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White Paper

Cisco's Packet over SONET/SDH (POS) Technology Support; Mission Accomplished

The explosive growth of Internet/intranet traffic is making its mark on the existing transport infrastructure. An unprece- dented shift has occurred in traffic content, pattern, and behavior. It has transformed the design of multiservice networks and created a commercial demand for IP networks that operate in excess of one gigabit per second. This white paper reviews these new bandwidth drivers, and explores the design techniques that best leverage new and existing infrastructure.

In this regard, Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) is the medium of choice. This reliable technology has existed since 1986, and the advent of technologies such as Asynchronous Transfer Mode (ATM) over SONET (and most recently, direct mapping of IP over SONET frames) has extended the useful life of this technology. But to design new networks, it's necessary to predict the future applications and provide a good migration path.

Until recently, ATM appeared to be the only viable method of aggregating voice and data traffic on very high- speed multiservice networks. But a new approach, packet over SONET, offers a new backbone architecture that preserves existing investments in SONET infrastructure and supports the deployment of IP-based video and voice applications.

Cisco's packet over SONET/SDH places the IP layer directly above the SONET layer, and while offering quality-of-service (QoS) guarantees, eliminates the overhead needed to run IP over ATM over SONET. (See Figure 1.) Cisco's packet over SONET/SDH better accommodates fast-growing Inter/intranet traffic, offers the first reliable way to create multiservice networks based on IP, and is being deployed by leading service providers such as GTE Internetworking, Qwest, Sprint, and UUNET. It is also promising to eliminate intermediate SONET Add Drop Multiplexers (ADMs). Advances in IP switching speeds, economy, and development of IP-based voice and video products have made this configuration possible only in the last year. Underlying the shift toward packet over SONET/SDH are the changing realities of business, network demand, and applications.

Figure 1: ATM IP Transport Infrastructures

The New Realities

The barriers between the inter-exchange carriers (IXCs) and the local exchange carriers (LECs) represented by the regional Bell operating companies (RBOCs), are breaking down. Within the next five years, LECs will be different in name, geographical coverage, and service offerings.

This change is partly a result of the Telecommunications Act of 1996, which is eliminating the artificial barriers between the intra-local access and transport area (LATA) and inter-LATA services and undercutting the access charges that make up the bulk of RBOC profits. But it is also driven by a merger mania created by the need for economies of scale and the demand for integrated services. Concentration in the services business means that network backbones will get bigger.

All that is occurring against a background of a transformation in usage patterns from connection-oriented, nailed up services to dynamic, connectionless IP services.

For public carriers, IP is critical for future revenue growth. (See Figure 2.) According to the Yankee Group, telco revenue will grow to $260 billion by 2000 from $190 billion in 1997, with data services, particularly IP, accounting for most of this increase. Market researcher CIMI Corp. expects that "from 2000 on, 80 percent of service provider profits will be derived from IP-based services."

Figure 2: Voice/Data Traffic Predictions

The change in revenue is also reflected in the changing protocol mix of enterprise and service provider networks. (See Figure 3.) In 1996, IP accounted for about 45 percent of enterprise network traffic; this year it will account for over 70 percent. IP has become the dominant routing protocol for data. Results of a 1996 survey by the Business Research Group of 450 companies with 1000 to 4500 network nodes are shown in Figure 3, which shows the changing protocol mix in enterprise networks.

Figure 3: Changing Protocol Mix

Shift in Network Traffic Pattern

As the protocols shift, network usage patterns are being turned upside down. The old rule was that network traffic was 80-percent local, and 20-percent wide area. Today, the ratio is closer to 50/50, and the growing number of applications that require all communication from clients to traverse the backbone to reach central servers means more stress on network backbones. Cisco expects that the 80/20 rule will eventually return, but with wide-area applications accounting for 80 percent of traffic. (See Figure 4.) This event has impacted the existing SONET/SDH infrastructure. SONET/SDH transport backbones are being stressed out not only by geometric growth of Internet traffic, but by a paradigm shift in network design that requires much more from WANs. The future platform of choice needs to maintain low latency and the resiliency of SONET, yet add dynamic throughput and performance topped with differentiated services in the networks.

Figure 4: Network Collapse

At the same time, a dramatic collapse of network Layers 2 and 3 is occurring through improved SONET/SDH transport, digital crossconnect, and routing integration. SONET/SDH platforms are melting into the digital cross- connects. As difficult as it may seem, vendors are considering the idea of supporting Layer 3 services in their ADM/crossconnect platforms, and major RBOCs are implementing digital crossconnects with full SONET/ADM and ring functionality.

