Within a system, multiple communications networks meet�from a chip handling internal connections all the way up to long-distance networks or networks formed from I/O buses and local networks. We will attempt to put the sundry communications technologies into perspective by organizing them by their potential data bandwidths and the distance they are intended to cover. What is usually covered by a discussion of networks is just one part of the whole picture.The attach image, with no claim to completeness, lists various technologies classified by their distance (point-to-point) and bandwidths.
The very highest bandwidths, over extremely short distances, are the internal communications within a chip.
We then encounter a number of technologies which we can group together under the rubric of intrasystems networks, for which the acronym SAN has been used; in that context it meant System Area Network. However, nowadays the use of the acronym has been preempted to refer to network storage, when it means Storage Area Network. To minimize confusion, we shall adopt this latter meaning for the acronym. In the category of intrasystems networks we find:
� Systems buses, connecting processors to memory and I/O, are characterized by bandwidths of several gigabytes per second and distances in the 10-20 cm range�less than a foot
� I/O buses, such as PCI, which connect the processor-memory complex to I/O controllers. InfiniBand might replace PCI for this usage in mid-range and high-end servers. Connecting the processor-memory complex to the I/O subsystems, which would themselves likely still be built around PCI or a derivative
� Connections between I/O controllers and the peripherals themselves, using SCSI or FC-AL
� Connections between the processor-memory complex and a peripheral subsystem, such as disk or tape susbystems or a communications concentrator. Fibre Channel is the current prime example, and it may be joined by InfiniBand. HPPI (High Performance Parallel Interface) offers the same level of performance as Fibre Channel, but its use in practice is limited, essentially, to supercomputers
Beyond the intrasystems networks we find the local area network category:
� Ethernet, offering three classes of bandwidth: traditional Ethernet at 10Mbits/ second, fast Ethernet at 100 Mbits/sec (both these are very widely used) and gigabit Ethernet at 1 Gbit/second, which is finding its place in the market
� Token Ring at 4 to 16 Mbits/sec, which has an advantage over Ethernet in that it offers deterministic behavior. However, it has not had the same marketplace acceptance as Ethernet, probably because it is not an open technology and because of higher prices
� Fibre Channel and FDDI (Fibre Distributed Data Interface) make an appearance in this category as well, often as a concentrator of lowerspeed local area networks (such a network is referred to as a backbone)
� ATM has pretensions in the field of local area networks as well, with an emulation of a LAN named Lane (for LAN Emulation)
We then come to metro area networks, which are networks within a single city using technologies such as:
� Fibre Channel
� DQDB (Distributed Queue Dual Bus), which uses a pair of unidirectional bus connections
� FDDI, based on a token-ring technique
� ATM
And finally we see the category of long-distance or wide-area networks with technologies such as SONET (Synchronous Optical NETwork), and SDH (Synchronous Data Hierarchy) which are standards (ANSI for SONET), and International Telecommunications Union (ITU�once known as CCITT). SONET and SDH both carry ATM packets. For private lines, Frame Relay is also worth mentioning; it extends the capabilities of X25.
In the image, we also show two technologies used to connect users to telecommunications networks: ADSL (Asymmetric Digital Subscriber Line) and HDSL (High Bit Rate Digital Subscriber Line), are standards in this area. They make it possible to sharply increase bandwidths using existing copper telephone connections, giving up to 52 Mbit/sec for distances up to 300 meters (a bit over 300 yards) and even 1.5 Mbit/sec at distances up to 3 kilometers (around 2 miles).
Alongside the convergence of the different technologies in the areas of distance and bandwidth, we also see a convergence between local and wide area networks. As an example, ATM technology is as well-suited to LAN usage as it is to WAN. And it incorporates the concept of quality of service, guarantee a user�once a connection is granted�a quality of service (in guaranteed bandwidth, for example) for the duration of that connection.
However, the largest portion of installed networks are IP networks; this domination can only be extended with the advent of the new IPv6 version of the protocol, which considerably extends the address space of an IP network. Given IP's commanding position, any new network technology must support IP if it is to succeed.
Source of Information : Elsevier Server Architectures 2005
