Alternate Access and the Local Loop
Bypass as we know it is primarily implemented in the local loop. The local loop is the connection between the business premise and the Local Exchange Carrier (LEC) central office. Service in the local loop was expanded because that is where the majority of transmission problems occur. This is due to the cable plants which have been in place for the past 30 years. Ironically, the local loop is typically the most expensive part of a circuit. In many cases, the local loop is more expensive than a circuit from an Inter-exchange Carrier (IXC) that may extend for hundreds of miles.
Alternate routing or media diversity can be used anywhere in the local loop. When a user selects an alternate technology which provides transmission facilities to the LEC’s central office, it is called service bypass. Facility bypass occurs when the user bypasses the LEC and connects directly to the IXC. Facility bypass could also occur when the user directly connects two facilities in the same general area or in a campus environment. There are many alternatives to alternate routing or media diversity in the local loop. Some of the examples include the following:
One alternative is a Metropolitan Area Network, or MANs. The fiber optic rings are strategically positioned in the densely populated metropolitan districts. The MANs provide subscribers increased protection from potential disasters. This added protection is accomplished by providing alternate path routing. Because of the nature of a ring topology, if a problem occurs at any particular point in the ring, the disrupted traffic is simply re-routed in the opposite direction.
A disadvantage with the MAN ring is the inability to gain access to the network and the limited area in which the ring covers. A fiber optic ring network will only benefit those users whose business resides in buildings where the districts would have a difficult time gaining access to the ring. Even if the subscriber is located near the ring, it may take months before the required facilities can be constructed--providing the MAN could obtain the appropriate right-of-way. In addition, if facilities are built to the subscriber point of presence, it would likely be a spur route off of the ring network. When a fiber route is installed as a spur off the ring instead of being a part of the ring, a single point of failure is created. The spur has no alternate route in which to direct traffic.
RBOC Routing Alternatives
In partial response to the proliferation of MANs, Regional Bell Operating Companies (RBOC) have begun planning and implementing necessary steps to compete. Some services available from the RBOCs include diverse routing and alternate routing. Diverse routing is the method of routing traffic through split cable facilities or duplicate cable facilities. This can be accomplished with different and/or duplicate cable sheaths. Alternate routing is the method of routing information via an alternate media such as copper cable or fiber optics. Many of the RBOCs have begun deploying counter-rotating fiber optic rings. These rings have fiber optic cables which transmit information in two different directions and in separate cable sheaths for increased protection. At the present time, these rings connect through one central switching office. Future expansion of the rings may incorporate a second central office in the circuit. Some RBOCs are offering alternate routes to different points of presence or alternate central offices for the same IXC.
The subscriber has many of the same disadvantages that occurred with the MAN. Diverse routing can be accomplished by using different cable sheaths, although the cable may be in the same conduit and is therefore subject to many hazards. The subscriber can duplicate the facilities by having alternate routes although the entrance to and from the customer premise may be in the same conduit. The subscriber can obtain diverse routing and alternate routing from the RBOC, including dual entrance facilities, however gaining such specialized routing requires a great deal of time and effort and a very resourceful budget. It is not uncommon for two users to pay the same tariff rate with one being served by an analog central office and the other being served by a digital central office.
The MANs and the RBOCs provide facilities for alternate and diverse routing, although the majority of services are transmitted over terrestrial media. These cable facilities are usually located in the ground or the basement. These ground-based facilities are at great risk due to aging infrastructures of cities. In addition, cable-based facilities usually share room with mechanical and electrical systems which can impose great risks due to human error and disastrous events.
Due to the aforementioned circumstances and basic cost-efficiencies, many companies are exploring short-haul microwave as a viable solution to telecommunications dilemmas.
Short-haul microwave is a proven industry that has been providing reliable service to thousands of users for many years. Digital and solid state technologies have improved the performance and reliability of microwave radio transmission. Leading manufacturers will guarantee an unfaded Bit Error Rate (BER) of 10E-12 and multi-year MTBF (mean time between failure). The majority of the RBOCs advertise 99.95% availability for DS1 services. In contrast, properly engineered microwave paths are designed for and achieve 99.999% availability.
In comparison to high leased lines costs, an entire short-haul microwave network can be installed with a cost payback of less than 18 months.
Microwave can provide both route and media diversity. A point-to-point microwave system can be installed between any two facilities as long as line of sight exists. If line of sight does not exist, a passive repeater can be installed to re-direct the microwave around the obstruction. Obtaining right-of-way for cable-based facilities can be a lengthy, if not impossible, process.
Microwave can be manufactured and installed quickly. The typical process to engineer, furnish, and install requires approximately 90 days. Microwave can also be packaged and licensed for emergencies or quick start applications. One RBOC recently conducted a test to determine the timeframe required to install a portable microwave and resume network operations. The portable microwave equipment was installed and traffic restored in 35 minutes.
Short-haul microwave usually resides at the top of a building or on a tower because it requires line of sight. The physical location of the equipment provides additional protection to the network. By locating the equipment on the tops of buildings, it is not subject to the possible hazards of basement-based facilities.
Short-haul microwave will provide bandwidths which range from 1.5 to 45 megabits. Microwave can be configured to transport 10 megabit Ethernet, 4 or 16 megabit Token Ring, ArcNet, Broadcast or digitally compressed video, and IBM Channel Extension, to name a few. Many of these applications are not available at full data rates through the RBOC or IXCs. Microwave can also be designed in fully redundant or non-redundant configurations necessary to provide maximum network availability.
Many approaches to route and media diversity exist. From that perspective, a customer must determine which alternative or combination of alternatives will provide the most adequate protection and availability to the network.
Rick Elliott is the Manager of Domestic Sales for Microwave Networks, Inc. in Houston.
This article adapted from Vol. 4 No. 2, p. 26.