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For Wide Area Network is a collection of links using protocols Level 2 or 3, capable of interconnecting smaller networks that are physically distant. Once, wide area networks were analog or digital.
The geographical connections using different technologies:
-DDC (direct digital), through TDM, frame relay, MPLS, ATM ....
Digital telephony, using ISDN, ATM, ...

ISDN has not been very successful in Italy, because the technology has come a bit 'too late. It provides a single connection at a fixed speed, which allows in voice and data transport. The telephone terminal must be digital.ISDN exist (although it is still used in some locations) in two versions:
2B + D-2 provides data channels at 64Kbps and 16Kbps 1canale signaling.
Provides 30-30B + D channels and 1 64Kbps to 16Kbps signaling channel.

TDM (Time Division Multiplexing)
TDM is a protocol that allows the existence of multiple streams of data over a single channel. All available bandwidth to each flow is given at regular intervals. If a data stream are assigned more than one time slot of the channel the band assigned to the flow will increase. At the moment this technology is only used for point to point connections.

SDH (Synchronous Digital Hierarchy)
SDH has been implemented in several versions, which allow the flow of data to travel at normal speed (from 1.5Mbps to 2.4 Gbps and beyond). SDH provides for the existence of a multiplexer (MUX) time division, which performs functions similar to the MUX of TDM, even if it provides for equal time intervals for each channel. The MUX and DEMUX must be synchronized with each other and very fast (especially to work in newer drives).

Format of the SDH frames
The SDH frame is a matrix consisting of 9 columns and 125 rows. Each cell is from 1B. This pattern is repeated every 125 microseconds (8Ktrame/secondo).
The payload (payload) is less of the total space available, as for each frame are inserted additional information necessary for the connection.
To increase the bandwidth in later versions has increased the number of bytes for each cell, while the time frame has not been modified, as is common to all the lines and allows their interoperability. PDH is a variant of SDH.

X.25 is a protocol that uses the virtual circuit level 2-3.Each virtual circuit is set manually (packets are routed dynamically as with IP).
X.25 networks has been designed for high noise and unreliable and for this requires the presence of ACK.

Frame Relay
It 'similar to X .25 but it's only level 2. Is no longer present the ACK and therefore does not goes to meet the problems of X.25.
The delay introduced by the Frame Relay and X.25 is lower than the performance is generally better.

ATM (Asynchronous Transfer Mode)
ATM is a communications protocol that relies on statistical multiplexing, which allows multiplexing of lines are not aligned temporally. ATM was designed to replace Ethernet and depopulation in wide area networks.
The main features are:
-Sophisticated reporting.
-Mechanisms of flow control very advanced.
-Dynamic allocation of the banda.
-Support for bursty traffic (block).
-Adaptable to both delay-sensitive applications, as well as those sensitive to the loss.
-Cell-switching of fixed size. Each cell is 53 octets, including 5 header and 48 data. The fixed size simplifies the switching. The small size allows it avoids problems of fragmentation and transmission A / V.
-Integration of the priorities.

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