SONET Offerings for Enterprises

Local telephone companies sell SONET (Synchronous Optical Networking) transport for connections between local customers and interexchange carriers. The speeds offered are at OC-3 (155 megabit), OC-12 (622 megabit), and OC-48 (2.5 gigabit) rates. The local telephone companies guarantee 50 millisecond network restoration in the case of a network failure or degradation. They run the SONET service to multiple local central offices. In the case of a failure at one CO (Central Office), service is immediately available from the backup CO. Matching SONET multiplexers are required at the customer premises and at the telephone company office. Another variation of SONET service protects customers from fiber cuts. This diverse routing scheme offers fiber from separate building entrances to the same CO.

Customers often opt for point-to-point SONET rather than bidirectional rings to save money. The major impediment on sales of these services is the cost to dig trenches for additional fiber runs from the customer to the incumbent carrier's fiber ring. Because it is lower in price, newer Ethernet services at gigabit or lower speeds ranging from 10 megabits to 500 megabits are gaining in popularity for data communications. However, customers with existing SONET service have the option to add Ethernet data that runs at 10, 50 or 100 megabits per second. This uses spare capacity on the SONET multiplexer for perhaps LAN-to-LAN connections in metro areas.

Third generation SONET

Connectivity to Ethernet: Transporting IP voice and Ethernet traffic on SONET-equipped links wastes capacity on carriers' networks. This is because SONET carries traffic in "chunks" at 64 kilobits per second in fixed-size frames called cells. However, IP and Ethernet traffic bits are in variable-size packets. In addition, SONET cells have high overhead (nonuser data such as monitoring and addressing), which adds to its inefficiency because less customer traffic is carried in each cell. This mismatch between frame size results in carriers stuffing zeros into many SONET frames.

Some manufacturers have developed SONET equipment that handles packet traffic more efficiently. For example, newer multiplexers have Gigabit Ethernet ports and ports that can interface directly with telecommunications services used in storage area networks. These SONET multiplexers have the capability to pick up and drop off Ethernet and IP traffic more efficiently at Ethernet speeds. However, they transport traffic to older SONET devices in SONET frames, which wastes capacity.

Second generation SONET

Second generation SONET also referred to as multiservice platforms, achieved higher speeds (up to OC-192 [Optical Carrier-192], 10 gigabits), took up less space by supporting more ports on each card, and gave carriers the capability to increase and decrease speeds remotely without taking the ring out of service. They also enabled carriers to drop off lower optical carrier streams to customers for enterprise SONET (Synchronous Optical Networking) services such as Ethernet and storage area network services. However, next-generation multiplexers do not interface directly to MPLS (Multiprotocol Label Switching) networks. In addition, although they carry Ethernet and storage area network services, they do so inefficiently, in SONET frames. Next-generation SONET devices can have internal add and drop multiplexers and digital cross-connect systems.

ADD and Drop Multiplexers (ADM)
Add and drop multiplexers add and drop channels from fiber rings at the edge of the network. They drop off and pick up channels to a particular central office or to a small metropolitan area from rings that connect the core to the access network. Add and drop multiplexers are less complex and handle fewer streams of traffic than digital cross connects.

Digital Cross Connects
Digital cross connects rearrange channels of traffic between multiple routes. A digital cross connect system has the same functionality as a switch. For example, multiple rings may connect at a carrier's point of presence (POP) in the core network in the northeast. The digital cross connect sends some of the traffic to, for example, New York, some to Pennsylvania, and the rest to New Jersey. It also accepts traffic from these states and connects it to other routes. The newest digital cross connects are all-optical. They switch colors (channels) of traffic without converting light signals carried on fiber to electrical signals and electrical signals back to light. This eliminates the need for conversation equipment in these devices, which leads to lower prices and higher-speed switching.

SONET Rings

SONET can run as a straight point-to-point line between sites, or in a ring topology. When fiber in a point-to-point arrangement is cut, service is lost. However, the higher speeds attainable on fiber make reliability critical. When a medium such as copper carries a conversation from one telephone subscriber, a copper cut only impacts one customer. Fiber cuts in networks can put hundreds of locations out of service. For this reason, the majority of telephone companies deploy bidirectional ring topology.

