层请求的大小。
And also handles the retransmission of lost blocks.
并且还处理丢失块的重传。
Quality of service is a compromise,
notably between reliability and delay.
服务质量是一种妥协，特别是在可靠性和延迟之间。
R. L. C defines three modes:
transparent mode, un-acknowledged mode and acknowledged mode.
R. L. C定义了三种模式：透明模式，未确认模式和确认模式。