KEY SONET Applications
With the break-up of ATT in the 80’s, the Baby Bells needed a communication protocol that would allow them to interoperate. To address this need and their need for a highly reliable network architecture, SONET was adopted as the standard for Telco wide area networking (WAN) operations.
Since those early days, network planners (from many different industries) looking for an efficient, easy to use, robust network architecture have discovered the elegance of SONET. The majority of today’s mission critical networks utilize SONET as an underlying transport technology.
Recently, the SONET and other related standards have been expanded to provide features for the transport of traffic other than traditional TDM-based voice traffic. The addition of functionality to efficiently transport packet-based traffic (ATM, Ethernet, MPLS, etc.) makes SONET an excellent network infrastructure for the transport of all types of mission critical traffic (voice, video, and data).
SONET provides a range of very desirable features. Following are a few of the most important to consider:
Ring Based Protection
In a ring-based topology, SONET can employ protection protocols that provide path failover protection in the case of a cable/fiber break of the ring by re-routing traffic around the other way. The protection protocols (UPSR and BLSR) provide for recovery after a break in less than 50 mSec.
Fig 2. SONET (UPSR) Protection Uses Both Directions Around Ring
Signal multiplexing
SONET utilizes time division multiplexing that allows multiple lower speed signals to be combined into a higher bit-rate (speed) signal by slicing the faster signal into “time slots”. This allows any combination of lower speed signals to be placed into the higher speed signal and accessed/added/removed at any location in the network. This makes SONET a flexible, any-to-any connectivity platform for any supported bit-rate signal.
Fig 3. SONET Pack signals low-speed signals into higher speed formats
High QoS
Time division multiplexing takes an input signal, breaks it into pieces, and transports the fragments to the destination with no loss, no change in order, and no variation in signal delay. This high quality of service is one of the reasons that SONET was chosen for transport of highly sensitive voice traffic.
Ethernet (packet) Support
The transport of Ethernet traffic (and other types of packet based traffic) has been a significant addition to traditional SONET networks. To carry this traffic effectively, several component features were added:
Generic Framing Procedure (ITU G.7041) – GFP allows SONET equipment to cost effectively map Ethernet packets into SONET signals. This approach is a standard now and allows differing vendor equipment to interoperate (one of the original requirements for SONET).
Virtual Concatenation (ITU G.707) – VCAT allows SONET equipment to bond together multiple signals in order to create a “pipe” that is correctly sized for the Ethernet signal. Using VCAT, multiple base-rate signals can be multiplexed to provide transport delivering the desired aggregate bandwidth. Granularity can be either at the VT1.5 level (1.5 Mbps per signal) or the STS-1 level (the basic signal in SONET, 45 Mbps).
Link Capacity Adjustment Scheme (LCAS) (ITU G.7042) – once a SONET circuit has been provisioned to carry Ethernet traffic, network operators frequently have the need to adjust the bandwidth capacity based on customer demands. LCAS (link capacity adjustment scheme) allows operators to make these changes without taking the circuit out of service.
Fig 4. Data Transport Over SONET
Analysts have been predicting the death of SONET for several years now. But with the latest enhancements to the standards, SONET is capable of transporting all types of traffic with high quality of service and minimal overhead. Driven by market forces and advancements in optical products, SONET gear is becoming a cost effective solution even compared to traditionally lower cost solutions such as Ethernet. Consider SONET when a highly reliable, manageable network is required to transport packet (Ethernet, MPLS, etc.) traffic and traditional TDM (DS1, DS33, etc.) signals as well.
Voice Transport – Inter-Office Facilities
The genesis of SONET was the need of the Baby Bells, in the wake of the break-up, to define a standard mid-span meet protocol. Today, SONET still plays a key role in inter-office transport of commercial voice traffic. Although packet networks, especially high quality MPLS networks, have recently emerged as a suitable alternative, the majority of core communications infrastructure remains SONET-based.
Telephony providers need to transport toll quality voice streams between central offices, ensure reliable communication to the signaling (SS7) network, and provide carrier grade (5 x 9’s reliable) customer access. SONET is the perfect transport medium to deliver high QoS signals across a metro or regional environment.
Voice companies now regularly include data (Internet) offerings in their portfolio of customer services. Once again, the reliability and manageability of next generation SONET equipment provides a converged transport medium that can handle traditional voice and new data traffic in the same infrastructure, providing a cost effective solution.
Utilities Network Transport & Substation Automation
The reliable and consistent delivery of power is a critical factor in the smooth operation of our modern, technology filled civilization. To support the population’s increasing needs, utility companies continually invest to improve the efficiency and robustness of the power delivery infrastructure.
Most utility companies have good network visibility and control of the power delivery equipment in large/central locations, but have significantly less access at the smaller, edge locations (substations). Control and monitoring down to the customer premise, although increasing, is still rare. But, advanced utility applications are moving control and responsiveness out to the edges of the network, requiring that network communication be extended out to these remote locations.
Many new utility application devices are becoming available that utilize network communications (typically Ethernet) to communicate. Extending the network out to these devices in the field is opening up many opportunities for utilities to manage critical events better, and to provide cost savings in the delivery of power to customers. Monitoring of remote equipment for anomalies to trigger preventative maintenance is becoming a cost (and trouble) saving application. Demand management (peak load management) applications are helping to better utilize generation capacity. Remote meter reading is eliminating operational costs. All of these types of applications are enabled by increased penetration of network communications.
