11-01-2012, 10:38 AM
Resilient Packet Ring Technology
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An Introduction to Resilient Packet Ring Technology
1.0 Introduction
An important trend in networking is the migration of packet-based technologies
from Local Area Networks to Metropolitan Area Networks (MANs). 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. Packetbased
transport technology is considered by many to be the only alternative for
scaling metro networks to meet the demand.
2.0 Ethernet in the Metro
Defined simply, an Ethernet service is any data service offered via an Ethernet
interface (10 Mbps, 100 Mbps, 1 Gbps Ethernet port). A key difference between
Ethernet services and legacy data services such as leased lines, Frame Relay or
ATM is the scalability of the service interface.
With legacy data services, physical interface requirements vary with the speed of
the service. Thus hardware required for a T1 service is completely different from
that required for DS-3 or OC-3 services.
With Ethernet service, on the other hand, a service provider can drop a Fast
Ethernet (100 Mbps capacity) or Gigabit Ethernet (1000 Mbps capacity) port to a
subscriber once and upgrade many times, without additional truck rolls beyond
the initial installation. Bandwidth and other service changes can be administered
remotely, simplifying and accelerating service provisioning.
Ethernet services are widely viewed as an offering that holds promise for rapid
acceptance in the marketplace. The question remains as to what infrastructure
can cost effectively scale to meet this demand.
3.0 The Limitations of SONET and Ethernet in Metro Rings
3.1 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.
Below are some of the disadvantages of using SONET Rings for data transport.
