1. Field of the Invention
The invention relates generally to the monitoring and testing of links in a communications network, such as a telephone network. More particularly, this invention relates to monitoring and testing a link between two or more locations in the telephone network.
2. Description of the Related Technology
Communication of data in a communications network often involves compliance with several transmission standards, such as frame relay, asynchronous transfer mode (ATM), integrated services digital network (ISDN), fiber distributed data interface (FDDI), and the Internet. These standards specify a variety of signal protocols and require conversion of signals from one protocol to another, and vice versa. Generally, a protocol refers to an agreed-upon format for transmitting data between two devices. The protocol determines, among other things, the type of error checking to be used, method of data compression, if any, and how a device indicates that it has finished sending or receiving a message. Several of the most significant protocols in use today are frame relay, ATM, ISDN, FDDI, and TCP/IP.
Frame relay is a packet-switching protocol for connecting devices on a wide area network (WAN). Frame relay networks support data transfer rates at 1.544 Megabits per second (Mbps) (also known as DS1 or T1 rate) and 44.736 Mbps (also known as DS3 or T3 rate). ATM is a packet-based network supporting data transfers between 25 and 622 Mbps. ATM offers a fixed point-to-point connection known as a “virtual circuit” (VC) between a source and destination. ATM is often transmitted over a physical medium known as a synchronous optical network (SONET) which employs fiber optic links. SONET defines a fiber optic transmission system offering optical channels from optical carrier 1 (OC-1) at 51 Mbps to OC-96 at 4.8 Gigabits per second (Gbps).
ISDN is an international communications standard for sending voice, data, and video over digital telephone lines. ISDN requires special metal wires and supports data transfer rates of 64 kilobits per second (kbps). FDDI is a set of American National Standards Institute (ANSI) protocols for sending digital data over fiber optic cable. FDDI networks support data rates up to 100 Mbps. FDDI networks are typically used as backbones for WANs. Finally, data traffic on the largest public network in the world, the Internet, conforms to Transmission Control Protocol/Internet Protocol (TCP/IP) standard which is a suite of communication protocols for connecting host computers on the Internet.
An open systems interconnection (OSI) model is often implemented to facilitate the interoperability of systems conforming to different standards. The OSI model provides a widely accepted structuring technique called “layering” whereby the communications functions are partitioned into a hierarchical set of layers. Each layer performs a related subset of the functions required to communicate with another system. Ideally, the layers are defined so that changes in one layer do not require changes in other layers. The OSI model defines the following: physical, data link, network, transport, session, presentation, and application layers. The following is a brief description of the function and purpose of each layer.
The physical layer defines the transmission of unstructured bit streams over physical links, involving parameters such as voltage swings and bit duration. The data link provides reliability to the bit stream by defining error detection and control bits. The network layer is responsible for establishing, maintaining, and terminating connections across one or more networks between two communicating systems. The transport layer is responsible for maintaining proper sequence and error-free delivery of data between two communicating systems. The session layer controls the dialogue between two communicating systems by specifying discipline (e.g., half- or full-duplex), grouping of data, and checkpoint mechanism for recovering lost data. The presentation layer defines data formats exchanged between applications by offering a set of transformation services, such as compression or encryption. Finally, the application layer defines the mechanism of accessing the OSI environment to support the exchange of information between two or more applications, such as file transfer and electronic mail.
As the number of communications networks increases, so does the complexity of managing, maintaining, and troubleshooting a malfunction in these networks. Service providers (e.g., telephone companies) may deploy one or more devices across the network to collect data from the network to analyze data protocol. For further details on such data collection, reference is made to a co-pending application entitled “SYSTEM AND METHOD OF ANALYZING NETWORK PROTOCOLS”, application Ser. No. 09/188,923 and filed on Nov. 9, 1998, (the '923 application) which is incorporated in its entirety by reference.
For these devices to collect data from the network, the network is expected to be operational. More particularly, it is expected that the network be active with data flowing through the collection point. If the network is down with no data flowing through the collection point, service providers have to dispatch technicians to the field to test the network at various points. Frequent dispatch of technicians increases maintenance cost and downtime, thereby raising cost of service to consumers. During this process, actual examination of a link at multiple locations may be necessary to isolate the source of the malfunction in the network. Moreover, the network user's operation is shut down or, in some cases, transferred to more costly back-up solutions.
Therefore, there is a need in communications network technology to provide service providers with the ability to maintain and troubleshoot their network in an efficient and cost-effective manner.