WAN vs LAN — The Basics
A LAN (Local Area Network) connects devices within a single building or campus — you own and control all the infrastructure. A WAN (Wide Area Network) connects sites across cities, countries, or continents — you typically lease connectivity from a carrier rather than building it yourself.
The key WAN challenge is balancing cost, performance, and reliability. Dedicated private circuits deliver predictable performance but cost significantly more than shared internet. Modern enterprises use a mix of technologies — private MPLS for critical traffic and cheaper internet connections for general traffic, managed intelligently by SD-WAN.
MPLS — Multiprotocol Label Switching
MPLS is a carrier-provided WAN technology that routes traffic based on short fixed-length labels rather than performing complex IP address lookups at every hop. When traffic enters the MPLS network at an edge router (PE router — Provider Edge), the carrier assigns a label. Core routers (P routers — Provider core) forward packets purely by swapping labels, making forwarding extremely fast. When traffic exits the MPLS network, the label is removed and normal IP routing resumes.
MPLS supports traffic engineering — the ability to pre-determine the exact path packets take through the carrier's network, ensuring latency-sensitive traffic (voice, video conferencing) always takes the lowest-latency path. MPLS also supports multiple VPNs over the same infrastructure through MPLS VPNs (Layer 3 VPN), where each customer's traffic is completely isolated from other customers despite sharing the carrier's physical infrastructure.
SD-WAN — Software-Defined Wide Area Network
SD-WAN applies software-defined networking (SDN) principles to WAN connectivity. Instead of being locked into a single expensive MPLS circuit, SD-WAN creates an overlay network on top of any combination of underlying transport links — MPLS, broadband internet, LTE/5G, or satellite — and manages them through a centralised software controller.
SD-WAN constantly monitors the performance of every available link (latency, jitter, packet loss) and automatically routes each application's traffic over the best-performing path in real time. Latency-sensitive applications (VoIP, video conferencing) are sent over the lowest-latency link; bulk transfers (backups, software updates) can use cheaper broadband links. If a link degrades or fails, traffic is instantly rerouted to an alternate path — often without users noticing.
The result is a dramatic reduction in WAN costs (replacing expensive dedicated MPLS with commodity internet) while maintaining or improving performance for cloud-based applications. SD-WAN also provides centralised visibility and management — a single dashboard shows traffic flows, application performance, and security posture across all branch sites simultaneously.
Most enterprises today are not choosing between MPLS and SD-WAN — they're replacing or augmenting MPLS with SD-WAN. A common architecture: keep a lower-bandwidth MPLS circuit for the most critical traffic (real-time voice, financial transactions) while adding cheaper broadband and LTE connections. SD-WAN intelligently manages all three, giving you MPLS-level performance for critical apps at a fraction of the cost of pure MPLS.
The tipping point: when most traffic is destined for cloud applications (Office 365, Salesforce, cloud ERP) rather than a corporate data centre, MPLS's hub-and-spoke model forces all that traffic through HQ — creating a bottleneck. SD-WAN enables direct internet breakout at each branch, dramatically improving cloud application performance.
Leased Lines
A leased line (also called a dedicated line or private circuit) is a dedicated, symmetric, point-to-point connection between two fixed locations, provided and maintained by a carrier. Unlike broadband, bandwidth is not shared with other customers — the full contracted capacity is available 24/7 with guaranteed uptime SLAs.
Common leased line standards include T1 (1.544 Mbps, North America) and E1 (2.048 Mbps, Europe), with higher-bandwidth options scaling through T3 (44.7 Mbps) and fiber-based services. Modern leased lines are typically delivered as Ethernet circuits (EoF — Ethernet over Fiber) at speeds from 10 Mbps to 10 Gbps.
Use cases: connecting headquarters to a data centre with guaranteed bandwidth, internet exchange connections for ISPs, point-to-point links between two buildings in the same city. Leased lines are significantly more expensive than broadband but provide the guaranteed performance and SLA that critical links require.
Other WAN Technologies
| Technology | Type | Key Characteristics | Use Case |
|---|---|---|---|
| DSL | Broadband | Uses telephone copper lines. ADSL asymmetric (faster download). VDSL faster but shorter range. Shared last mile. | Small office, home office broadband |
| Cable (DOCSIS) | Broadband | Uses coaxial TV cable. Shared neighbourhood segment — performance varies with congestion. High speeds available. | Home/SMB broadband |
| Fiber (FTTH/FTTP) | Broadband | Fiber to the premises. Symmetrical speeds available. Lowest latency of broadband options. Gold standard for business. | Business broadband, ISP backhaul |
| LTE / 5G | Cellular WAN | Wireless WAN. Used as primary connectivity in remote sites or as failover backup. High latency on LTE vs fiber. | Remote sites, WAN failover, IoT |
| Satellite | Satellite | Global coverage. High latency (~600ms geostationary, ~40ms LEO/Starlink). LEO satellites dramatically improved usability. | Remote/rural sites with no terrestrial option |
| Metro Ethernet | Carrier Ethernet | Ethernet connectivity across a metropolitan area via carrier fiber. Scalable bandwidth. Point-to-point or multipoint. | Connecting multiple sites in a city |
| Frame Relay | Legacy packet-switched | Legacy WAN technology — largely replaced by MPLS and broadband. Uses permanent virtual circuits (PVCs). Still appears on older Network+ exam versions. | Legacy corporate WANs (historical) |
| ATM | Legacy cell-switched | Uses fixed 53-byte cells. Very low latency. Legacy carrier backbone technology, largely replaced. May appear on older exams. | Legacy carrier backbones (historical) |
MPLS = label-based switching, traffic engineering, QoS, private carrier network. Key terms: PE router, P router, LSP (Label Switched Path), MPLS VPN.
SD-WAN = software overlay over any WAN links, centralised controller, application-aware routing, reduces cost vs pure MPLS.
T1 = 1.544 Mbps (24 DS0 channels × 64 Kbps). T3 = 44.7 Mbps (28 T1s). E1 = 2.048 Mbps (30 channels, used in Europe).
Frame Relay and ATM are legacy technologies — they appear in older study materials and may appear on exams as "what did MPLS replace?"
WAN Connectivity Concepts
Exam Scenarios
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