Previously, we saw that, to emit a transmissionrequest, terminals must first register with the eNodeB.过去，我们看到终端必须首先向前发送传输请求eNodeB注册。
In this video, we’ll see how that works.在这段视频中，我们将看到它是如何工作的。
In a network, rules must define how several users sharethe same channel preventing interferences.在网络中，规则必须定义多个用户如何共享同一信道以防止干扰。
These rules are called access protocols.这些规则称为访问协议。
They serve to define who can talk, when, and how.它们用来定义谁能说话，什么时候，怎么说话。
There are 2 groups of access protocols.
有两组访问协议。
In the first group, “contention-based access” protocols,we speculate that the chances of a collision are very small.在第一组基于竞争的访问协议中，我们不太可能推测碰撞。
Everyone can transmit whenever they want.每个人都可以随时传输。
Because there can still be interferences, we create a wayto know whether the message was received or not.由于仍然存在干扰，我们创造了一种了解新闻是否被接收的方法。
For example, using acknowledgements.例如，使用确认。
If a transmission fails, we can try again later.如果传输失败，以后可以再试。
For example Wi-Fi works like this.例如，Wi-Fi就是这样。
The other group is called “reservation-based access” protocols.另一组称为基于预留访问协议。
The resource is broken into small elementsthat are allocated to senders.资源给发件人的配给发件人的小元素。
For example, each user will have the right to a turn totalk for one millisecond.例如，每个用户都有权转向谈话一毫秒。
This allocation can be static, when it is defined at the start;or dynamic - it can be modified according to needs.当开始定义时，分配可以是静态的；或动态的 - 可根据需要进行修改。
LTE belongs to this group.
LTE属于该组。
And we have already seen that the resources are dividedby time and by frequency and that they are allocated dynamically.我们已经看到资源按时间和频率划分，它们是动态分配的。
On the downlink, there’s no problem, since alltransmissions are made by the eNodeB.在下行链路中，没有问题，因为所有的传输都是由eNodeB完成的。
On the uplink, terminals must first send a request beforebeing allocated resources.在上行链路中，终端必须在资源分配前发送请求。
These requests go on the uplink control channel onwhich each terminal has the right to a turn to speak.每个终端都有权在上行链路控制信道上转向发言。
This assumes that terminals are registered with the base station.假设终端注册为基站。
For that, they must have been able to transmitonce to make themselves known.为此，他们必须能够传输一次才能让自己知道。
The solution in LTE was tointroduce a bit of contention within the uplink.LTE解决方案是在上行链路中引入一些争议。
On the uplink, a group of resources is reserved so thatnew terminals can indicate their presence.在上行链路上保留一组资源，以便新终端能够指示它们的存在。
This group of resources constitutes a new physicalchannel called PRACH, or Physical Random Access Channel.该组资源构成称为PRACH或物理随机接入新的物理信道。
It is made up of 6 adjacent resource blocks pairsand occurs every one to 20 millisecondsdepending on the eNodeB parameters.它由六个相邻的资源块组成，每1到20毫秒发生一次，具体取决于eNodeB参数。
The access to this channel is done using a particular access techniquethat is called CDMA Code Division Multiple Access.
使用称为CDMA为了完成对该信道的访问，编码了多址的特定接入技术。
In CDMA, transmitters encode their transmissions with sequences.
在CDMA发射机用序列编码其传输。
The mathematical properties of thesesequences are such that, if 2 devices transmitsimultaneously with different sequences, the receiver canstill distinguish between the two transmissions.如果发送两个设备，这些序列的数学特性在不同序列的同时仍然可以区分两种传输。
Another detail about this channel:on the allocated one millisecond, only one symbol istransmitted and this symbol uses all 6 resource blocks.
另一个关于信道的细节：配的一毫秒内，只传输一个符号，并使用所有六个资源块。
So, when a terminal transmits on this channel,it actually send only one symbol.因此，当终端在信道上发送时，它实际上只发送一个符号。
But, the base station detects that something wastransmitted, and infers that a new terminal wants to connect.
然而，基站检测到发送了一些内容，并推断出新终端想要连接。
Even though the terminal does not transmit data, the basestation can identify the particular sequence used by themobile and associate this terminal with this sequence.即使终端不发送数据，基站也可以识别移动平台使用的特定序列，并将终端与序列相关联。
There are 64 sequences available, so, there couldbe up to 64 simultaneous requests.有64个序列可用，所以最多可以同时有64个请求。
We will explain later what happensif 2 users use the same sequence at the same time.稍后，我们将解释如果两个用户同时使用相同的序列会发生什么。
Let's first see hat happens when a new terminal enters the cell.让我们先看看当新终端进入小区时会发生什么。
We saw previously that, to allocate resources, the eNodeBgives each terminal an RNTI.我们之前看到，为了分配资源，eNodeB为每个终端提供RNTI。
This is the procedure: the terminal that arrives listensto the eNodeB to identify when the Randomaccess channel is scheduled.这是程序：到达的终端监听eNodeB以识别何时调度随机接入信道。
It then chooses, at random, one of the 64 sequences,and transmits on the random-access channel, usingthis sequence at the right moment.然后，它随机选择64个序列中的一个，并在随机访问信道上，在适当的时刻使用该序列进行发送。
The eNodeB detects this transmission and the sequence.eNodeB检测到该传输和序列。
It does not yet know the identity of the mobile, butwill use this sequence to temporarily identify theterminal and will then assign it an RNTI.
