Replacing Legacy Data Circuits with LTE Cellular Technology

3600 Pair Copper Cable
With the deregulation of the majority of the nation’s local telephone companies and the dramatic improvement in wireless data/voice services, Local Exchange Carriers (LECs) no longer have governmental oversight to motivate them to maintain certain traditional land line circuits. Technologies like Voice over IP (VoIP) and Virtual Private Network (VPN) have shifted traditional telephony point-to-point data circuits services to the Internet. Today, the “last mile” delivery of land line dedicated Internet services to the premise are predominantly delivered on T-(X) carrier circuits over two pairs of copper cable. Newer technology such as fiber optics and coax cable are offered as competing technologies; however, the most widely available medium by far is the LEC copper wire infrastructure. This legacy infrastructure is aging and diminishing in capacity. Without the monopoly the LECs once enjoyed, it is no longer profitable for these companies to maintain, replace, repair failing embedded infrastructure.

Anatomy of a T1

The DSO circuit provides 64 Kilobits per second (Kbit/s) of bandwidth and is the basic building block of legacy land line based data circuits. This technology uses Time Division Multiple Access (TDMA) introduced in the early 1960s by Bell Labs, to transport voice or data traffic over four copper wires by packetizing traffic into bit streams sent in channelized framed cyclical time slots synchronized by a common timing clock. This protocol is transparent to the end user, but actually is sharing the available transport medium with other traffic sent in round robin fashion. TDMA is the predecessor to current digital transport technologies such as Digital Subscriber Line (DSL), Pulse Code Modulation (PCM) and Code Division Multiple Access (CDMA used in 3G cellular radio networks).

A “T” carrier multiplexes multiple DSO circuits to deliver the desired bandwidth. A standard T1 circuit combines 24-DSO 64Kbit/s channels providing 1.544 Megabits per second (Mbit/s) of bandwidth which can be further multiplexed delivering progressive bandwidth. A fractional T1 circuit is provisioned to provide less than the 24 64Kbit/s channels where the full T1 bandwidth is not required.

A T1 circuit starts at the customer premise and is routed to a local cross-connection box where the building wiring is connected to the LEC cable system. The circuit can then pass through many more cross-connection points jumping from cable to cable. In some cases, the circuit will be multiplexed and digitally mixed with other circuits until it reaches the local Central Office (CO). Here the “Local Loop” is terminated to a large switch that handles both voice and data traffic. Depending on the purpose of the circuit, it can then be routed to a location that contains a Point of Presence (POP) for a particular carrier or service like Internet. COs serve a specific local geographic area and feed a Main Central Office that often contain the POP gateways.

The ups and downs of land line data circuits

Dedicated land line based circuits are expensive. The cost of the circuit is calculated by the distance of the “Local Loop” from the customer premise to the POP of the carrier or service in addition to the bandwidth requirement. This “Loop” charge is collected by the LEC and usually is provided for a minimum contract period. Depending on the contractual arrangement, the customer can pay a substantial provisioning fee for installation of the circuit. In the days of regulation and monopolies of the LECs and Long Distance service providers, Service Level Agreements (SLA) were mandated for land line based circuits by government oversight agencies. Today much of that oversight is gone. LECs are competing with other companies, who in most cases are reselling LEC circuits. LECs have cut their field service staff and raised the SLA response times to increase profitability in the legacy copper circuits.

Circuits go down for a variety of reasons but the most common reason is a break or degradation of the copper cable infrastructure. Copper based data circuits require specially conditioned lines that can respond to certain frequency ranges and timing continuity protocols. Over time, legacy copper cable systems degrade from exposure to natural elements. In low lying areas, pressurized nitrogen must be maintained in buried cables to mitigate water intrusion, and aerial cables are exposed to constant UV radiation and weather. The explosion in fiber optic cable installation and thefts of copper cable have made legacy cable cuts commonplace.

Central Telephone Office
When a circuit goes down, the source of the problem may have to be traced through the entire physical “Loop.” If bad wire(s) are found, the connections are routed through another connection on the cable. The problem arises, however, that while there is a lot of copper infrastructure in the country, there may not be enough in the exact location it is needed. This is particularly true in urban core and rural areas. The repair of the circuit may require extensive re-engineering and re-routing, thus extending downtime. Replacing and adding copper cable capacity is no longer economically feasible, and upgrading infrastructure to fiber optic systems is a slow expensive process.

The Long Term Evolution (LTE) cellular network

The first digital data devices were introduced for the cellular network in 1992. Cellular Digital Packet Data (CDPD) allowed mobile devices to transport data at 9.6 Kbit/s. Today, the first 4G LTE devices are reliably delivering 20 Mbit/s (an increase of over 1800% throughput over CDPD). The LTE network in place today can deliver 100 Mbit/s capability, and the roadmap for the next generation advanced LTE global network leads to 1 Gigabit per second within the next 10 years.
As carriers implement features of the LTE standards like class of service pricing models with SLAs, the customer will be able to choose the bandwidth and reliability previously offered in land line circuits at a fraction of the cost. Multi-Protocol Label Switching (MPLS) networks which deliver end-to-end Ethernet connectivity on legacy land line technologies such as T1, Frame Relay, and DSL will migrate to the lower-cost cellular wireless networks. These migrations will be driven by benefits that include increased up time, lower latency, and a true one-vendor solution. Cloud-based applications tied directly to the carrier backbone will replace traditional network management systems with embedded technologies requiring very little overhead and intervention. Traditionally, there are substantial lead times and associated costs to provision a land line data circuit. In the LTE cellular data network, hardware can be activated on the network instantaneously with little or no cost or lead time.

There will always be applications for land line based data circuits, and advances in technology will continue to increase the capacity of land based transport media; however, deployment of these technologies will be reserved for larger network requirements that justify the associated costs and maintenance. The advantages and cost savings that LTE wireless networks offer will certainly drive many applications to wireless data access in the future. 

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