The network edge

Desktop computers, servers, mobile devices, etc. are referred to as end systems because they sit at the edge of the Internet.

End systems are also referred to as hosts because they host (that is, run) application programs (e.g., a Web browser, a Web server, an e-mail client, etc.).

Hosts may act as clients, servers, or both depending on the application being run.

Access networks

An access network is the portion of the network that connects an end system to the first router (or edge router) that provides access to the Internet backbone.

After that point, traffic enters the core network that carries data between different networks.

Simplified view:

End system → Access network → Edge router → Internet core

Example of different types of access networks (mobile, home, and enterprise):

Access networks

Home access networks

Technology	Infrastructure	Customer equipment	Provider equipment	Characteristics
DSL (Digital Subscriber Line)	Uses existing telephone lines for Internet access. The local telephone company (telco) also acts as the ISP.	Splitter: separates telephone and data signals arriving at the home forwards the data signal to the DSL modem DSL modem: uses the telephone line to exchange data with the DSLAM located in the CO (telco's Central Office) converts digital data into high-frequency tones for transmission receives analog signals and converts them back into digital data	DSLAM (Digital Subscriber Line Access Multiplexer): located in the CO separates the data and phone signals sends the data into the Internet serves thousands of households	Internet and telephone services share the same line using frequency-division multiplexing. Data and telephone signals are encoded at different frequencies: high-speed downstream channel medium-speed upstream channel two-way telephone channel Access is said to be asymmetric because the downstream and upstream rates are different.
Cable Internet Access	Uses existing cable television infrastructure. The cable television provider is also the ISP. Uses both fiber and coaxial cable referred to as Hybrid Fiber Coax (HFC).	Cable modem: communicates over upstream and downstream frequency channels access is asymmetric	CMTS (Cable Modem Termination System): located at the cable head end transforms analog signals from cable modems into digital signals	Shared broadcast medium: downstream channel is shared among users: more simultaneous users reduce individual download rates upstream channel is also shared a multiple access protocol is needed to coordinate transmissions and avoid collisions
FTTH (Fiber To The Home)	Delivers optical fiber directly from a Central Office (CO) to homes. Two main types: direct fiber: each home has its own fiber connection from the CO shared fiber: a single fiber from the CO is split among nearby homes	Optical Network Terminator (ONT): installed at each home users access the Internet through a router connected to the ONT	Splitter: splits one fiber connection among multiple homes A single shared fiber runs from the splitter to an Optical Line Terminator (OLT) at the CO. OLT: connects the fiber network to the Internet through a router	Shared fiber can use two architectures: AON (Active Optical Network): uses switched Ethernet PON (Passive Optical Network): uses optical splitters without active switching (example: Verizon Fios)

Technology

Infrastructure

Customer equipment

Provider equipment

Characteristics

Image

DSL (Digital Subscriber Line)

Uses existing telephone lines for Internet access.

The local telephone company (telco) also acts as the ISP.

Splitter:

separates telephone and data signals arriving at the home
forwards the data signal to the DSL modem

DSL modem:

uses the telephone line to exchange data with the DSLAM located in the CO (telco's Central Office)
converts digital data into high-frequency tones for transmission
receives analog signals and converts them back into digital data

DSLAM (Digital Subscriber Line Access Multiplexer):

located in the CO
separates the data and phone signals
sends the data into the Internet
serves thousands of households

Internet and telephone services share the same line using frequency-division multiplexing.

Data and telephone signals are encoded at different frequencies:

high-speed downstream channel
medium-speed upstream channel
two-way telephone channel

Access is said to be asymmetric because the downstream and upstream rates are different.

Cable Internet Access

Uses existing cable television infrastructure.

The cable television provider is also the ISP.

Uses both fiber and coaxial cable referred to as Hybrid Fiber Coax (HFC).

Cable modem:

communicates over upstream and downstream frequency channels
access is asymmetric

CMTS (Cable Modem Termination System):

located at the cable head end
transforms analog signals from cable modems into digital signals

Shared broadcast medium:

downstream channel is shared among users: more simultaneous users reduce individual download rates
upstream channel is also shared
a multiple access protocol is needed to coordinate transmissions and avoid collisions

FTTH (Fiber To The Home)

Delivers optical fiber directly from a Central Office (CO) to homes.