To construct these mission critical management and communications networks, utilities have typically relied upon SONET-based technologies. SONET can transport traditional DS-0/1/3 based services in a fail-safe (protected) manner eliminating the need for costly leased lines. Next-generation SONET equipment supports the delivery of Ethernet services in a high QoS model also. This capability to develop a single, converged infrastructure that supports traditional and new traffic (Ethernet) is critical for utilities struggling to develop cost-effective network infrastructures.
TDM and Ethernet Customer Services
Traditional DS1/DS3 services still provide the majority of revenue for commercial network operators. But many of their customers are migrating to an all data (Ethernet/IP) infrastructure. To cost effectively deliver the traditional services and enable their customers to seamlessly migrate to Ethernet, network operators must build a single converged transport network. SONET remains the best technology fit for these applications.
Fiber to the Tower
With the increase of data services being delivered to mobile devices, the large wireless operators are faced with rapidly increasing bandwidth demands. Their current infrastructure, commonly based on leased-lines from the local exchange carrier, cannot scale effectively to meet the new needs. Additionally, the current cost of those lines is a large part of overall operations expenses, driving the wireless operators to search for alternatives.
Commercial wireline network operators are racing to deploy solutions that can displace/supplement the tradition LEC-based T1 services with higher speed data services. The ability to serve traditional T1’s to the cell site while also providing high quality Ethernet services (10 – 50 Mbps to each tower) is a huge new opportunity in the market place.
A variety of technologies are being investigated for “Fiber to the Tower Solutions.” Although a native Ethernet solution is viewed as the end-goal solution, current issues with timing, latency, and QoS cause many operators to adopt more conservative approach – SONET provides an excellent alternative. Delivering a predictable, high-quality-of-service infrastructure capable of transporting T1’s and large Ethernet circuits without jitter, loss, or large delays, SONET is a solution proven in today’s marketplace.
Fig 8. SONET – Tower to Switching Center
In step with the sky-rocketing demands for bandwidth across the network, the user demand for storage (disk space) is increasing as well. To address these needs, many enterprises have adopted advanced disk technologies such as storage area networks (SAN) or network attached storage (NAS). But providing disk space is only half the solution – how do you backup this huge amount of data?
Previously, off-site tape backup were considered the most cost effective and secure model for large scale backups. But with the falling price of disk drive space, and the increasing network speeds available in the WAN, many Enterprises are looking to direct disk replication services to provide backup (cold), failover (warm stand-by), and even real-time access (disk mirroring).
To reliably support these WAN-based services, many disk vendors have adopted standard protocols ensuring the successful replication of data. FICON/ESCON protocols, used for many years, are now being replaced by Fiber Channel as the primary transport protocol in the WAN.
The chief requirement for successful storage replication/transfer applications is a reliable and robust network. Although simple Ethernet networks have been used on small to medium scale replication networks, large scale jobs require more – they require a high quality of service to eliminate packet loss. SONET provides an ideal transport to meet these needs.
Older SONET equipment can readily transmit data services across TDM and/or data circuits. This architecture, although efficient, required equipment on both ends to provide a suitable interface to the WAN. Newer “next generation” SONET equipment natively supports FICON/ESCON and Fiber Channel interfaces eliminating the need for this extra equipment.
Fig 9. Data Replication Using CPE devices for WAN Interface
Fig 10. Data Replication Using “Next Generation SONET Equipment
Multi-Point (E-LAN) Ethernet on SONET
Many network operators have significant investments in SONET equipment, and ring-based fiber plants. They face decreasing or flat needs for traditional TDM services, while the customer demand for Ethernet services is sky-rocketing. Faced with this situation, network designers are looking for alternative technologies that can leverage their outside plant footprint, and ideally, their existing equipment investment to deliver Ethernet services more efficiently.
To meet this need, a technology combining the best of the packet transport, with the reliability of a SONET platform was needed. Combining attributes from both these technology sets yielded Resilient Packet Ring (RPR).
Relying on the ring topology common to SONET networks, RPR implements a dual-counter-rotating ring protocol to provide the sub 50 mSec resiliency found in SONET, but with several improvements. Failures in the ring are handled by sending traffic the opposite way around the ring, similar to a SONET path switched ring. But RPR does not reserve 1 ring just for protection. RPR allows the users to send live traffic around both rings in normal conditions doubling the capacity when compared to a SONET ring. Additionally, the protection mechanism can be configured to protect packet ordering even during a ring-fault event.
Fig 11. RPR Protection
To deliver packets effectively around a ring, some method of controlling the bandwidth used by individual nodes must be employed. With a “fairness mechanism”, a single node could consume all the bandwidth and prevent any other node from communicating. RPR implements just such a mechanism. Communicating amongst themselves, the nodes in an RPR calculate the maximum bandwidth that each can consume without affecting the ability of the other nodes to communicate. Each node then throttles its own traffic to ensure fairness around the ring.
The third major enhancement RPR brings is a strict Quality of Service (QoS) capability. Prioritizing traffic and delivering delay sensitive traffic in a timely manner are critical for the delivery of advanced services over a packet network. The ability to deliver voice and (particularly) video services reliably is a step forward rivaled only by (more) complex MPLS infrastructures.
The ability to deliver traffic with correct prioritization, in an infrastructure that guarantees each element will be able to send/receive an adequate share of bandwidth and will be protected from path failures makes RPR a very attractive transport protocol.
