Data Transmission

Calculate Data Transfer Rates on a Network

Routing Cost Formula

The cost of transmitting data along a network link is calculated as:

Link cost = 1 Mbps ÷ Transmission speed of the link

A lower cost means a faster (and therefore cheaper) link. A higher-speed link has a lower cost because data travels more quickly.

Unit Conversions

Unit	Equivalent
1 byte	8 bits
1 Kbps (kilobits per second)	1,000 bps
1 Mbps (megabits per second)	1,000 Kbps = 1,000,000 bps
1 Gbps (gigabits per second)	1,000 Mbps
1 KB (kilobyte)	1,000 bytes = 8,000 bits
1 MB (megabyte)	1,000 KB = 8,000,000 bits

Worked Example 1 — Link Cost

A network link has a transmission speed of 100 Mbps. Calculate its cost.

Cost = 1 Mbps ÷ 100 Mbps = 0.01

Worked Example 2 — Comparing Links

Link	Speed	Cost
A → B	10 Mbps	1 ÷ 10 = 0.1
A → C	100 Mbps	1 ÷ 100 = 0.01
B → D	50 Mbps	1 ÷ 50 = 0.02
C → D	20 Mbps	1 ÷ 20 = 0.05

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Calculate Lowest Cost Routes on a Network

To find the lowest cost route between two nodes, add up the costs of all links on each possible path and choose the path with the smallest total cost.

Worked Example — Simple Routing

Using the link costs above, find the lowest cost route from A to D:

Route 1: A → B → D = 0.1 + 0.02 = 0.12

Route 2: A → C → D = 0.01 + 0.05 = 0.06

Lowest cost route: A → C → D (cost = 0.06)

This makes sense — the route through C uses faster links overall.

Worked Example — Larger Network

Consider a network with nodes A, B, C, D, E:

Link	Speed (Mbps)	Cost
A → B	100	0.01
A → C	50	0.02
B → D	20	0.05
B → E	200	0.005
C → D	100	0.01
D → E	50	0.02

Find the lowest cost route from A to E:

Route	Links	Total Cost
A → B → E	0.01 + 0.005	0.015
A → B → D → E	0.01 + 0.05 + 0.02	0.08
A → C → D → E	0.02 + 0.01 + 0.02	0.05

Lowest cost route: A → B → E (cost = 0.015)

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Routing with Delay Factors

In real networks, links may experience delays due to congestion, distance, or equipment. A delay factor is a multiplier applied to the base cost.

Adjusted link cost = Base cost × Delay factor

A delay factor of 1.0 means no delay. A factor greater than 1.0 increases the effective cost.

Worked Example — Delay Factors

Using the same network, but now with delay factors:

Link	Base Cost	Delay Factor	Adjusted Cost
A → B	0.01	3.0	0.01 × 3.0 = 0.03
A → C	0.02	1.0	0.02 × 1.0 = 0.02
B → E	0.005	5.0	0.005 × 5.0 = 0.025
C → D	0.01	1.5	0.01 × 1.5 = 0.015
D → E	0.02	1.0	0.02 × 1.0 = 0.02

Find the lowest cost route from A to E (with delays):

Route	Adjusted Costs	Total
A → B → E	0.03 + 0.025	0.055
A → C → D → E	0.02 + 0.015 + 0.02	0.055

Both routes have equal adjusted cost (0.055). Either could be chosen — in practice, a router might use additional criteria (e.g. hop count, reliability) to break the tie.

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The Internet as a World-Wide Communications Infrastructure

What is the Internet?

The Internet is a global network of interconnected networks that uses a standardised set of protocols (TCP/IP) to enable communication between billions of devices worldwide.

Key Characteristics

Characteristic	Description
Decentralised	No single authority controls the entire Internet
Packet-switched	Data is broken into packets that travel independently
Fault-tolerant	If one route fails, packets can take alternative paths
Scalable	New networks and devices can be added easily
Open standards	Uses publicly available protocols (TCP/IP, HTTP, etc.)

