Ethernet is king of corporate networking and Sonet reigns in the carrier space, but both are less than perfect. Today, the 10 Gigabit standard is propelling Ethernet to speeds equal to Sonet, but are we moving networking forward or just strapping a rocket engine to a Studebaker?
Ethernet inadequacies include a lack of resiliency, QoS (Quality of Service) and bandwidth management. Sonet has never addressed its lack of packet support and adjustable bandwidths successfully: Sonet assigns data to a virtual tributary, and each VT can carry only the data assigned to it. If no data is being carried, the VT is empty and cannot be used by other VTs that may have excess data. Although this setup works for voice and video--both of which typically have constant data rates--it is inefficient for the bursty traffic normally associated with corporate networks.
RPR (Resilient Packet Ring) can help transform that Studebaker into a Ferrari by accentuating Sonet's and Ethernet's positives and attenuating their negatives. Although RPR was designed for carrier, metropolitan and campus networks, it can be implemented anywhere resiliency and efficient bandwidth usage are needed. RPR is not meant to replace Sonet but to ride on it and add value to it. Sonet will remain the king for long-haul networks.
And though carriers will be the first use RPR, primarily because of the cost savings, vendors are already designing RPR-based products for the enterprise. For example, AT&T has announced its Managed OptEring Service, which runs over an RPR network.
Looks Can Be Deceiving
RPR looks like a Layer 1 technology, but it's a Layer 2 implementation designed to run on Layer 1 standards such as Sonet/SDH. Although RPR can operate over Gigabit or 10 Gigabit Ethernet, Corrigent Systems and other vendors say they prefer using Sonet as Layer 1.
The reason for developing RPR as a Layer 2 protocol was simple: cost. Because RPR runs on top of existing technologies, carriers, service providers and enterprise customers don't need to scrap expensive network infrastructures--they can just add RPR equipment to the mix. And RPR traffic can travel on Sonet/SDH networks using Sonet VTs, so RPR can be added to networks without affecting current Sonet traffic.
Including RPR on an Ethernet network is slightly different and may require a new network on top of the existing Ethernet. RPR rides on Ethernet's Layer 1 and can encapsulate Ethernet frames within an RPR frame. However, most Ethernet devices expect Ethernet to be the Layer 1 and the Layer 2 protocol of the frame, and won't carry RPR. Instead, the RPR equipment must be the core of the network, with Ethernet devices feeding in to or out of the RPR network.
Because RPR is a Layer 2 protocol, it defines a new MAC (Media Access Control) format into which data packets are placed (see chart "The RPR Header"). The RPR MAC header comprises 24 bytes. The first two bytes are for the ring control field, which is divided into seven subfields: TTL (time to live), P (parity), WE (wrap eligible), FT (frame type), FE (fairness eligible), RI (ring ID) and SC (service class). Service class determines the packet's priority on the network, while frame type determines whether the frame contains user data, fairness requests or control data for other nodes. Control frames contain node changes and other data. RPR supports network device and node autodiscovery; new network nodes announce themselves to their direct neighbors with control messages and distribute changes in their settings or topologies.
The next two fields are six bytes each and contain 48-bit destination and source addresses. These are hardware addresses as defined in section 5.2 of IEEE Std 802-1990. The Extended Ring Control takes up the next two bytes. The first eight bits contain the TTL Base. This is the original value of the TTL and is not decremented. The last byte contains the EF (extended frame), FF (flooding form), PS (past source), SO (strict order) and three reserved bits at the end, which should be 0s.
FYI
Vendors' desire to enable Ethernet in the MAN was one driver for their exceptional cooperation on the RPR spec. Yankee Group estimates that retail metro Ethernet services will hit $10 billion by 2006, and infrastructure equipment vendors, which expect carrier spending to pick up, want a piece of that pie.
The EF field indicates whether the frame is a Base Data Frame or an Extended Data Frame. The Base Data Frame is used by all data traffic that travels from start to finish on the same ring. If data needs to travel from one ring to another to get to its destination, an Extended Data Frame, which includes the original source address and final destination address after the HEC (header error check), is used. The FF bits indicate whether data is flooded unidirectionally, bidirectionally or not at all. The PS bit is used during a wrap function to indicate the frame has traveled past the source address on its way back to the destination. SO is used when frames need to be kept in order.
The next two bytes contain the HEC, or header CRC (cyclical redundancy check). The protocol field takes up two bytes. When the value of the field is less than 1,535, the field indicates the length of the frame. If the value is equal to or greater than 1,536, it indicates the MAC client protocol. This value is designated by the IEEE Type Field Register. The protocol bytes determine type or length, never both. Finally, after the actual payload, a four-byte FCS (frame check sequence) is placed at the end.
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