Carriers, Service Providers, and How Traffic Is Carried

Carriers , Service Providers, and How Traffic Is Carried

In telecommunications, transferring information from one point to another is based on transferring signals that by nature can be continuous and discrete, or analog and digital. The typical Public Switched Telephone Network (PSTN) of the early 1960s is primarily an analog network. Discrete or digital signal terminology is based on the base 2 numerical system (1 and 0), where a digital signal has at least two recognizable states that can be interpreted as 1 and 0.

Carriers build networks that differ from each other based on the way that the carrier switches the traffic and reroutes it to the requested destination. The carrier's switching systems can be classified as follows :

• Circuit switching The physical resources of the time, space, and frequency spectrum are dedicated to a single call for the duration of the call. The network handles the resources based on physical availability, thus restricting and blocking the call requests that exceed the physical limitations of the network.

• Message switching Each switch can store and delay incoming messages, and rather than block the requests, the switch uses the available bandwidth by using rate adoption algorithms, multiplexing, or both. Packet switching is a special case of message switching, where messages are formalized and length-restricted by using maximum transmission or receive unit (MTU, MRU) conventions, or by using standards-based message exchanges to minimize the content-based calculations.

• Packet switching Packet switching is slightly different from message switching. Message switching passes messages from the sender to the receiver through intermediate nodes. Packet switching relies on a large number of trunks attached to the switches and thus balances the traffic load around busy trunks and lines.

• Cell switching Cell switching is primarily based on Asynchronous Transfer Mode (ATM), where a standard, fixed-length 53-byte packet that is called a cell can deliver data, voice, and video in a uniform fashion. Traditional switching methods are defined by the two dimensions of the physical conduit and the subchannel (subinterface). Cell switching uses additional dimensions that define the physical conduit, virtual path, and virtual channel, where the intermediate dimension of the virtual path is composed of all the virtual channels that are pointing in the same direction.

Cell switching terminology can be a bit confusing because the term asynchronous is part of the name . In asynchronous data communications, the synchronization is not necessary because the synchronization is part of the frame format. Each character (5 or 8 bits long) requires a start and a stop bit and, in some cases, the stop bit can be 1.5 times longer than the start bit. Considered a reliable form of communication, the asynchronous transfer creates high overhead and thus restricts high bandwidth rates. In the case of synchronous data communications, precise synchronization is mandatory.

NOTE

Have you ever heard a technician from the central office (CO) say, "The line cannot sync"? This is the most common reason that the first layer does not come up and/or the clocking is not available.

Synchronous data transmission offers high-speed rates because instead of start and stop procedures, both customer premises equipment (CPE) and local exchange carrier (LEC) equipment must synchronize their exchange, which is based on special frames or bits that indicate the beginning and end of transmitted user data. ATM is called asynchronous because of the sporadic presence of user data. If user data needs to be sent, it is transformed into cells and sent over the line. If there is no user data, only keepalive cell types are sent and received.

The choices that need to be made in analog versus digital communications are related to analog or digital signaling. Historically, the analog versus digital arguments seemed to be a waste of time, because digital technologies are vigorously gaining new market shares. The new technologies of cable, WAN satellite, and wireless LANs (WLANs) are reinventing some of the advantages of analog transmission. The advantages of digital transmission over analog includes better quality of service (QoS), simplified maintenance, synergy with digital switching, powerful digital signal processing, simplified multiplexing, and it is well matched to fiber- optic and device technology. The disadvantages of digital transmission include the need for larger bandwidth, accurate network synchronization, and a costly interface with existing analog networks.

Carrier systems can be split into analog carrier systems and digital carrier systems.

Analog carrier systems use active or passive repeaters to recreate the signal, so any losses or attenuations are corrected while the signal is propagated along the line. Analog systems carry voice, data, and video information, and they exchange supervisory and maintenance information between each other. These carriers are best suited for voice signals; they operate over multi-pair cables, N-carriers, coaxial cables, and L carriers. There are a growing number of radio-channel carriers whose market share is increasing faster than the classic analog carriers.

Digital carrier systems are based primarily on the regeneration of signals. Besides regenerators, these systems use multiplexing and demultiplexing techniques to use broadband signals over a single circuit. The two most significant methods are frequency-division multiplexing (FDM) and time-division multiplexing (TDM).