计网第一章The Network Edge，the access network，physical media

The Network Edge

the applications and end systems are at the “edge of the network,”

the computers and other devices connected to the Internet are often referred to as end sys-
tems. They are referred to as end systems because they sit at the edge of the Internet

host = end system

Hosts are sometimes further divided into two categories: clients and servers.
clients tend to be desktops, laptops, smartphones, and so on, whereas
servers tend to be more powerful machines that store and distribute Web pages,
stream video, relay e-mail, and so on

the access network

—the network that physically connects an end system to the first router (also known as the “edge router”) on a path from the end system to any other distant end system.

1.Home Access: DSL, Cable, FTTH（ fiber to the home）, and 5G Fixed Wireless
the two most prevalent types of broadband residential access are
digital subscriber line (DSL) and cable

1.1 DSL连电话线，把数据用调制解调器等连入co
On the customer side, a splitter separates the data and telephone
signals arriving to the home and forwards the data signal to the DSL modem.
On the telco side, in the CO, the DSLAM separates the data and phone signals and sends
the data into the Internet. Hundreds or even thousands of households connect to a
single DSLAM.
The DSL standards define multiple transmission rates, including downstream
transmission rates of 24 Mbs and 52 Mbs, and upstream rates of 3.5 Mbps and
16 Mbps;— frequency-division multiplexing


1.2 cable用有线电视的混合光纤同轴接入网络–混合光纤同轴电缆 （HFC）

While DSL makes use of the telco’s existing local telephone infrastructure,
cable Internet access makes use of the cable television company’s existing cable
television infrastructure. A residence obtains cable Internet access from the same
company that provides its cable television.

Cable internet access requires special modems, called cable modems. As
with a DSL modem, the cable modem is typically an external device and con-
nects to the home PC through an Ethernet port. At the cable head end, the cable modem termination
system (CMTS) serves a similar function as the DSL network’s DSLAM—
turning the analog signal sent from the cable modems in many downstream
homes back into digital format

1.3 FTTH Internet access光纤到户

active optical networks (AONs) and passive optical networks (PONs)

AON is essentially switched Ethernet

FTTH using the PON distribution architecture. Each home has
an optical network terminator (ONT), which is connected by dedicated optical
fiber to a neighborhood splitter. The splitter combines a number of homes (typi-
cally less than 100) onto a single, shared optical fiber, which connects to an optical
line terminator (OLT) in the telco’s CO. The OLT, providing conversion between
optical and electrical signals, connects to the Internet via a telco router. At home,
users connect a home router (typically a wireless router) to the ONT and access the
Internet via this home router.
PNO图解：

2. Access in the Enterprise (and the Home): Ethernet and WiFi

2.1 Ethernet Internet access

3.Wide-Area Wireless Access: 3G and LTE 4G and 5G

Mobile devices such as iPhones and Android devices are being used to message,
These devices employ the same wireless infrastructure
used for cellular telephony to send/receive packets through a base station that is oper-
ated by the cellular network provider.

physical media

1. Twisted-Pair Copper Wire双绞线
   For over a hundred years it has been used by telephone networks.

2. Coaxial Cable同轴电缆
   Coaxial cable is quite common in cable television systems.

3. Fiber Optics光纤

4. Radio
   Terrestrial Radio Channels，Satellite Radio Channels
   Radio channels carry signals in the electromagnetic spectrum. They are an attrac-
   tive medium because they require no physical wire to be installed, can penetrate
   walls, provide connectivity to a mobile user, and can potentially carry a signal
   for long distances.