它还不知道移动设备的身份，但会使用此序列临时识别终端，然后为其分配RNTI。
However, at this point this RNTI is only known by the eNodeB.
但是，此时此RNTI仅由eNodeB知晓。
So, the eNodeB cannot use it as an address to send a message to the terminal.
因此，eNodeB不能将其用作向终端发送消息的地址。
Instead, it will use another identifier: the RA-RNTI orRandom Access RNTI.相反，它将使用另一个标识符：RA-RNTI或随机接入RNTI。
This identifier is calculated using the sequencealready used by the mobile.使用移动设备已使用的序列计算此标识符。
So, the terminal, just like the eNodeB,can calculate this RA-RNTI.
因此，终端，就像eNodeB一样，可以计算该RA-RNTI。
Now, the eNodeB transmitsa message to the mobileto send a few parameters, including the assigned RNTIand it will use the RA-RNTI as thedestination address of this message.现在，eNodeB向移动台发送消息以发送一些参数，包括分配的RNTI，并且它将使用RA-RNTI作为该消息的目的地地址。
The eNodeB also allocates a resource to that terminal sothat it can send a connection request.eNodeB还为该终端分配资源，以便它可以发送连接请求。
When the terminal receives this message, it can transmitits transmission request on the allocated resource usingthe given RNTI.当终端接收到该消息时，它可以使用给定的RNTI在分配的资源上发送其传输请求。
The rest of the connection procedure is described inanother week of this MOOC.其余的连接程序在本MOOC的另一周描述。
I said I would explain what happens when 2UEs access the contention channel with the same sequence.我说我会解释当2个UE以相同的顺序访问争用信道时会发生什么。
Each terminal goes through the random-access procedurethinking it is the only one.每个终端都经过随机访问程序，认为它是唯一的一个。
The eNodeB will note one request, thinking it is the onlyone and will then go through the procedure we just saw:it will assign an RNTI and allocate a transmission resource,and send this information to the mobile.eNodeB将记录一个请求，认为它是唯一的一个，然后将通过我们刚刚看到的过程：它将分配RNTI并分配传输资源，并将此信息发送到移动设备。
The 2 phones will receive this message thinking it is meant for them,each will transmit a connectionrequest and an interference will occur.2个电话将收到此消息，认为它是针对他们的，每个电话都会发送连接请求，并且会发生干扰。
As for interference, 2 cases are possible.
至于干扰，可能有2种情况。
In the first case, the base station is not ableto decode either of the messages and will not respond.在第一种情况下，基站不能解码任何一个消息而不会响应。
The terminals will then repeat the procedure later.然后终端将重复该过程。
In the other case, the base station can decode one of the messages.在另一种情况下，基站可以解码其中一个消息。
That’s what we see here.这就是我们在这里看到的。
It believes there is only one user and continues the procedure.它认为只有一个用户并继续该过程。
This is a problem because we don’t know which of the 2terminals was registered.这是一个问题，因为我们不知道哪两个终端已注册。
To solve this problem, the standard dictates that theeNodeB must send to the UE an echoof its connection request.为了解决这个问题，标准规定eNodeB必须向UE发送其连接请求的回声。
Because the connection request contains the identity of the mobile the terminal can compare its identity to the one in this copy.因为连接请求包含移动设备的标识，所以终端可以将其标识与该副本中的标识进行比较。
If it is its own identity,it continues the connection procedure.如果它是自己的标识，则继续连接过程。
Otherwise it stops the procedure and starts again later.
否则它会停止程序并稍后再次启动。
In a nutshell, LTE is a reservation-based protocol.简而言之，LTE是一种基于预留的协议。
Allocation is managed dynamically by the eNodeB.分配由eNodeB动态管理。
The registered terminals have theright to use the control channel to send their transmission requests.注册终端有权使用控制信道发送其传输请求。
To register in a new cell, the terminals use therandom-access channel.
为了在新小区中注册，终端使用随机接入信道。
This channel is contention based.此信道是基于竞争的。
And a protocol is used to resolve ambiguities resulting from collisions.并且协议用于解决由冲突导致的模糊。