2.1.7 Internet Infrastructure and Access
The Internet is supported by many independent networks, access systems and communication links. A user does not connect directly to one single worldwide machine. Data first enters an access network, then travels through interconnected provider networks towards its destination.
This section focuses on the infrastructure named in the syllabus: modems, the Public Switched Telephone Network (PSTN), dedicated lines and cell phone networks. Internet Service Providers and exchange points are included as supporting context.
By the end of this section, you should be able to:
- describe the Internet as a collection of interconnected networks;
- explain how a user first connects through an access provider;
- describe the role of a modem in an Internet connection;
- explain how the PSTN has supported dial-up and broadband access;
- describe the purpose and characteristics of a dedicated line;
- trace mobile Internet access through a cell tower and mobile provider;
- compare access methods for different situations.
The Internet as a connected system
The Internet is the largest internetwork. Home networks, school networks, mobile networks, cloud-provider networks and Internet Service Provider networks all remain separately managed, but they exchange data using agreed protocols.
Two parts of an Internet journey
| Part | Main question | Typical infrastructure |
|---|---|---|
| Access network | How does the user or organisation first get online? | Modem, telephone or fibre line, WiFi hotspot, cell tower |
| Provider and backbone networks | How does traffic travel between different organisations and regions? | ISP networks, routers, dedicated links, fibre backbones and exchange points |
Common mistake
The Internet is not one centrally owned network. It is a connected collection of networks operated by many organisations.
How a user gets online
The first part of the journey depends on the access method.
| User situation | First connection path |
|---|---|
| Home broadband | Device → home router/modem → access line → ISP |
| Public WiFi hotspot | Device → WAP → wired Internet connection → ISP |
| Mobile data | Phone → cell tower → mobile provider network → Internet |
| Organisation with a dedicated circuit | Organisation router → permanent leased connection → provider network |
Exam tip
Trace the access method in order. For mobile access, for example: device → cell tower → mobile network → Internet.
Modems and the PSTN
PSTN: the Public Switched Telephone Network, originally designed for voice calls but later also used to carry Internet traffic.
Classic dial-up access
Traditional telephone lines carried analogue voice-frequency signals. A dial-up modem converted a computer's digital data into audio-frequency signals for transmission through the telephone network. A modem at the receiving end reversed the process.
- The modem dialled a provider's access number.
- A temporary connection was established through the PSTN.
- Digital data were represented using signals that the telephone line could carry.
- The receiving modem recovered the digital data.
- The telephone line was normally occupied while the connection was active.
Broadband use of telephone infrastructure
Telephone companies later upgraded much of their infrastructure and introduced broadband services. A broadband modem uses the signalling method required by the access line, while the provider connects the customer to its Internet network.
| Feature | Dial-up | Broadband over access infrastructure |
|---|---|---|
| Connection | Established when required | Normally continuously available |
| Typical data rate | Very limited | Much higher |
| Telephone use | Traditional line may be occupied | Voice and data can be separated by the technology used |
| Modem role | Modulates and demodulates audio-frequency signals | Encodes and decodes data for the broadband access technology |
Common mistake
A modem is not the same as an ISP. The modem handles signalling on the access link; the ISP provides connectivity to the wider Internet.
Dedicated and leased lines
A dedicated line, often called a leased line, is a permanent communication link reserved for one customer or organisation rather than set up only when required.
Why an organisation may use one
- Predictable capacity is available at all times.
- The connection is not established separately for every session.
- Business sites can exchange data through a permanent link.
- Service-level agreements may specify availability and repair targets.
- Traffic can be carried between an organisation and its provider or between sites.
Trade-offs
| Benefit | Related drawback |
|---|---|
| Permanent availability | The organisation pays for the service continuously. |
| Predictable data rate | Higher cost than ordinary shared consumer access. |
| Suitable for important business traffic | Installation may take time and depend on provider coverage. |
| Can connect separate sites | A backup route may still be required if the line fails. |
Example
A design company transfers large project files continuously between two offices. A dedicated line is suitable because the company needs permanent, predictable capacity. The higher monthly cost is justified by the operational requirement.
Cell phone networks and Internet access
A mobile device can connect without using a local fixed line. Its cellular radio communicates with a nearby base station or cell tower. The mobile network then carries the data towards an Internet gateway operated by the mobile provider.
Important characteristics
- The user can remain connected while moving between coverage areas.
- The radio capacity of a cell is shared among connected users.
- Performance depends on signal strength, congestion, radio technology and provider coverage.
- The mobile company acts as the user's Internet access provider.
- The network manages handover when the device moves from one cell to another.
Example
A field engineer uploads inspection images from a rural road. Mobile access is useful because it does not require a nearby wired connection. However, the transfer may slow or fail where coverage is weak.
Common mistake
Mobile data does not travel directly from the phone to a website. It first enters the mobile provider's network through a cell tower.
Provider networks and traffic exchange
After entering an access network, traffic may pass through several provider networks. The exact commercial structure is complex, but three ideas help explain the journey.
Internet Exchange Point (IXP): a location where separate networks connect and exchange traffic directly.
| Network role | Contribution |
|---|---|
| Access provider | Connects homes, organisations or mobile users. |
| Regional or transit provider | Carries traffic between access networks and larger destinations. |
| Backbone or large content network | Operates high-capacity links over large distances. |
| IXP | Allows participating networks to exchange traffic without sending it through an unnecessary third network. |
Routers are used throughout these networks, but their detailed forwarding process was covered in 2.1.6 Routing, Ethernet and Collision Management. Here, the important point is that access networks and provider networks are connected by routers and communication links.
Exam tip
For the syllabus hardware question, prioritise the named infrastructure: modem, PSTN, dedicated line and cell phone network. ISP hierarchy and IXPs are helpful supporting context rather than substitutes for those items.
Interactive: Internet Infrastructure Explorer
Choose how the user first gets online and then send a request. The hierarchy view emphasises provider roles; the mesh view emphasises multiple connected routes.
Practice
Explain the infrastructure
- Explain the role of a modem in an Internet connection.
- Describe how a dial-up connection used the PSTN.
- Explain two differences between dial-up and broadband access.
- Describe the purpose of a dedicated line.
- Trace mobile Internet access from a phone to the wider Internet.
- Explain the difference between an ISP and an IXP.
Choose an access method
- A temporary visitor needs Internet access in a railway station.
- A hospital needs a permanent, predictable link to a remote data centre.
- A delivery driver needs access while moving around a city.
- A historical question asks how computers sent data through analogue telephone lines.
For each situation, name the relevant infrastructure and explain why it is suitable.
Review
| Infrastructure | Main function | Important implication |
|---|---|---|
| Modem | Adapts digital data to the signalling method used by the access link | It is access hardware, not the service provider |
| PSTN | Telephone infrastructure that has carried Internet access | Supported dial-up and later broadband services |
| Dedicated line | Permanent reserved connection | Predictable service but higher cost |
| Cell phone network | Provides wireless Internet access through cellular infrastructure | Supports mobility but depends on coverage and shared capacity |
| ISP | Connects customers or networks to the Internet | May be fixed-line, mobile or organisational |
| IXP | Allows separate networks to exchange traffic | It is a connection facility, not an access provider |
Quick self-check
- Can I explain what a modem changes and why?
- Can I describe how the PSTN supported Internet access?
- Can I justify a dedicated line for an organisation?
- Can I trace a mobile connection through a cell tower?
- Can I distinguish an ISP from an IXP?