Resilient Packet Ring Technique
An important trend in networking is the migration of packet-based technologies from local Area Networks to Metropolitan Area Networks. The rapidly increasing volume of data traffic in metro networks is challenging the capacity limits of existing transport infrastructures based on circuit-oriented technologies like SONET and ATM. Inefficiencies associated with carrying increasing quantities of data traffic over voice-optimized circuit-switched networks makes it difficult to provision new services, and increases the cost of building additional capacity beyond the limits of most carriers’ capital expense budgets. Packet-based transport technology, a natural fit with the now ubiquitous IP protocol, is considered by many to be the only alternative for scaling metro networks to meet the demand.
The emerging solution for metro data transport applications is Packet Ring technology. It offers two key features that have heretofore been exclusive to SONET: efficient support for ring topology and fast recovery from fiber cuts and link failures. At the same time, Packet Ring technology can provide data efficiency, simplicity, and cost advantages that are typical to Ethernet. Even though there is currently no standard for Packet Rings operating at Gigabit speeds and higher, many vendors are developing and introducing Packet Ring technologies to address this emerging market.
To be a viable contender for data transport in the MAN, Packet Ring technology should provide support for multi-Gigabit data speeds and integrate seamlessly with existing Ethernet and SONET networks. Packet Ring solutions should be available in various form factors and link speeds, and at prices that are competitive with Ethernet. Finally, an industry standard that defines the link layer.
Packet Rings needs to be developed to achieve vendor interoperability and customer acceptance.
LIMITATIONS OF SONET AND ETHERNET
SONET
Most metro area fiber is in ring form. Ring topology is a natural match for SONET-based TDM networks that constitute the bulk of existing metro network infrastructure. However, there are well-known disadvantages to using SONET for transporting data traffic (or point-to-point SONET data solutions, like Packet over SONET [POS]). SONET was designed for point-to-point, circuit-switched applications (e.g. voice traffic), and most of limitations stem from these origins. Here are some of the disadvantages of using SONET Rings for data transport:
Fixed Circuits.
SONET provisions point-to-point circuits between ring nodes. Each circuit is allocated a fixed amount of bandwidth that is wasted when not used. For the SONET network that is used for access in Figure 2 (left), each node on the ring is allocated only one quarter of the ring’s total bandwidth (say, OC-3 each on an OC-12 ring). That fixed allocation puts a limit on the maximum burst traffic data transfer rate between endpoints. This is a disadvantage for data traffic, which is inherently bursty.
Waste of Bandwidth for Meshing.
If the network design calls for a logical mesh, (right), the network designer must divide the OC-12 of ring bandwidth into 10 provisioned circuits. Provisioning the circuits necessary to create a logical mesh over a SONET Ring is not only difficult but also results in extremely inefficient use of ring bandwidth. As the amount of data traffic that stays within metro networks is increasing, a fully meshed network that is easy to deploy, maintain and upgrade is becoming an important requirement.
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