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Carrier Ethernet Over SONET – WDM Networks

Carrier Ethernet Over SONET – WDM Networks

The broadband networks of US telecommunications carriers are fiber optics networks using SONET (Synchronous Optical Network) and WDM (Wavelength Division Multiplexing) technologies. SONET uses TDM (Time Division Multiplexing) to multiplex data streams of different rates in electrical domain and then converts the electrical signals to optical signals to be transported over optical fiber. SONET transmission systems payload can transport 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). WDM enables multiple optical signals with different wavelengths to be multiplexed and transported over a single optical fiber. Since each SONET signal uses a different wavelength, hence multiple SONET signals can be transported over an optical fiber using WDM.

Ethernets are defined according to data rates. There are 10Mb/s, 100 Mb/s, 1 Gb/s and 10 Gb/s Ethernet. Since SONET is optimized for TDM traffic (Voice), it does not match the optimal rate for transporting a typical Ethernet data stream. To increase the efficiency of transporting Ethernet over SONET, two new technology standards are introduced, namely Virtual Concatenation and Generic Framing Procedure. Virtual Concatenation allows SONET channels to multiplex in arbitrary arrangement to address the bandwidth mismatch between Ethernet and SONET. Custom-sized SONET pipes are created using multiple STS-1/STS-3 as well as virtual tributaries (VT-1.5/VT-2). Each SONET channel is routed independently and transparently through the network. This enables SONET pipes to be right-sized for Ethernet transport. The capability of dynamic bandwidth allocation of virtual concatenation channel, LCAS (Link Capacity Adjustment Scheme), is also developed.

Generic Framing Procedure (GFP) is a simple adaptation scheme that extends the ability of SONET to carry different types of traffic. Frame based GFP permits the transport of frame oriented traffic for connections where efficiency and flexibility is important, such as Ethernet and IP over PPP. Transparent GFP is used for applications that are sensitive to latency. In this encapsulation, all code words from the physical interface are transmitted. Transparent GFP is primarily targeted at Storage Area Networks (SANs) where latency is very important and the delays associated with frame based GFP cannot be tolerated. With Virtual Concatenation and GFP, the required inter-working function for Ethernet (LAN) to the Telco network (WAN) is eliminated. This reduces the complexity of establishing a ubiquitous Ethernet between the customer networks and the Telco SONET network.

Since SONET payload can transport data streams from T1 to OC-192, the Telco SONET network can provide carrier Ethernet services at various rates from 1.5 Mb/s to 10 Gb/s using GFP and virtual concatenation. The first type of carrier Ethernet is E-LINE or Private Lease Line. E-LINE is a point-to-point Ethernet connection between any two locations within a metropolitan area. E-LINE provides reliability, rate guarantee and security of SONET network as well as adjustable bandwidth using LCAS protocol. There is also E-LINE that is similar to Virtual Lease Line where many customers share the same transport bandwidth using Ethernet over MPLS protocol (EoMPLS) with lower QoS (Quality of Services) than Private Lease Line. Hence, E-Line can be used to create a Private Network or a Virtual Private Network (VPN).

The second type of carrier Ethernet is E-LAN. E-LAN is an Ethernet service that can connect multiple locations. E-LAN resembles a multi-point service with any-to-any connection. E-LAN is used to provide interconnection capability between multiple organization sites within a metropolitan area. From the subscriber standpoint, E-LAN makes the carrier Ethernet looks like a LAN. Just like E-LINE, E-LAN can also be shared by many customers using the EoMPLS protocol. Hence E-LAN can be used to create a Private LAN or a Virtual Private LAN Service (VPLS).