15-06-2012, 05:31 PM
INVERSE MULTIPLEXING
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Introduction
Over the past ten years, local area networks (LANs) have become increasingly popular, to the point that they are now an indispensable part of a company’s infrastructure. Not only have LANs grown in popularity, but also in size. From a network of only a few computers, most companies now have a computer on every desk, each being a part of the local network. And as they have grown in size and the number of computers attached to them, LANs have also seen a large growth in the amount of bandwidth they use. It is not uncommon for a LAN backbone to have data rates of 100 Mbps up to 1 Gbps. This increase in bandwidth has not only occurred as a natural result of an increase in the size of networks, but also because today’s modern applications have evolved to require these large amounts of bandwidth. In addition, today’s LANs have also experienced a convergence of applications that they must support. Local area networks are now being used to transport voice and video traffic together with the traditional data traffic that they have already supported. And in the case of voice and video applications, not only is there a need for more bandwidth, but there is also a need for guaranteed levels of service because these applications are very sensitive to latency and delay.
T1 Inverse Multiplexing – What is it?
While the idea of inverse multiplexing may be new to many IT administrators, traditional multiplexing is not. Therefore, we will first consider how traditional multiplexing works before describing inverse multiplexing. In traditional multiplexing, multiple streams of data are combined into one single but larger data pipe. At the other end of the pipe, the data stream is demultiplexed into the original streams of data. There are a number of ways this is accomplished, depending on whether a signal is analog or digital. Generally, analog circuits use frequency division multiplexing (FDM), while digital circuits use time division multiplexing (TDM). In frequency division multiplexing, multiple streams of data can be transported simultaneously because each uses a different frequency width. In time division multiplexing, multiple streams of data are combined by using alternating time slots. For optical networks, a new form of multiplexing has come into existence. Wavelength division multiplexing (WDM) uses different wavelengths of light in order to transport multiple streams of data. In any case, traditional multiplexing is based on the concept of combining multiple streams of data into a single, larger data stream.
Inverse Multiplexing over ATM
ATM networks use a cell based technology that supports voice, video and data at a wide range of transmission speeds (Techguide, 19). Because it is capable of supporting these various types of traffic simultaneously while guaranteeing different levels of quality (quality of service, or QoS), there has been an increase in the popularity of ATM among LAN backbones, and in some situations, even to the desktop. However, the availability of ATM on the WAN has been very limited, and in cases where it has been available, its use has generally been prohibited by the high costs. Therefore, in 1997 the ATM Forum defined a standard for Inverse Multiplexing over ATM, or IMA (Techguide, 20).
Bit-Based ATM Inverse Multiplexing
Bit-based ATM inverse multiplexing uses the ATM Forum’s Cell-Based Transmission Convergence sublayer. This specifies how an ATM cell stream can be transported at the bit level, instead of at the cell level. After determining the “start of cell”, cells are subsequently transported across the media bit-by-bit (Bit by Bit, 4). This can use the existing DS3 or OC3c UNI interfaces already available on a company’s ATM switch. These ATM interfaces are used as a DTE port, from which the data is transported over the WAN using multiple T1s. A bit-based ATM imux converts the traffic rate to the lower NxT1 rate using buffers. This can provide network managers the ability to have an ATM link over their WAN without using a new UNI or changing their existing ATM equipment (Bit by Bit, 6).
Transparent LAN Service
One of the benefits of inverse multiplexing, is its ability to make possible Transparent LAN Service using existing T1 circuits. A service provider as a convenience can offer transparent LAN services to its customers. Customers who need to interconnect various locations at native LAN speeds, but who do not have experience with WAN products or protocols, can be offered this service. The service provider can accomplish this by installing and maintaining inverse multiplexers at the customer’s sites and then scale the bandwidth to their needs.