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14
Communication in Networks
Node Problem. In contrast, a station which senses only RTS can start a transmission. As long
as it does not collide with CTS, it can finish its transmission successfully and this property
solves Exposed Node Problem. In spite of these improvements, a collision can still occur when
two stations send RTS at the same time to a mutual receiver. CSMA/CA is used in WLAN
802.11. Unlike the CSMA/CD, CSMA/CA is also efficient under heavy traffic. There are
some other CSMA methods for wireless LANs which can slightly change from CSMA/CA to
improve the throughput.
Logical Link Control Layer
The LLC sublayer acts as an interface between the MAC sublayer and the Network Layer.
This sublayer multiplexes the MAC protocols while transmitting and decoding while receiving.
It encapsulates the Network Layer data packets into the frames. It also provides optional
flow and error control in addition to the Transport Layer. In wireless networks this control is
used with retransmission of the erroneous packets, however in LAN only error detection and
canceling of the packet are provided. In most of the network protocols IEEE 802.2 standard
is used as the LLC protocol. Now, we will explain this standard in detail.
IEEE 802.2
This standard provides a common interface for upper layers to deal with any kind of MAC
protocols. It provides three types of operation for data communication.
• Type 1 is a connectionless service in which data is simply transmitted regardless of
whether destination station accepted the packet. It allows multicast transmission.
• Type 2 is connection-oriented service. First a logical connection is established between
two stations. The connection is maintained and terminated by using special packets
that can be understood by stations. Sequence numbers are added to the packets for
ordered transmission. Error control and flow control are provided but this service does
not allow broadcast or multicast addresses.
• Type 3 is acknowledged connectionless service for point-to-point communication only.
The LLC header consists of 4 fields:
• 8-bit DSAP (Destination Service Access Point) is the SAP(Service Access Point) where
the packet is intended.
• 8-bit SSAP (Source Service Access Point) is the SAP(Service Access Point) where the
packet is originated.
• 8-bit control field for use in auxiliary functions such as flow control.
• Information field that carries LCC information.
IEEE 802.2 is independent of the particular medium and commonly used.
Ali Osman ISIK
Master of Science Thesis