When I consult a server using my smartphone, or a Web site, for example, what do the packets that arrive at my terminal pass through?

例如，当我使用智能手机或网站查询服务器时，到达终端的数据包通过了什么？

That is the question we will answer in this video.

这是我们将在本视频中回答的问题。

The Internet network itself cannot manage mobility.

互联网本身无法管理移动性。

Each packet is intended for an address and an address is generally linked to a physical location.

每个数据包用于地址，地址通常链接到物理位置。

The solution adopted for 4G networks, was to converge packets destined for mobile subscribers to a single network gateway called the PGW, or Packet Gateway.

4G网络采用的解决方案是将发往移动用户的数据包汇聚到称为PGW或分组网关的单个网络网关。

That means that when I consult a server, the packet – constituting, for example, a piece of a web site – will be transmitted through the Internet to my mobile operator’s P-Gateway, then from the P-Gateway to the base station using the mobile operator’s own IP network, and from the base station to my terminal over the radio channel.

这意味着当我咨询服务器时，数据包 - 例如，构成一个网站 - 将通过互联网传输到我的移动运营商的P网关，然后从P网关传输到基站。 移动运营商自己的IP网络，并通过无线电信道从基站到我的终端。

So, the function of the P-Gateway is to route data to the terminal and also to take care of a certain number of security functions.

因此，P-Gateway的功能是将数据路由到终端并且还处理一定数量的安全功能。

It is a gateway and it must be protected against attacks from outside.

它是一个网关，必须保护它免受来自外部的攻击。

In the opposite direction, when I send a request from my smartphone, it’s the same thing.

在相反的方向，当我从我的智能手机发送请求时，它是一样的。

From the terminal to the eNode B, from the eNode B to the P-Gateway, from the P-Gateway to the server.

从终端到eNode B，从eNode B到P-Gateway，从P-Gateway到服务器。

If we have a network with a very large number of subscribers like in China, there can be several PGWs because one PGW does not have enough capacity to manage all packets.

如果我们拥有像中国这样拥有大量订户的网络，则可能存在多个PGW，因为一个PGW没有足够的容量来管理所有数据包。

Several PGW can also be necessary to have redundancy and ensure reliability.

多个PGW也可能需要冗余并确保可靠性。

However, P-Gateways are alimited in number.

但是，P-Gateways的数量有限。

If you think about it for a second, the typical number of eNodeBs for a French network is around 10,000.

如果您考虑一下，法国网络的典型eNodeB数量大约为10,000。

In certain cases, in dense areas, the coverage of an eNodeB is just several hundred meters around that eNodeB.

在某些情况下，在密集区域，eNodeB的覆盖范围仅在eNodeB周围几百米处。

On the other hand, there are just a few P-Gateways, in 4G network.

另一方面，在4G网络中只有少数P网关。

If we consider the transfer to a terminal,when this terminal is moving, we can see we’ll have to notify the P-Gateway and route data NOT to the former eNodeB where the subscriber was before, but to the new eNodeB.

如果我们考虑转移到终端，当这个终端移动时，我们可以看到我们必须通知P-Gateway并且不将数据路由到用户之前的前eNodeB，而是路由到新的eNodeB。

This rerouting requires the exchange of signaling messages.

这种重新路由需要交换信令消息。

To avoid frequent rerouting at the level of the P-Gateway, we’ll introduce intermediary equipment called the Serving Gateway.

为了避免在P-Gateway层面频繁重新路由，我们将引入称为服务网关的中间设备。

The Serving Gateway serves a given geographic zone,typically one French region.

服务网关服务于给定的地理区域，通常是一个法国区域。

Now, we have an additional hop:server- P-Gateway, P-Gateway - S-Gateway,  S-Gateway -eNodeB.

现在，我们还有一个额外的跃点：服务器P网关，P网关 - S网关，S网关-eNodeB。

That seems slightly more complex, but the advantage is that, when the terminal moves, very often it remains under the same S-Gateway, and that it’s only the S-Gateway that has to manage  this mobility, this rerouting.

这似乎稍微复杂一点，但优点是，当终端移动时，它经常保持在同一个S网关下，并且只有S网关必须管理这种移动性，这种重新路由。

So, an S-Gateway enables not only the collection of data sent by the mobile terminals to various eNodeB, but also the distribution of data coming from servers to the eNodeBs where the terminal is, which is what we represented here.

因此，S-Gateway不仅能够收集移动终端发送到各种eNodeB的数据，还能够将来自服务器的数据分发到终端所在的eNodeB，这就是我们在此处所代表的。

The Serving Gateway and the Packet Gateway are two very important elements of the core of the 4G network.

服务网关和分组网关是4G网络核心的两个非常重要的元素。

Something to note is that the packets that make up the data stream – for example, the video stream I look at on my smartphone – go through the P-Gateway – it’s the entrance – and the Serving  Gateway – which is linked to the place, the region where I am located.

需要注意的是，组成数据流的数据包 - 例如我在智能手机上看到的视频流 - 通过P-Gateway - 它是入口 - 和服务网关 - 链接到该地点， 我所在的地区。