The Three-Tier Structure

The Internet is organised into three tiers of Internet Service Providers (ISPs):

Tier	Role	Examples
Tier 1	Global backbone providers. Own and operate major long-distance fibre-optic links that span continents. Peer with other Tier 1 providers (exchange traffic at no cost).	BT, AT&T, NTT, Lumen
Tier 2	Regional ISPs. Purchase transit from Tier 1 providers and sell connectivity to Tier 3. May also peer with other Tier 2 providers.	Sky, Virgin Media, regional carriers
Tier 3	Local ISPs. Purchase connectivity from Tier 2 providers and sell directly to end users (homes and businesses).	Local broadband providers, mobile operators

How tiers connect:

• Peering — two networks of similar size agree to exchange traffic directly, usually at no cost. Common between Tier 1 providers.
• Transit — a smaller network pays a larger network to carry its traffic to the rest of the Internet. Tier 3 pays Tier 2, Tier 2 pays Tier 1.
• Internet Exchange Points (IXPs) — physical locations where multiple ISPs connect their networks to exchange traffic efficiently.

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Protocols and Standards

TCP/IP Protocol Suite

The TCP/IP model is the standard framework for Internet communication, organised into four layers:

Layer	Name	Protocols	Function
4	Application	HTTP, HTTPS, FTP, SMTP, DNS	Provides services directly to the user (web, email, file transfer)
3	Transport	TCP, UDP	Ensures reliable delivery (TCP) or fast delivery (UDP). Manages port numbers.
2	Internet	IP, ICMP	Handles addressing and routing. IP addresses identify source and destination.
1	Network Access	Ethernet, Wi-Fi	Handles physical transmission of data over the network medium

Key Protocols

Protocol	Full Name	Purpose
HTTP/HTTPS	HyperText Transfer Protocol (Secure)	Transfers web pages between browser and server. HTTPS encrypts the connection using TLS/SSL.
FTP	File Transfer Protocol	Transfers files between computers. Supports upload and download.
SMTP	Simple Mail Transfer Protocol	Sends emails from client to server and between servers
POP3	Post Office Protocol v3	Downloads emails from server to client (removes from server)
IMAP	Internet Message Access Protocol	Accesses emails on the server without downloading (syncs across devices)
DNS	Domain Name System	Translates domain names (e.g. www.example.com) into IP addresses (e.g. 93.184.216.34)
TCP	Transmission Control Protocol	Reliable, connection-oriented data transfer with error checking and retransmission
UDP	User Datagram Protocol	Fast, connectionless transfer without guaranteed delivery (used for streaming, gaming)
IP	Internet Protocol	Addresses and routes packets across networks

DNS Resolution Process

When you type a URL into a browser:

1. Browser checks its local cache for the IP address
2. If not cached, the OS checks the hosts file
3. If not found, a request is sent to the local DNS resolver (usually provided by your ISP)
4. The resolver checks its cache; if not found, it queries a root DNS server
5. The root server directs the resolver to the appropriate Top-Level Domain (TLD) server (e.g. .com, .co.uk)
6. The TLD server directs the resolver to the authoritative DNS server for the specific domain
7. The authoritative server returns the IP address
8. The resolver caches the result and returns it to the browser
9. The browser connects to the web server using the IP address

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Why Standards Matter

The Importance of Standards

• Interoperability — devices and software from different manufacturers can communicate because they all follow the same standards
• Competition — standards prevent vendor lock-in and allow multiple companies to compete, driving innovation and lowering prices
• Reliability — standardised protocols have been extensively tested and are well-understood
• Global reach — the Internet works worldwide because all connected networks use TCP/IP

Challenges in Developing Standards

• Competing interests — different companies may want standards that favour their products
• Speed of change — technology evolves faster than standards bodies can work (e.g. the long development of HTML5)
• Legacy compatibility — new standards must often maintain backward compatibility with older systems
• Global agreement — standards must work across different countries, languages, and legal frameworks

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Internet Services

Service	Description	Protocols Used
World Wide Web (WWW)	System of interlinked hypertext documents accessed via web browsers	HTTP/HTTPS, HTML, CSS, JavaScript
Email	Electronic messaging between users	SMTP (sending), POP3/IMAP (receiving)
VoIP	Voice over IP — telephone calls over the Internet	SIP, RTP, UDP
Cloud computing	On-demand access to computing resources (storage, processing, software) over the Internet	HTTPS, various APIs
Streaming	Real-time delivery of audio/video content without downloading first	HTTP (adaptive streaming), UDP (live)
File sharing	Transfer of files between users or systems	FTP, BitTorrent, HTTP
Instant messaging	Real-time text communication	XMPP, proprietary protocols