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Unit 2: Wireless System Architecture

may occur in the Physical Layer. Originally, this layer was intended for point-to-point Notes
and point-to-multipoint media, characteristic of wide area media in the telephone system.
Local area network architecture, which included broadcast-capable multiaccess media,
was developed independently of the ISO work in IEEE Project 802. IEEE work assumed
sublayering and management functions not required for WAN use. In modern practice,
only error detection, not flow control using sliding window, is present in data link
protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2
LLC layer is not used for most protocols on the Ethernet, and on other local area networks,
its flow control and acknowledgment mechanisms are rarely used. Sliding-window flow
control and acknowledgment is used at the Transport Layer by protocols such as TCP, but
is still used in niches where X.25 offers performance advantages. Simply, its main job is to
create and recognize the frame boundary. This can be done by attaching special bit patterns
to the beginning and the end of the frame. The input data is broken up into frames.
z z Network Layer: The Network Layer provides the functional and procedural means
of transferring variable length data sequences from a source host on one network to a
destination host on a different network, while maintaining the quality of service requested
by the Transport Layer (in contrast to the data link layer which connects hosts within the
same network). The Network Layer performs network routing functions, and might also
perform fragmentation and reassembly, and report delivery errors. Routers operate at this
layer—sending data throughout the extended network and making the Internet possible.
This is a logical addressing scheme – values are chosen by the network engineer. The
addressing scheme is not hierarchical. It controls the operation of the subnet and determine
the routing strategies between IMP and insures that all the packs are correctly received at
the destination in the proper order.
z z Transport Layer: The Transport Layer provides transparent transfer of data between end
users, providing reliable data transfer services to the upper layers. The Transport Layer
controls the reliability of a given link through flow control, segmentation/desegmentation,
and error control. Some protocols are state and connection oriented. This means that the
Transport Layer can keep track of the segments and retransmit those that fail. The Transport
layer also provides the acknowledgement of the successful data transmission and sends
the next data if no errors occurred. Some Transport Layer protocols, for example TCP,
but not UDP, support virtual circuits provide connection oriented communication over an
underlying packet oriented datagram network .Where it assures the delivery of packets
in the order in which they were sent and assure that they are free of errors .The datagram
transportation deliver the packets randomly and broadcast it to multiple nodes. Notes: The
transport layer multiplexes several streams on to 1 physical channel. The transport headers
tells which message belongs to which connnection.

z z The Session Layer: This Layer provides a user interface to the network where the user
negotiates to establish a connection. The user must provide the remote address to be
contacted. The operation of setting up a session between two processes is called “Binding”.
In some protocols it is merged with the transport layer. Its main work is to transfer
data from the other application to this application so this application is mainly used for
transferred layer.
z z Presentation Layer: The Presentation Layer establishes context between Application
Layer entities, in which the higher-layer entities may use different syntax and semantics
if the presentation service provides a mapping between them. If a mapping is available,
presentation service data units are encapsulated into session protocol data units, and
passed down the stack.This layer provides independence from data representation (e.g.,
encryption) by translating between application and network formats. The presentation
layer transforms data into the form that the application accepts. This layer formats and
encrypts data to be sent across a network. It is sometimes called the syntax layer. The
original presentation structure used the basic encoding rules of Abstract Syntax Notation

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