From the data side, low-speed drops no longer exist. Demand for bandwidth has pushed major internetworking vendors to migrate to SONET rates of OC-3-12-48. Cisco 7000 and 12000 gigabit switch router (GSR) series have also pushed the limits by supporting SONET features such as automatic protection switching (APS) and the capability to interpret and provision various SONET overhead bytes. With the right optics in place, who needs a plain vanilla SONET box?

Finally, changes at the edge of the network are making optimization of core switches, routers, and backbones for IP increasingly important. Despite early hopes that ATM would eventually provide service all the way to the desktop, the edge of the network is dominated by Ethernet, which has proven to be the most scalable, economical, and manageable LAN protocol with data rates from 10 Mbps to 1 Gbps, port costs as low as $40, and a wide variety of network management tools and techniques familiar to most network managers.

With Ethernet now accounting for more than 80 percent of the installed base of network ports and network interface cards (NICs) (see Figure 5), it makes sense to optimize the core of the network to handle IP traffic.

Figure 5: Source IDC

Benefits of Cisco's Packet over SONET

To grasp the benefits of packet over SONET/SDH, it's necessary to compare it with the traditional ATM over SONET/SDH architecture.

ATM has numerous strengths. It can operate over SONET links at speeds up to OC-12 (622 Mbps); it also provides QoS guarantees, which makes it suitable for voice and video, and it can accommodate multiple services and protocols. For telco networks carrying thousands of voice conversations, it's still the way to go.

Historically, IP could not operate at very high speeds or provide QoS, but those shortcomings no longer exist. The Cisco 12000 GSR breaks the old performance limitations of IP, scaling up from OC-3 to OC-48 today, with OC-192 speeds becoming available in the next 12 to 15 months. Cisco's new IP QoS techniques can easily be delivered via packet over SONET using the three precedence bits in the IP header, (See Tables, next section.) allowing deployment of voice, video, and other isochronous services.

Layer 3 Quality of Service

With the telecommunication deregulation in full swing, incumbent local exchange carriers (ILECs) will need to ward off competitive risks within their in-regions by raising the bar on the level of service offerings and to focus on providing value-added services. Simply providing more bandwidth is not a competitive way of doing business. Service providers can "up the ante" by giving the customers guaranteed and differentiated services through IP-based QoS products. This scenario allows the customers to rely on their network for their mission-critical applications and the service providers to increase their revenue earnings. Cisco high-end routers support this level of service through the Layer 3 QoS offering. With the three precedence bits in the IP header, it is possible to provide differentiated classes of services by utilizing Random Early Detection (RED) and Weighted RED (WRED). As packets enter the network, their precedence is set by the edge routers, and it is used to determine the queuing of packets through the network.

Table 1:

Committed Access Rate

  • Used at the edge of the network for rate-limiting, controls the amount of traffic going through a given interface, that is, DS3 interface with T1 throughput

Random Early Detection (RED)

  • A congestion-avoidance mechanism that controls the packet flow before congestion occurs; that is, TCP operates based on closing and opening of its window;

  • Manipulates flow through the TCP sessions before the actual congestion takes place

  • Typically is supported at the core of the network.

Weighted RED

  • A congestion-avoidance mechanism designed to provide preferential treatment for premium-class traffic under congestion situations while concurrently maximizing network throughput and capacity utilization and minimizing packet loss and delay

  • The network operator may define up to six classes of service

Need for Efficient Bandwidth Utilization

For IP traffic now on ATM networks, packet over SONET/SDH also offers a 25- to 30-percent gains in efficiency because the overhead required (in the ATM cell header, IP over ATM encapsulation, and segmentation and reassembly [SAR] functionality) is eliminated.

Figure 6: Relative Efficiencies of ATM and Packet over SONET

High-end routers, such as the Cisco 12000 and 7500 series, that connect IP networks directly to SONET rings eliminate the need for expensive intermediate ADMs. By combining the support for SONET APS and long-reach and intermediate-reach optical interfaces, APS provides protection against fiber cuts or module failure. The end result is tremendous cost saving for the service providers by deploying and managing lower platforms.

In summary, packet over SONET/SDH is the best way to build an optimized infrastructure based on the dominant protocol. Other benefits depend on user and application, and that requires taking a closer look at different networking environments, users, and applications now common today.

