LEO NETWORK QUICK START

Figure 1 plots a typical architecture of emerging LSNs, which has been used by today’s operational LSNs such as SpaceX’s Starlink and OneWeb. Overall, the entire LSN can be divided into a space segment which consists of a considerable number of LEO satellites, together with a terrestrial segment that contains many geo-distributed ground stations around the world.

图1展示了新兴低地轨道卫星网络（LSNs）的典型架构，该架构已被当前运营的LSNs（如SpaceX的Starlink和OneWeb）广泛采用。整体来看，整个LSN可以分为两个部分：空间段和地面段。空间段由大量LEO卫星组成，而地面段则包含分布于全球的多个地面站。

On the user side, to access the LSN, a user needs to purchase and deploy a dedicated satellite terminal (i.e., a dish), together with a home router which connects to the user’s terminal (e.g., a smartphone or a laptop) via a WiFi or Ethernet interface. On the ground station side, the LSN exchanges traffic with the terrestrial Internet through a set of geographically distributed gateways behind ground stations. When the LSN provides Internet services for terrestrial users, user traffic is forwarded to LEO satellites via the satellite terminal, then to a ground station, and finally to the gateway and terrestrial Internet (and vice versa).

在用户端，为了接入LSN，用户需要购买并部署专用的卫星终端设备（即天线盘），以及一台家庭路由器。家庭路由器通过WiFi或以太网接口连接到用户终端设备（如智能手机或笔记本电脑）。

在地面站端，LSN通过位于地面站后的多个地理分布式网关与地面互联网交换流量。当LSN为地面用户提供互联网服务时，用户流量会通过卫星终端传输至LEO卫星，再传输到地面站，最终到达网关并接入地面互联网（反向亦然）。

If the user is close to an available ground station (i.e., both the user and ground station can be covered by a single satellite), satellites can use the well-known “bent-pipe” routing mechanism to transparently forward user traffic to the corresponding ground station. Otherwise, for users in remote areas far away from available ground stations, the LSN can use inter-satellite links (ISLs) to route user traffic to the ground station [4].

如果用户靠近可用的地面站（即用户和地面站均可被同一颗卫星覆盖），卫星可以利用众所周知的“弯管”（bent-pipe）路由机制，将用户流量透明地转发至相应的地面站。否则，对于远离可用地面站的偏远地区用户，LSN可以利用星间链路（ISLs）将用户流量路由至地面站。

Note

LEO Mobility. Unlike traditional geostationary (GEO) satellite networks, one key property of emerging LSNs is that: a portion of the network infrastructure, i.e., LEO satellite routers, are moving at a high velocity related to the earth. Such unique LEO mobility can result in a series of network instability issues such as handovers (i.e., ground nodes have to frequently disconnect with the outgoing satellite, and connect to a new incoming satellite), channel quality variations and link rate adaptation, and path fluctuations which can further affect the performance of end-to-end connections. In the remainder of this paper, to avoid ambiguity we use end host to refer to the "user terminal" and use terminal to represent the "satellite terminal" plotted in Figure 1. In addition, because SpaceX’s Starlink is currently the largest operational LSN with more than 4 million subscribers , and OneWeb only provides services for businesses and is currently unavailable for individual academic users, in this paper we carry out our performance study based on Starlink.

LEO移动性 与传统的地球同步轨道（GEO）卫星网络不同，新兴低地轨道卫星网络（LSNs）的一个关键特性在于：部分网络基础设施（即LEO卫星路由器）相对于地球表面以高速移动。这种独特的LEO移动性会导致一系列网络不稳定问题，例如切换（即地面节点需要频繁断开与离轨卫星的连接，并与新进入轨道的卫星建立连接）、信道质量变化、链路速率自适应以及路径波动，这些问题会进一步影响端到端连接的性能。

为了避免歧义，本文其余部分中将“用户终端”称为端主机（end host），而将“卫星终端”称为终端设备（terminal），如图1所示。此外，由于SpaceX的Starlink目前是最大的运营LSN，拥有超过400万订阅用户，而OneWeb仅为企业提供服务且当前不向个人学术用户开放，因此本文的性能研究基于Starlink进行。

LEO and GEO

• LEO: Low Earth Orbit
• LSNs: LEO Satellite Networks
• GEO: Geostationary Earth Orbit
• GSNs: GEO Satellite Networks

LEO的“卫星路由器”相对于地球表面以高速移动，这点很🥚疼 ->

1. 频繁切换（即: 地面节点需要频繁断开与离轨卫星的连接，并与新进入轨道的卫星建立连接）
2. 信道质量变化
3. 链路速率自适应
4. 路径波动

术语更改

在后文中，我们统一术语：