Gigabit Ethernet has replaced ATM as the LAN backbone technology of choice among most network professionals, and now the technology is leapfrogging into the metropolitan-area network as the driver behind a new type of IP-based data service that's cheaper and more flexible than SONET-based alternatives.
Service providers such as Yipes Communications Inc., Vitts Networks Inc. and Telseon Inc. are using new carrier-class optical Gigabit Ethernet switches as the foundation for MANs that offer IP-based LAN-to-LAN data connections within a metropolitan area for a corporation or customized connections to multiple customers in a single building.
These IP MANs are attractive to application service providers and Web-caching service providers that are looking for a fast, flexible, inexpensive way to deliver Internet, intranet and extranet services to their corporate customers.
Gigabit Ethernet-based MANs offer several advantages over traditional SONET rings, including:
- The connection between the LAN and WAN is easy because Ethernet is running on both sides of the WAN access device.
- Prices are significantly lower than comparable bandwidth on a SONET/ATM network.
- Bandwidth can be turned up in 1M bit/sec increments within minutes of a customer request, compared with having to wait weeks for an additional 1.5M bit/sec T-1 line.
Gigabit Ethernet switch vendors, including Riverstone Networks, Extreme Networks and Foundry Networks, began delivering carrier-class gigabit switches early this year, with other vendors expected to follow suit. These carrier-class switches, also known as aggregation switches, differ from enterprise switches in that they provide connectivity to optical core networks, including SONET, and they provide access to customers.
Catch the wave
Wave division multiplexing (WDM) plays a major role in this scheme by complementing Gigabit Ethernet technology. Full-duplex Gigabit Ethernet requires a fiber pair - one for transmitting and the other for receiving. This can become economically unsound when running numerous gigabit channels to points of presence (POP).
Using WDM, for example, Extreme substantially reduces the number of fiber pairs required to connect to POPs. It uses a 10G bit/sec WDM module to combine eight full-duplex Gigabit Ethernet channels onto a fiber pair, with each fiber carrying four channels of bidirectional traffic.
In the MAN backbone, dense WDM (DWDM) is now replacing traditional SONET switches to provide a substantial increase in bandwidth and dramatically reduces the time it takes to provision a circuit.
Elegy to SONET
SONET has serious drawbacks for packet-oriented traffic. A telecommunications industry technology for highly scalable, large bandwidth traffic requirements, SONET is a time-division multiplexing transport technology that scales up to 10G bit/sec.
It was developed in the 1980s for high-volume voice traffic, and although it can support any kind of traffic, it is optimized for voice. SONET is a hierarchical digital transport technology for transmission over fiber and is organized into 64K-byte (Digital Service, level zero) voice channels.
Its frame format contains large and expanding payload capacities. The payload portion of a SONET frame, called the Synchronous Payload Envelope, is organized into a byte format that expands from 87 bytes (octets) for OC-1 to 16,704 bytes for OC-192. This large payload format can be a limitation because any unused channel capacity is wasted. Packets must be mapped to a SONET frame format, then stripped from the SONET frame on arrival at their destinations.
Data must be packed into these rigid channels, resulting in wasted channel capacity where packets do not exactly fit the channel. This inefficiency becomes more pronounced with increased data traffic on the network.
The high cost of SONET equipment is financially challenging to companies that must scale their networks to meet the demand of exploding data traffic. Other downsides to SONET are its long time cycles and lack of flexibility for provisioning SONET connections to subscribers. Because SONET is voice-oriented, it does not support flexible billing and management features that service providers require to offer value-added services, such as VPNs and Web hosting.
Ode to Gigabit Ethernet
By contrast, Gigabit Ethernet technology addresses data-intensive traffic and is highly-scalable. It also supports Layer 3 through Layer 7 IP packet processing required for developing value-added services, and bandwidth management and provisioning. Gigabit Ethernet switches are an order of magnitude lower in cost than SONET.
By using WDM, Gigabit Ethernet can duplicate the robust dual-ring architecture of SONET by provisioning multiple Gigabit Ethernet channels over WDM fiber to create a scalable, robust, data-optimized optical ring.
Looking ahead, the deployment of 10G bit/sec Ethernet will flexibly aggregate native data traffic across high-speed optical MANs, providing an end-to-end Ethernet network.
Some of these switches address all requirements for MANs and service provider networks, while others address specific requirements. As the MAN and service provider network markets solidify, vendor switches will undoubtedly increase to address all current requirements.
Dawn of Gigabit MAN
Gigabit Ethernet technology is becoming the technology of choice for today's data-intensive MANs. It is an economically sound, large-bandwidth solution that provides access flexibility and connectivity with SONET or emerging DWDM-optic networks.
Extreme has demonstrated that it can replace SONET rings with Gigabit Ethernet switches using DWDM to provide an end-to-end Ethernet network that supports voice, video and data.
Other vendors position their switches on the edge of a core SONET network, enabling MANs and service providers to provision network services to subscribers, while maintaining the SONET core for high-volume traffic. While some switch vendors provide Ethernet only to the POP, others support mixed media to provide connectivity to virtually all subscriber network media. MANs are migrating from a voice-optimized network to a data-optimized network that will support value-added business services.