Packet over SONET/SDH Deployments

For telcos, competitive local exchange companies, Internet service providers (ISPs), campus LAN managers, and others with dark or dim fiber, packet over SONET/SDH offers considerable advantages over ATM. As a key supplier of the packet-over-SONET/SDH technology, Cisco sees a huge increase across the board on deployment of this technology worldwide in the enterprise market as well the service provider market.

Sprint, for example, is deploying packet over SONET using the Cisco 12000 series GSRs to boost its Internet backbone speed to 622 Mbps. This setup "will increase bandwidth 400 percent by running live traffic over full line speed OC-12 connections, providing faster access to Web pages, real-time applications, and file transfers for its customers worldwide," according to Dominick DeAngelo, Vice President, Sprint Internet & Intranet Services. "Sprint will be able to provide increased fault tolerance, and to offer future quality of service and multicasting products without affecting router or network performance."

The other leading IP carriers are doing the same:

Packet over SONET/SDH Applications:
Lighting the Glass

While the early adopters of Packet over SONET/SDH tend to be the high-end network service providers in the Internet arena, many other types of users and applications can benefit from the technology.

The most important applications for packet over SONET/SDH include:

Although this may seem to be a very diverse set of applications, they all provide a rapid return on investment, as well as scalability, manageability, and improved reliability.

One of the many success stories of this technology is its deployment across both the Atlantic and the Pacific Oceans. Today, Cisco's packet over SONET technology spans both oceans (See Figure 7), enabling leading-edge ISPs and enterprise users to maximize the efficiency of their backbone infrastructure.

Figure 7: Packet over SONET Trans-Oceanic Deployment

In recent years, DS3 has become the most common drop interface at the customer premises, replacing T1 services. Figure 8 shows the data network as an overlay to the SONET interoffice transport facility. The Cisco 12000 GSR provides the capability to connect these networks together through the high-speed channelized interconnect. The traffic that originates in the data network rides a specific Synchronous Transport Signal (STS), which traverses through the channelized high-speed interface to the SONET transport LATA. The SONET gateway node or the digital crossconnect in the central office peels off each individual STS and drops or passes it through the network, as required. High-speed channelized optical interfaces on the Cisco 12000 series efficiently connect the two networks together. Key benefits for the service providers in this application include:

Figure 8: Aggregation and Backhauling of the Packet over DS3 Services to the Internet Backbone by Utilizing Metropolitan SONET/SDH Ring Network and High-Speed Channelized Connections on the High-End Routers

One of the most important applications for packet over SONET/SDH is lighting the dark or dim fibers available in many campuses and enterprises and in rights-of-way owned by utilities. With Cisco's packet over SONET/SDH line cards, an ISP or enterprise network designer can scale the speed of its interconnecting SONET links without experiencing the overhead tax associated with other transmission methods.

In the application shown in Figure 9, the edge routers aggregate traffic into the Cisco 12000 GSRs. Since the Internet backbone is a distance away from the end users, the Cisco 12000 connects to the existing SONET/SDH infrastructure and the traffic travels through the access and interoffice SONET/SDH rings in order to reach the Internet backbone. Full redundancy and protection is available throughout the network in the Cisco 12000 as well in the transport network elements.

Figure 9: Packet over SONET/SDH Used for Aggregation of Traffic
from Edge Routers

Today's networks are not homogenous; the application in Figure 10 represents a typical telco/ISP interworking, which includes both a multiservice and an IP optimized network. On the left side of the figure, the Cisco 12000 GSR interfaces with the ATM cloud to route digital subscriber line access multiplexer (DSLAM) traffic to the Internet backbone. The IP traffic can directly traverse the fiber between the Cisco 12000 routers at OC-12/-48 optical rates with SONET APS. This scenario provides reliable private peering and gives the ability to bypass the congested network access points (NAPs). For route diversity, the data can also ride the interoffice SONET ring and traverse the telco ring infrastructure to get to the Internet backbone shown on the right side of the figure.

Figure 10: Cisco 12000 Providing Private Peering and Route Diversification over SONET Interoffice Facility

Implementing Packet over SONET/SDH

Packet over SONET/SDH promises to offer significant advantages by providing efficient bandwidth utilization, higher performance, and greater simplicity. But it will be the implementation details that determine whether packet over SONET becomes a ubiquitous service.

