NOTA WAN

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WAN technologies are generally represented by the three lower layers of the OSI
model, namely, the network, data link, and physical layers.

Dedicated leased line - one single user, no share
Circuit-switches - use the telephone communication infrastructure dialyp/ISDN/ASDL
Packet-Switches - shared bandwidth technology. Users connect to a service
provider network and share the bandwidth. X.25 dan Frame Relay
Cell-Switches - Cell-switched connections are very similar to packet-switched
connections with the exception that cell-switched frames have a fixed size
whereas packet-switched frames vary in size. For this reason, cell-switched
connections tend to be faster than packet-switched connections, especially under
heavy traffic.

Karakter - pasal cell
The ATM protocol breaks up transmitted data into 53‐byte cells. A cell is
analogous to a packet or frame, except that an ATM cell is always fixed in length
and is relatively small and fast, whereas a frame's length can vary.
ATM is packet-switching digital transmission technology that sends voice, video,
and data signals using 53-byte, fixed-length cell relay between end points over a
virtual circuit. An ATM cell contains a 5-byte header and a 48-byte payload (user
data), which is processed individually (asynchronously). ATM cells are queued
before being multiplexed over the connection-oriented transmission path.
Packaging data into smaller fixed-size cell units allows jitter reduction and
prevents data-queuing delays known as contention. By preventing contention in
applications where timely delivery of data is crucial — such as voice and video
applications — jitter is reduced and overall performance is improved.

Fizikal/Transfer Medium
ATM is a popular Layer 2 WAN protocol that establishes data-link layer
communications over physical Layer 1 circuits. Data bit rates of 155 Mbps over
CAT5 cable and 622 Mbps using fiber-optic cable are possible, with ATM network
speeds approaching 10 Gbps. SONET/SDH,
Data rates are scalable and start as low as 1.5Mbps, with speeds of 25Mbps,
51Mbps, 100Mbps, 155Mbps, and higher. The common speeds of ATM networks
today are 51.84Mbps and 155.52Mbps; both of them can be used over either
copper or fiber‐optic cabling. An ATM with a speed of 622.08Mbps is also
becoming common but is currently used exclusively over fiber‐optic cable. ATM
supports very high speeds because it's designed to be routed by hardware rather
than software, which makes faster processing speeds possible.

Cara Operasi
ATM is designed to switch these small cells through an ATM network very quickly.
It does this by setting up a virtual connection between the source and
destination nodes; the cells may go through multiple switching points before
ultimately arriving at their final destination. The cells may also arrive out of
order, so the receiving system may have to reassemble and correctly order the
arriving cells.
An ATM network is comprised of multiple ATM switches interconnected by pointto-point links, which transmits data cells to destination ATM network interface
adapters (known as the ATM endpoints). Examples of ATM end-points include
CSUs, routers, switches, computers, and video coder-decoders (codecs).
Two types of ATM switch interfaces are known as either UNI or NNI. UNI interfaces
interconnect ATM end systems to ATM switches. NNI interfaces connect ATM
switches. UNI and NNI interfaces are also classified by the owner of the interface.
The telephone company is assigned publicly owned interfaces, with private

equipment being assigned to the end user and is known as customer premises
equipment (CPE).
Further, all such data can be supported with a very small set of network
protocols, regardless of whether the network is local, metropolitan, or wide area
in nature. In terms of user access rates,
Advantage
The small cell size reaps several advantages. First, it can accommodate any form
of data—digital voice, facsimile, data, video, and so on. Second, the fixed length
of the cell offers the network switches the advantage of predictability, as
compared to a variable-length frame. Third, the fixed cell size facilitates the
implementation of switching functions in hardware (i.e., silicon), which enables
processes to be accomplished at significantly greater speed than does software,
especially if processing variable-size frames. These last two considerations yield
decreased delay, as data move through the switching systems and across the
transmission links in frequent little blasts. Long, and especially variably long,
frames occupy the attention of the network for relatively long periods of time,
causing delay as other data wait to be processed.
ATM is the first network technology to offer truly guaranteed bandwidth on
demand, as the bandwidth can vary during the course of the call [7]. True
enough, other services offer bandwidth that can vary with each call, but none
can offer the ability to adjust the amount of bandwidth required to support a call
once the call is established and to guarantee that it will be available when
required. A high-quality videoconference, for example, might require 1.544-Mbps
(T1) capacity as a rule. Yet, with the sophistication of contemporary compression
techniques, that call might require much less bandwidth much of the time. Once
that call is set up, a full T1 is dedicated to it, regardless of the actual bandwidth
requirement moment by moment. ATM is not so rigid; it can adapt dynamically to
the bandwidth actually required.
ATM networks provide for error detection of the header only and not the payload.
ATM networks make no provision for error correction. The main concern is to
deliver a cell to only the addressee. If the address is corrupted, the cell is
discarded, and the endpoint is responsible for determining that fact and
recovering from the loss through a request for retransmission. The advantages of
this simplified approach to error control are increased speed of switching,
reduced latency, and lowered cost, as the ATM switches require less memory and
processing power.

ATM differs from more common data link technologies like Ethernet in several
ways. For example, ATM utilizes no routing. Hardware devices known as ATM
switches establish point-to-point connections between endpoints and data flows
directly from source to destination. Additionally, instead of using variable-length
packets as Ethernet does, ATM utilizes fixed-sized cells. ATM cells are 53 bytes in
length, that includes 48 bytes of data and five (5) bytes of header information.
The performance of ATM is often expressed in the form of OC (Optical Carrier)
levels, written as "OC-xxx." Performance levels as high as 10 Gbps (OC-192) are

technically feasible with ATM. More common performance levels for ATM are 155
Mbps (OC-3) and 622 Mbps (OC-12).
ATM technology is designed to improve utilization and quality of service (QoS) on
high-traffic networks. Without routing and with fixed-size cells, networks can
much more easily manage bandwidth under ATM than under Ethernet, for
example. The high cost of ATM relative to Ethernet is one factor that has limited
its adoption to backbone and other high-performance, specialized networks.

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