Different technologies are used for MAN and WAN networks.
PBX
Short for Private Branch Exchange. A telephone switching system used inside an organization.
Handles internal calls between extensions within the organization and connects to the public telephone network for external calls. Manages a shared pool of lines and routes calls internally.
PON
Short for Passive Optical Network. A single fiber from the provider splits passively into multiple fibers serving separate customers. No active electronics between the provider and customer.
Leased Lines
A dedicated, full-time point-to-point link between 2 entities. Always available. Not shared. Provided by a telecom provider.
Fixed carrying capacity: 64 kb/s, 2 Mb/s, or 155 Mb/s. Common speeds: 64 kb/s (few computers), 2 Mb/s (large office).
Very high fixed monthly cost. Bandwidth is wasted when there is no data communication.
Both analog and digital variants exist. Analog is old and less common today. Digital is dominant today.
Used in tie lines of PBXs (leased lines between 2 PBXs), corporate networks.
X.25
ITU standard defining the interface between user equipment and a public packet-switched data network. Introduced in the 1960s. Became the dominant standard for wide-area data communication.
Data is broken into packets and forwarded hop-by-hop through a shared network. Multiple customers’ traffic shares the same physical infrastructure.
Connection-oriented. A virtual circuit (logical path) is established before data transfer begins. All packets for a session follow that path.
Every intermediate switch checks each packet for errors and retransmits before forwarding. Designed for unreliable physical links. Introduces significant processing overhead and delay at each hop. Became a liability once physical links improved.
Supports both:
- Permanent Virtual Circuits
Aka. PVCs. The virtual circuit is configured once between 2 routers and stays up indefinitely. - Switched Virtual Circuits
Aka. SVCs. The circuit is established on-demand. Used for the duration of a session, then torn down. More common in X.25
Specs:
- Maximum speed: ~64 kb/s
- Max packet payload: 128 bytes
- Supports permanent virtual circuits (PVCs)
- No multicast support
- Per-node checking makes it unsuitable for real-time traffic. Only suitable for non-real-time data.
Less expensive than leased lines.
Very expensive compared to later technologies. Does not support multicasting. Used heavily by banks and airlines. Largely obsolete today.
Frame Relay
Direct replacement for X.25. Operates at the data link layer. Designed in the late 1980s. Widely deployed through the 1990s.
Per-node error checking removed. Delegated to upper-layer protocols. Frames forwarded hop-to-hop without per-node verification.
Connection-oriented. Uses virtual circuits. Most deployments use PVCs over SVCs.
Does not support multicasting.
Properties:
- Acts as link layer for IP. Carries IP datagrams.
- No per-node error control. Assumes reliable links.
- End-to-end congestion control only.
- Designed purely for data. Inter-frame delay is unpredictable.
- Not suitable for voice or video.
Specs:
- Max speed: 2 Mb/s
- Max payload: 1600 bytes
Committed Information Rate
Aka. CIR. The data rate a provider guarantees for a specific virtual circuit. Negotiated at VC setup. Billed on CIR, not actual usage. Traffic may burst above the CIR, but only the CIR is guaranteed.
ATM
Short for Asynchronous Transfer Mode. Cell-switched. Charged based on cell count. Uses optical fiber as transmission medium. Developed in 1980s and 1990s.
Speed is typically 155 Mb/s. Suitable for voice, video and data on a single network. Fixes X.25 and Frame Relay’s switching delay unacceptable for real-time traffic.
Connection-oriented. Every switch on the path maintains per-connection state. Supports both PVCs and SVCs.
Scalable. Flexible. Supports multicasting. Does not support variable-length frames.
ATM was designed to be a universal networking technology that would replace everything. Originally intended to operate at Layer 3, replacing IP. However when ATM was being deployed, Ethernet was already widely used. And was getting faster. Ethernet was cheaper, simpler, and already familiar to everyone.
ATM ended up being used only to interconnect IP backbone routers across WANs. Demoted to Layer 2, acting purely as a high-speed, connection-oriented link layer between backbone routers. The routers don’t care about ATM’s QoS classes or VC structure in any meaningful way; they just see a fast link.
Cell
Fixed size of 53-bytes. 5-byte header and 48-byte data. Simple header enables faster switching.
Fixed size cells enable faster processing at hardware level. This enables bounded delay. Inefficient for large data transfers. Constant 10% overhead per cell.
Each cell carries:
- Virtual Path Identifier
Aka. VPI. Identifies a virtual path. A bundle of virtual circuits sharing the same physical route through the network. Switches route based on VPI alone at intermediate nodes. - Virtual Circuit Identifier
Aka. VCI. Identifies a specific virtual circuit within a virtual path. Only meaningful in the context of a VPI.
At each switch, the VPI/VCI pair is replaced with a new pair on the outgoing link. Separating path-level from circuit-level routing lets intermediate nodes forward on VPI alone, without inspecting individual VCIs. Enables faster switching.
Quality of Service
Supports per-VC QoS guarantees via service classes.
- Constant Bit Rate (CBR)
Fixed, guaranteed bandwidth for a given virtual circuit. Used for uncompressed voice and video. - Variable Bit Rate (VBR)
Bandwidth varies within defined bounds. Peak and sustained rates are both specified. 2 sub-classes: real-time VBR and non-real-time VBR. - Available Bit Rate (ABR)
No guaranteed bandwidth. Sender adjusts rate based on congestion feedback from the network. - Unspecified Bit Rate (UBR)
No guarantees. Best-effort delivery. Cells dropped under congestion.
Metro Ethernet
Ethernet technology extended to MAN and WAN distances. Provided by telecom carriers as a managed service. IEEE 802.3ah standard.
As LANs, MANs, and WANs use the same technology, requirement for separate WAN hardware and expertise is eliminated. Customer’s existing Ethernet switches and routers connect directly. Lower cost. Higher speeds (10 Mbps to 10Gbps or more).
Ethernet First Mile
Aka. EFM. IEEE 802.3ah. A set of physical layer standards defining how Ethernet signals are carried over the link between the carrier’s network and the customer’s premises.
Called “first mile” because it’s the segment where the provider’s service begins, from the user’s perspective.
EFM over Copper
Aka. EFMC. Runs Ethernet over existing copper telephone lines. DSL modulation encodes Ethernet frames so they can travel over copper at MAN distances without requiring new cabling.
- EFMC short range: 10 Mb/s at 750 m.
- EFMC long range: 2 Mb/s at 2,700 m.
EFM over Fibre
Aka. EFMF. Fiber-based connections to customer premises. Supports point-to-point links and PONs.
- Dual-fiber
Separate fibers for TX and RX. Standards: 100BASE-LX10, 1000BASE-LX10. - Single-fiber
One fiber for both directions using separate wavelengths. Standards: 100BASE-BX10, 1000BASE-BX10.