MAN & WAN

Different technologies are used for MAN and WAN networks.

Leased Lines

A dedicated, full-time point-to-point link between 2 entities. Always available. Not shared.

• Fixed carrying capacity: 64 kb/s, 2 Mb/s, or 155 Mb/s.
• Fixed monthly cost — independent of usage.
• Types: analog or digital. Most modern lines are digital.
• Common speeds: 64 kb/s (few computers), 2 Mb/s (large office).

Used in:

• Voice: tie lines between PBXs.
• Data: corporate networks, Internet access.
• Combined voice + data.

X.25

X.25 — ITU standard for user interface to a packet data network.

• Introduced: 1960s. Became standard for data communication.
• Used by: banks, airlines.
• Speed: ~64 kb/s.
• Very expensive relative to modern networks.

Frame Relay

WAN virtual-circuit technology. Replacement for X.25. Designed in late 1980s. Widely deployed in 1990s.

• Relays frames point-to-point. Carries IP datagrams — acts as link layer for IP.
• No error control — assumes network is reliable.
• End-to-end congestion control.

Virtual circuit model:

• Permanent VCs (PVCs) — aggregate traffic between two routers.
• Multiple VCs per physical link.
• CIR (Committed Information Rate) — guaranteed data rate per VC. Negotiated at setup. Customer billed per CIR.

ATM (Asynchronous Transfer Mode)

ATM — cell-switched WAN technology. Fixed 53-byte cells (5-byte header, 48-byte data).

Why cells:

• Uniform size → fast hardware processing.
• Simple header → low switching delay.
• Suitable for voice, video, and data on a single network.
• X.25/Frame Relay switching delay unacceptable for real-time traffic.

Properties:

• Scalable and flexible.
• Charging based on cell count — not fixed bandwidth tiers.
• Transmission medium: optical fiber.

Deployment reality:

• Original vision: end-to-end desktop-to-desktop ATM.
• Actual use: ATM as switched link layer connecting IP backbone routers.

Virtual circuits in ATM:

• Each cell carries a VC identifier — not destination address.
• Every switch on the path maintains per-connection state.
• Resources (bandwidth, buffers) allocated per VC.

Addressing:

• VPI (Virtual Path Identifier) + VCI (Virtual Circuit Identifier).
• Two-part addressing speeds up routing and switching.

Technology Comparison

MPLS (Multi-Protocol Label Switching)

MPLS — connection-oriented switching using labels applied at the edge of an MPLS domain. “Multiprotocol” — applicable to any Layer 2 protocol.

• Based on IP and routing protocols: BGP-4, OSPF, IS-IS.
• Resides in service provider/carrier networks only — not private networks.
• IP used to signal MPLS connections.

Protocol stack (bottom to top):

1. Physical
2. Layer 2 (PPP, ATM, Frame Relay, …)
3. MPLS
4. IP or Multi-Service
5. Application

Major applications:

• Network scalability.
• Traffic engineering.
• VPNs.

Additional capabilities: IP Multicast, IP CoS, RSVP.

MPLS hybrid position:

Principle: Route at the edge; switch at the core.

Convergence benefit: MPLS enables a single IP/MPLS network to consolidate PSTN, IP, Frame Relay, and ATM networks.

Metro Ethernet / Ethernet First Mile (EFM)

EFM — connects homes and offices via Ethernet. IEEE 802.3ah standard.

EFMC (EFM over Copper):

• Uses DSL modulation with Ethernet frames.
• EFMC SR: 10 Mb/s at 750 m.
• EFMC LR: 2 Mb/s at 2,700 m.

EFMF (EFM over Fibre):

• Point-to-point or Passive Optical Networks (PON).
• Dual-fiber standards: 100BASE-LX10, 1000BASE-LX10.
• Single-fiber standards: 100BASE-BX10, 1000BASE-BX10.

• X.25 — early standard; slow (~64 kb/s); expensive.
• Frame Relay — X.25 replacement; virtual circuits; no error control; CIR billing.
• ATM — fixed 53-byte cells; low delay; supports voice/video/data; VPI+VCI addressing.
• MPLS — label switching at provider core; supports traffic engineering, VPNs, scalability; hybrid of IP flexibility and ATM performance.
• EFM — Ethernet to the home/office over copper (DSL) or fibre (PON).
• Dominant trend: MPLS and Ethernet replacing legacy WAN technologies.