Multiplex communications – Pathfinding or routing – Through a circuit switch
Reexamination Certificate
1999-12-03
2004-09-07
Lee, Andy (Department: 2663)
Multiplex communications
Pathfinding or routing
Through a circuit switch
C370S524000, C370S523000
Reexamination Certificate
active
06788678
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to an interface between the channel units of an incumbent local exchange carrier and a competitive local exchange carrier. More particularly, the invention pertains to an interface for a competitive local exchange carrier interfacing to an incumbent local exchange carrier's digital subscriber loop under the United States Telecommunications Act of 1996.
BACKGROUND OF THE INVENTION
In accordance with the United States Telecommunications Act of 1996, incumbent local exchange carriers (ILECs) that, up to that time, had a monopoly on local telephone service in a given geographic area were required to provide to competitive local exchange carriers (CLECs) access to their connections to subscribers. Accordingly, CLECs could then offer competitive local telephone services to the original ILEC's customers.
The purpose of the Telecommunications Act of 1996 was to foster competition for local telephone service by assuring that a CLEC could provide services to telephone subscribers with quality comparable to that provided by the ILEC. Accordingly, ILECs need to provide an interface between its central office terminals (COTS) and a CLEC's remote terminal (RT). Despite the name, the CLEC's remote terminal (RT) typically is located in the ILEC's Central Office (CO). The CLEC then typically transports the communication channels via its own network facilities from its RT to its own office where it maintains its own telecommunications equipment, including switching fabric.
There are three common schemes in use today for interfacing between an ILEC's COT and a CLEC's RT. The first is an analog VF interconnection as illustrated in FIG.
1
. In short, a twisted wire pair
12
is provided between the ILEC's COT
14
and the CLEC's RT
16
and the communication data is transmitted between the two terminals in analog form.
At least one goal of the Telecommunications Act of 1996 is to assure that a CLEC can provide the same grade of service (GoS) as the ILEC to telecommunication subscribers. However, many, if not most, ILEC telecommunications networks utilize digital loop carrier (DLC) systems
18
between their central office terminals, e.g., COT
14
, and remote terminals, e.g., RT
20
, located closer to the subscribers' telephone
22
. Typically, remote terminal
20
is located within about two miles of the particular subscriber and the signaling between the central office terminal
14
and the remote terminal
20
is digital, termed a digital loop carrier or DLC
18
. The DLC portion
18
is a 4-wire portion comprising two twisted wire pairs
18
a
and
18
b
, one of which carries downstream signals and the other of which carries signals upstream. For those subscribers that do not have digital service, a CODEC
24
in the RT interfaces with the subscriber's telephone equipment
22
and converts the signals between analog and digital. In the U.S., for example, in the digital loop carrier portion
18
of the network, voice (and data) is sampled at the rate of eight KHz and digitized into eight bit samples, yielding a 64 Kbps (8 KHz times 8 bits) data rate. A digitized voice channel (64 Kbps) is called a DSO. In many if not most digital loop carrier systems, 24 digitized voice channels (DSOS) are time division multiplexed into one signal called a DS1 signal.
When an ILEC provides local area telephone services to its subscribers using digital loop carrier systems between its RTs
20
and COTs
14
, the signaling between terminals is all digital. However, when a CLEC provides the local area service and interconnects to the ILECs COT via an analog VF interconnection, a digital to analog conversion must be performed in the ILEC's channel unit before the data can be transmitted over the analog interface. Specifically, a channel unit
26
in the COT
14
must convert signals that are to be transmitted to the CLEC RT
16
over the interface
12
from digital to analog and convert signals received from the CLEC RT
16
from analog to digital.
The CLEC's switching fabric
29
typically is located in the CLEC's central office
31
at a remote location from the ILEC's central office
27
. Accordingly, the CLEC's RT interfaces with the CLEC's Central Office via another link
33
, which is commonly a digital carrier link, such as another DLC. Therefore, the CLEC's RT
16
must convert signals that are to be transmitted to the ILEC's COT
16
over the interface
12
from digital to analog and convert signals received from the ILEC's COT
16
from analog to digital. Accordingly, the ILEC's COT channel unit
26
and the CLEC's RT channel unit
28
both contain CODECs
25
for converting upstream channels through the interface
12
from analog to digital and converting downstream channels from digital to analog.
Further, the interface between the ILEC COT
14
and the CLEC RT
16
is a single twisted wire pair
12
in which upstream and downstream data are carried on the same wire pair, whereas the digital loop carrier portion of the system is a four wire system with the upstream and downstream signals being transmitted on separate twisted wire pairs. Therefore, each channel unit
14
,
16
also includes a hybrid circuit
23
for converting between 2-wire (the interface side) and 4-wire (the DLC) transmission modes.
Specifically, there are at least three aspects of this link which degrade the service when signals are exchanged between the ILEC and the CLEC. First, there is an increase in background (or quantization) noise. Each time an analog to digital conversion occurs, quantizing noise is introduced because of the finite granularity of the digital code representing each sample of the analog signal. When the ILEC provides service, there is only one analog/digital conversion (at the ILEC's RT
20
or the ILEC's digital switch). However, when the CLEC provides analog-to-digital service, there are two analog-to-digital conversions namely, at the ILEC's RT and at the CLEC's RT
16
. Accordingly, the background noise when service is provided by a CLEC can be twice that of when service is provided directly by the ILEC.
Secondly, additional noise is introduced by the repeated use of robbed bit signaling (RBS). Robbed bit signaling allows the digital voice or data bit stream to carry signal states necessary for supervision of telephone circuits. A robbed bit is sent in the least significant bit (LSB) position of every 6
th
voice/data sample. In essence, one out of every 48 bits transmitted is “robbed” for supervisory purposes. When a CLEC provides service, the signals will contain an extra set of robbed bits since the CLEC will add its own robbed bit in its own network while the signal will still contain the robbed bit from the ILEC's network.
Thirdly, the back to back channel unit configuration used for interconnecting the CLEC and ILEC networks introduces two extra sources of echo. Specifically, when the interconnection between the ILEC and the CLEC is two wire, there must be a hybrid circuit in each channel unit to convert between 2- and 4-wire, thus introducing two more sources of echo in the link. The extra echoes can be avoided by using a 4-wire rather than a 2-wire interface.
A second option for the interface between the ILEC's COT and the CLEC's RT is a digital cross-connect system (DCS) I/O. A DCS I/O has a plurality, e.g., 24, of DS1 interfaces and a time slot interchange matrix that can be used to cross connect DSO channels. However, a DCS I/O interface is expensive. Further, it provides only DS1 interfaces. Thus, bandwidth is wasted when less than 24 channels are being interconnected.
Another existing interface is the DS1 interconnection. However, like the DCS I/O solution, this type of interface also has 24 channels and is expensive. Accordingly, bandwidth is wasted when less than 24 channels are being interconnected.
Accordingly, it is an object of the present invention to provide an improved interface between the chan
Adams, III Robert L.
Brown Ronald R.
Rice D. Reagan
Lee Andy
Lucent Technologies - Inc.
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