domingo, 14 de febrero de 2010
Numerous technologies for transport and encapsulation of data have been advocated in the metropolitan market. A characteristic of these networks, as noted earlier, is that they are called upon to support a variety of older and new traffic types and rates. Overall, however, there is a trend toward using a common optical layer for transporting digital data.
SONET/SDH has been the foundation for MANs over the last decade, serving as the fundamental transport layer for both TDM-based circuit switched network and most overlay data networks. While SONET/SDH has evolved into a very resilient technology, it remains fairly expensive to implement. Inherent inefficiencies in adapting data services to the voice-optimized hierarchy and an inflexible multiplexing hierarchy remain problematic. More importantly, capacity scaling limitations—OC-768 may be the practical limit of SONET/SDH—and unresponsiveness to bursty IP traffic make any TDM-based technology a poor choice for the future.
Many service providers favor ATM because it can encapsulate different protocols and traffic types into a common format for transmission over a SONET infrastructure. Meanwhile the data networking world, which is overwhelmingly IP-oriented, favors packet over SONET (POS), which obviates the costly ATM intermediate layer. Advancements in IP, combined with the scaling capacity of gigabit and multigigabit routers, make it possible to envisage an IP-based network that is well suited for carrying primarily data traffic, and secondarily voice. Nevertheless ATM remains strong in the metropolitan area. It can accommodate higher speed line interfaces and provide managed virtual circuit services while offering traffic management capabilities. Thus ATM edge devices are commonly used to terminate traffic, including VoIP, DSL, and Frame Relay.
Gigabit Ethernet (GE) is a proven technology for easy migration from and integration into traditional Ethernet. It is relatively inexpensive compared to other technologies that offer the same transmission rate, but does not provide quality of service (QoS) or fault tolerance on its own. When confined to point-to-point topologies, collisions and carrier sense multiple access (CSMA) are not of concern, resulting in more effective use of the full bandwidth. Because the optical physical layer can support much longer distances than traditional Category 5 cable, Gigabit Ethernet over fiber (1000BASE-LX, for example) can be extended into the wide-area realm using DWDM.
The latest advancement in Ethernet technology, 10 Gigabit Ethernet, is being driven by a need to interconnect Ethernet LANs operating at 10, 100, or 1000 Mbps. Ten Gigabit Ethernet can be used for aggregating slower access links, in the backbone networks, and for WAN access. Using 1550-nm serial lasers, distances of 40 to 80 km (25 to 50 mi) are possible with 10 Gigabit Ethernet over standard SM fiber. With such technology, service providers can build simple Ethernet networks over dark fiber without SONET or ATM and provision high-speed 10/100/1000 Mbps services at very low cost. In addition, a very short reach (VSR) OC-192 interface can be used to connect 10 Gigabit Ethernet to DWDM equipment over MM fiber.
Ethernet offers the technical advantages of a proven, adaptable, reliable, and uncomplicated technology. Implementations are standard and interoperable, and cost is much less than SONET or ATM. Architecturally, Ethernet's advantage is its emerging potential to serve as a scalable, end-to-end solution. Network management can also be improved by using Ethernet across the MAN and WAN.
Clearly, as traditional circuit-switched services migrate to IP networks and data grows, networks must evolve to accommodate the traffic. However, IP may need to become as complex as ATM to replace its functionality. Thus, both ATM and IP are candidates for transport directly over DWDM. In either case, the result is simplified network infrastructure, lower cost due to fewer network elements and less fiber, open interfaces, increased flexibility, and stability. The question is, in which format will IP travel over an optical network: IP over ATM over SONET, IP over SONET (as POS), or IP over Gigabit Ethernet or 10 Gigabit Ethernet? (See Figure 3-1)
Fibre Channel is the predominant data link technology used in storage area networks (SANs). Fibre Channel is an economical replacement for the Small Computer System Interface (SCSI) protocol as a high-speed interface for applications such as data backup, recovery, and mirroring. Fibre Channel interfaces are available at 100 MBps today; 200 MBps interfaces should be available in the near future, and 400 MBps interfaces are in testing.
Fibre Channel comes without the very short distance limitations of SCSI; it also avoids the termination restrictions of SCSI because each node acts as an optical repeater. Fibre Channel can be implemented in a point-to-point, arbitrated loop, or mesh topology using a switch. As shown in Figure 3-1, Fibre Channel, like other protocols, can be carried directly over the optical layer using DWDM.
Dynamic Packet Transport
Dynamic Packet Transport (DPT) is a Cisco protocol that provides an alternative to SONET for more efficient transport of data in ring architectures. DPT supports basic packet processing, fairness, multicasting, Intelligent Protection Switching (IPS), topology discovery, Address Resolution Protocol (ARP), routing, and network management. DPT can run over dark fiber, SONET, or WDM.
DPT's chief advantage over SONET is its ability to reuse bandwidth that would have otherwise been lost. Bandwidth is consumed only on traversed segments, and multiple nodes can transmit concurrently. DPT is based on bidirectional counter rotating rings (see Figure 3-2). Packets are transported on both rings in concatenated payload, while control messages are carried in the opposite direction from data.
FDDI is at this point a legacy technology. Having served a need at one time, it has been replaced by more advanced technologies. Although FDDI is capable of scaling to the metropolitan area, it is also a shared media technology with a relatively low capacity by current standards. This limitation, along with falling availability of FDDI interfaces on network equipment, is causing FDDI to be replaced by Gigabit Ethernet, or ATM. Nevertheless, it is also a protocol that can be transparently transported over the optical layer using DWDM.
Support for Legacy Traffic
In spite of the disproportionate growth of data traffic versus voice, legacy traffic won't suddenly vanish. Networks must support diverse low speed connections in addition to newer, higher speed data connection. Thus DWDM must be complimented by electrical (TDM/FDM) multiplexing to ensure efficient use of lightwaves. At the same time, legacy traffic must be augmented with high-capacity data transport without impacting efficient IP transport.
For ISPs, the situation is different: All of their traffic is IP. ISPs need rapid build-out of networks and favor packet-over-lightwave or Gigabit Ethernet, rather than ATM or SONET. Other requirements of this market include load sharing strategies for resilience, leverage of dark fiber, and simpler datacom-like management.
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