Two main types:

direct fiber: each home has its own fiber connection from the CO
shared fiber: a single fiber from the CO is split among nearby homes

Optical Network Terminator (ONT):

installed at each home
users access the Internet through a router connected to the ONT

Splitter:

splits one fiber connection among multiple homes

A single shared fiber runs from the splitter to an Optical Line Terminator (OLT) at the CO.

OLT:

connects the fiber network to the Internet through a router

Shared fiber can use two architectures:

AON (Active Optical Network): uses switched Ethernet
PON (Passive Optical Network): uses optical splitters without active switching (example: Verizon Fios)

Local area network (LAN)

A LAN (Local Area Network) is a network that connects devices within a small area, such as a home, school, or office.

The LAN is connected to an edge router, which links the local network to the Internet backbone.

Laptop  ─┐
Phone ───┤── LAN → Edge router → Internet
Printer ─┘

The most widely used LAN technologies are Ethernet and Wireless LANs (Wi-Fi).

Wireless wide area network (WWAN)

A WWAN (Wireless Wide Area Network) is a network that connects end systems over a large geographic area using wireless links.

The WWAN uses a mobile network operator's infrastructure to connect devices to an edge router, which provides access to the Internet backbone.

Unlike Wi-Fi networks, which cover short distances within homes or offices, WWAN technologies (e.g., 4G, 5G) provide much wider coverage, often extending across several kilometers or more.

Physical media

Physical media are the channels through which bits are transmitted between devices.

They allow end systems at the network edge to connect to the Internet through access networks.

Categories:

guided media: signals travel through solid materials (e.g., copper, fiber)
unguided media: signals travel through air or space (e.g., Wi-Fi, satellite)

Physical wires

Medium	Material	Physical structure	Speed	Distance	Electromagnetic interference resistance	Cost	Uses
Twisted-pair	Copper wire	Two insulated copper wires twisted together to reduce interference Several pairs are combined into one cable	Up to 10 Gbps	Up to 100 m for LANs	Low to moderate	Low	Most common network cable Affordable and easy to install Commonly used in Ethernet LANs
Coaxial cable	Copper core with shielding	Central copper conductor surrounded by insulation and a metal shield, all concentric around a common axis Shielding reduces interference	Up to multi-Gbps	Hundreds of meters or more	Moderate to high	Moderate	Used for cable TV and cable Internet Better shielding than twisted-pair cable
Fiber optics	Glass or plastic fiber	Made of a thin glass or plastic core surrounded by cladding Protected by outer coating and cable layers for strength and protection Transmits data as pulses of light through the core	Very high (Gbps to Tbps)	Several km to hundreds of km	Very high (immune to EMI)	Moderate to high	Very fast and reliable Low signal loss over long distances Used for Internet backbones and long-distance links

Terrestrial radio channels

Terrestrial radio channels use electromagnetic waves to transmit signals through the air.

Signal quality depends on factors such as distance, frequency, antenna characteristics, and the propagation environment.

Factors affecting performance:

path loss and shadow fading (signal weakening due to distance and obstacles)
multipath fading (caused by signal reflections)
interference (from other signals or transmissions)

Classification by range:

short-distance (1–2 meters) → wireless headsets, keyboards
local-area (10–hundreds of meters) → Wi-Fi
wide-area (tens of kilometers) → cellular networks

Satellite radio channels

Communication satellites use radio/microwave links to communicate between ground stations.

A satellite:

receives a signal from a ground station on one frequency band
processes or amplifies the signal using a transponder
retransmits the signal on another frequency band

Types of communication satellites:

geostationary satellites (GEO):
- appear fixed above one point on Earth
- orbit at 36,000 km altitude
- have high latency due to their distance (about 500–600 ms round-trip delay)
- useful in areas without terrestrial broadband access
- support data rates of hundreds of Mbps
low-earth orbiting satellites (LEO):
- orbit much closer to Earth and move relative to the surface
- circle the globe many times per day
- can communicate with each other and with ground stations
- require many satellites (a constellation) to provide continuous coverage

Published on 03/08/2026

Previous What is a protocol? All ⏎ Next The network core

Keyboard shortcut	Description
`←`	Previous item
`→`	Next item
`⇧` + `←`	Previous page
`⇧` + `→`	Next page
`f`	Show/hide furigana