In the bidirectional SONET/SDH ring, one set of fiber strands is used for sending and receiving; the other is the protect ring (spare ring). If one set of fiber strands is broken, the spare (protect) ring reroutes traffic in the other direction. In addition, if one multiplexer on one set of fibers fails, the backup multiplexer on the fiber running in the other direction automatically takes over.

SONET continued

SONET was developed to aggregate (multiplex) and carry circuit switched traffic such as T-1, E-1, T-3 and E-3 as well as slower rates from multiple sources on fiber-optic networks. SONET transports traffic at high speeds called OC (optical carrier). The international version of SONET is synchronous digital hierarchy (SDH). SDH carries traffic at synchronous transport mode (STM) rates. See the table below for optical carrier and synchronous transport mode speeds. Interfaces in the equipment make SONET and SDH speeds compatible with each other. The same SONET equipment can be used for both OC and SDH speeds.

Europe's time division hierarchy is based on E1 (2-megabit) and E3 (34-megabit) signals. E1 circuits carry 30 channels at 64 kilobits per channel. E3 circuits carry 512 channels at 64 kilobits per channel. Traffic that is carried between cities in Europe or in undersea cables is often referred to as being carried at STM-1 or STM-16 rates.

SONET (Synchronous Optical Network):

First introduced in 1994, SONET is a North American standard for multiplexing slower streams of traffic onto fiber-optic cabling and transporting it at optical carrier (OC) speeds. The international standard for the same functions is synchronous digital hierarchy (SDH). SONET/SDH was a major innovation in enabling carriers to carry enormous amounts of voice and data traffic reliably on fiber networks. As SONET equipment prices dropped, large enterprises adopted it as well.

SONET equipment transports high-speed traffic on fiber-optic network between the following:
- Central offices in metropolitan areas (the metropolitan core)
- Remote terminals (digital loop carriers) in metropolitan networks (metropolitan access networks) and central offices
- Long-haul backbone networks and metropolitan areas
- Points of presence (POPs) in long-haul, core networks
- Enterprises and data centers where backup data is stored
- Enterprises and points of presence (POPs) that carry their long distance traffic
- Enterprises to separate central offices for redundancy in case of a central office failure or a fiber cut

SONET also can carry ATM and IP traffic and television signals. However, as increasing amounts of traffic is data rather than voice and more of the data and a growing percentage of the traffic is IP based, lower-priced gear is becoming available to transport IP traffic more efficiently and at lower costs on redundant fiber rings. These rings found in MPLS (Mulitprotocol Label Switching) networks and some metro-area networks are based on dense wavelength division multiplexing.

ATM Characteristics

When ATM was developed in the early 1990s, its speed provided a key advantage over T-1 and T-3 services, which are based on time division multiplexing. It was also faster than routers available at that time. ATM's speed is due to its fixed-size cells, switching in hardware and asynchronous technology, which does not depend on timing. Rather, cells are forwarded based on priority and arrival time.

Fixed-Sized Cells - Less Processing: ATM packages data into discrete groups called cells. These cells are of a fixed size. Handling fixed-sized cells requires less processing than older routers with variable-sized packets. The ATM switch does not have to look for bits telling it when the cell is over. Each cell is 53 bytes long. Five of the 53 bytes contain header information. This includes bits that identify the type of information contained in the cell (for example, voice, data , or video) so that the cell can be prioritized. The remaining 48 bytes are the "payload" - user data such as voice, video, or sales proposals.

Switching in Hardware - Less Address Lookup: ATM cells are switched in hardware. This means that an ATM switch does not have to look up each cell's address in software. Rather, an ATM switch sets up a route through the network when it sees the first cell of a transmission. It puts this information into its hardware and sends each cell with the same header routing information down the virtual path previously established. For example, all cells with XXX in the header use route 234. Using the same path for each cell makes ATM a connection-oriented service.

Asynchronous Switching - Improving Network Utilization: With asynchronous switching, every bit of network capacity is available for every cell. This is different than synchronous multiplexing technology such as T-1/E-1 and T-3/E-3. With T-3 multiplexing, every one of the 672 input transmissions is assigned a time slot. If device A has nothing to send, its slot is sent through the network empty. ATM has no synchronous requirements. It statistically multiplexes cells onto the network path based on quality-of-service information in the header. With ATM, network capacity is not wasted forwarding empty cells.


Source: The Essential Guide to Telecommunications, 4th Edition by Annabel Z. Dodd