To be successful, packet over SONET/SDH implementations must provide support for Layer 3 switching, multicast/broadcast controls, traffic management, and congestion control features that enable efficient network bandwidth utilization. They must also offer QoS that enables customers to support mission-critical or delay-sensitive applications, such as voice and video.

As data becomes the dominant part of the backbone payload and as routers scale up to higher rates, seamless integration with SONET/SDH networks and equipment from multiple vendors becomes a requirement. Despite minor differences between various aspects of SONET and SDH, interoperability between these standards is vital to service providers. To ensure interoperability, Cisco allows the setting and display of SONET/SDH section, line, and path overhead bytes, and this interoperability has been extensively tested with a wide variety of the SONET/SDH transport platforms.

In particular, data product support of APS means fault tolerance, even in multivendor networks. Another important standard for Packet over SONET implementation is RFC1619, Point-to-Point Protocol (PPP) over SONET/SDH, an Internet Engineering Task Force (IETF) protocol for encapsulating IP datagrams on SONET/SDH circuits.


Clearly, network managers need to optimize their networks for the dominant applications. With increasing competition driving mergers and acquisitions in the service business, carriers need to provide differentiated services with real QoS and disperse with the "best efforts" mentality if they want to have satisfied customers. Simply throwing bandwidth at the problem isn't enough.

Figure 11 reflects the dramatic increase in the number of Web users, which is expected to reach 160 million by the year 2000. This increase is one more reason for service providers to optimize their network for the dominant routed protocol, IP.

Figure 11:

World Wide Web Users (millions)

Despite indications of a religious war between packet over SONET and ATM, with "net-heads" on one side and "Bell-heads" on the other, the two technologies can and should coexist. It's still important, however, to mark out separate territories based on the strengths and weaknesses of each technology.

As envisioned, ATM is a ubiquitous, end-to-end service offering extensive QoS capabilities that support the different requirements of various voice and data services. But ATM has not achieved ubiquitous deployment, and there are no indications that it ever will. Ethernet and LAN switching continues to dominate enterprise networks and the desktop, and the combination of Ethernet switching, Fast Ethernet, and Gigabit Ethernet have ensured a steady advance of economical bandwidth.

For carrier, ISP, CLEC, "right of way," and enterprise networks, the optimum network design must support the dominant protocol. Today, over 70 percent of data traffic is IP, and as other legacy applications such as time-division multiplexing (TDM), voice, and video become increasingly packetized, the proportion of IP traffic will increase.

Cisco's packet over SONET/SDH takes advantage of the existing SONET/SDH infrastructure to better support IP, and is optimized for the Ethernet infrastructure that now extends to millions of desktops. It offers scalability to enable the continued growth of IP traffic, more efficient bandwidth utilization, and support for new IP applications such as voice, multicast, and video.

As the light at the end of the tunnel---or fiber conduit--- packet over SONET unifies existing infrastructure with IP, supports a richer set applications, and delivers a dramatically lower cost of ownership.

Network Design in a Nutshell

Based on the new capabilities of packet over SONET/SDH, Cisco has developed some new rules of thumb for network design.

    1. Optimize your network for the dominant protocol---IP.

    2. For enhanced, end-to-end, optimized TCP traffic, buffering is an important issue. The longest distance common in global networks is between Europe and Asia going through the United States, Sydney and Stockholm, a route that has a 500-ms delay, and at OC-12 it requires 32-MB buffer. Network equipment must support optional expandable buffers to deal with various network topologies, and typically, delay times bandwidth gives the needed buffer.

    3. Seamless migration between data platforms, routers, and the transport network element is a necessity. Today's high-end routers, such as the Cisco 12000 gigabit switch router, scales up to OC-3-12-48. Support for automatic protection switching (APS) is required to ensure reliability and interoperability.

    4. Overlay networks are a controversial subject, but the reality is that most networks, especially the enterprise and campus environments, must support multiple protocols, including IP, IPX, DECnet, and so on. Many network managers have concluded that in order to keep up with the pace of IP growth and maintain the needed performance and packet throughput, it may be beneficial to push off the multiprotocol services to a new overlay network, and over time, move services to this new network.

    5. For service providers, QoS brings the service differentiation offering that your company has been looking for. By selling QoS, your valued customers can be at ease in handing off their mission-critical services to your network, and your company can increase its revenue by selling differentiated services. Major data companies such as Cisco support Layer 2 as well as Layer 3 QoS.

Posted: Mon Jul 3 11:18:20 PDT 2000

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