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The broadband networks of US telecommunications operators are fiber optic networks that use SONET (Synchronous Optical Network) and WDM (Wavelength Division Multiplexing) technologies. SONET uses TDM (time division multiplexing) to multiplex data streams of different speeds in the electrical domain and then converts the electrical signals into optical signals to be transported over fiber optics. The payload of SONET transmission systems can carry data streams of rates from T1 (1,544 Mb / s), T3 (44,736 Mb / s), OC-3 (155.52 Mb / s) to OC-192 (10 Gb / s) s). WDM allows multiplexing and transporting multiple optical signals with different wavelengths through a single optical fiber. Since each SONET signal uses a different wavelength, multiple SONET signals can be carried over an optical fiber using WDM.
Ethernet networks are defined according to data rates. There is 10 Mb / s, 100 Mb / s, 1 Gb / s and 10 Gb / s Ethernet. Since SONET is optimized for TDM (voice) traffic, it does not match the optimal speed for carrying a typical Ethernet data stream. To increase the efficiency of Ethernet transport over SONET, two new technology standards are introduced, namely Virtual Concatenation and Generic Framing Procedure. Virtual concatenation allows SONET channels to be multiplexed in an arbitrary arrangement to address bandwidth mismatch between Ethernet and SONET. SONET custom size pipes are created using multiple STS-1 / STS-3 as well as virtual tributaries (VT-1.5 / VT-2). Each SONET channel is independently and transparently routed through the network. This allows SONET pipes to be sized appropriately for Ethernet transport. The virtual concatenation channel dynamic bandwidth allocation capability, LCAS (Link Capacity Tuning Scheme), is also developed.
The Generic Framing Procedure (GFP) is a simple adaptation scheme that extends SONET’s ability to carry different types of traffic. Frame-based GFP enables frame-oriented traffic transport for connections where efficiency and flexibility are important, such as Ethernet and IP over PPP. Transparent GFP is used for latency-sensitive applications. In this encapsulation, all the code words of the physical interface are transmitted. Transparent GFP is primarily intended for storage area networks (SANs), where latency is very high and the delays associated with frame-based GFP cannot be tolerated. With virtual concatenation and GFP, the interworking functionality required for Ethernet (LAN) to Telco (WAN) is eliminated. This reduces the complexity of establishing a ubiquitous Ethernet between customer networks and the Telco SONET network.
Since the SONET payload can carry data streams from T1 to OC-192, the Telco SONET network can provide carrier Ethernet services at various speeds from 1.5 Mb / s to 10 Gb / s using GFP and virtual concatenation. The first type of Ethernet carrier is E-LINE or Private Lease Line. E-LINE is a point-to-point Ethernet connection between two locations within a metropolitan area. E-LINE provides reliability, speed assurance and security of the SONET network, as well as adjustable bandwidth using the LCAS protocol. There is also E-LINE which is similar to Virtual Lease Line, where many clients share the same transport bandwidth using Ethernet over the MPLS protocol (EoMPLS) with lower QoS (Quality of Service) than Private Lease Line. Therefore, E-Line can be used to create a private network or virtual private network (VPN).
The second type of Ethernet carrier is E-LAN. E-LAN is an Ethernet service that can connect multiple locations. E-LAN resembles a multipoint service with any connection. E-LAN is used to provide interconnection capability between various organization sites within a metropolitan area. From the subscriber’s point of view, E-LAN makes the Ethernet carrier look like a LAN. Like E-LINE, many clients can also share E-LAN using the EoMPLS protocol. Therefore, E-LAN can be used to create a private LAN or virtual private LAN service (VPLS).