LAN Protocol ArchitectureLAN Protocol Architecture

LAN Protocol ArchitectureLAN Protocol ArchitectureLower layers of OSI modelIEEE 802 reference model, is a

standardized protocol architecture for LANs, which describes:◦Physical layer.◦Logical link control (LLC) sub-layer,◦Media access control (MAC) sub-layer.

IEEE 802 v OSIIEEE 802 v OSI

IEEE 802 Protocol LayersIEEE 802 Protocol LayersPhysical layer, includes such functions as:

◦ Encoding and decoding of signals◦ Preamble generation and removal (for synchronization)◦ Bit transmission and reception

Logical Link Control, functions:◦ Assemble data into a frame with address and error-detection

fields◦ Disassemble frame and perform address recognition and error

detection◦ Govern access to the LAN transmission medium◦ Interface to higher levels and performs flow and error control

Logical Link Control Logical Link Control LLC is concerned with transmission of link-

level PDUs between two stationsLLC has two special characteristics:

◦Must support multiaccess, shared medium (no primary node as in multidrop line)

◦Relieved of some link access details by MAC layer

Addressing involves specifying source and destination LLC users◦Referred to as service access points (SAP)◦Service users are typically higher level protocols

LLC ServicesLLC ServicesThe operation and format of this standard is based on

HDLC (High Level Data Link Control).Provide three different services for attached devices:

◦ Unacknowledged connectionless service: Datagram-style service. No flow-control and no error-control mechanisms. Reliability depend to some higher layer of software.

◦ Connection mode service: Similar to that of HDLC. Provides flow- and error-control.

◦ Acknowledged connectionless service Datagrams are to be acknowledged. No prior logical connection is set up.

LLC ProtocolLLC ProtocolIs modeled after HDLC, and has similar functions and

formats.LLC protocol operation:

◦ LLC use asynchronous balanced mode of operation of HDLC to support connection mode LLC service (type 2 operation)

◦ LLC supports an unacknowledged connectionless service using unnumbered information PDUs (type 1 operation)

◦ LLC supports acknowledged connectionless service using unnumbered information PDUs (type 3 operation)

◦ LLC permits multiplexing using LLC service access points (LSAPs)

Media Access ControlMedia Access ControlKey parameters of MAC technique is where and how:Where

◦ Central Adv.: Greater control and simple access logic at station (no

coordination complexities) Disadv.: Single point of failure and potential bottleneck

◦ Distributed: opposite of central.How

◦ Synchronous: specific capacity dedicated to connection◦ Asynchronous: in response to demand; can be subdivided

into three categories (round robin, reservation, contention)

Asynchronous MAC TechniquesAsynchronous MAC TechniquesRound robin:

◦ Each station is given the opportunity to transmit. The right to transmit passes in a logical sequence.

Reservation: a node will reserves future slots from the medium time to transmit, this is good for stream traffic.

Contention◦ Good for bursty traffic◦ All stations contend for time◦ Distributed control◦ Simple to implement◦ Efficient under moderate load◦ Tend to collapse under heavy load

MAC Frame FormatMAC Frame FormatMAC layer receives data from LLC layerThe fields of MAC frame:

◦MAC control: protocol control like priority.◦Destination MAC address◦Source MAC address◦LLC: data from next higher layer.◦CRC: FCS for error detection.

MAC layer detects errors and discards frames

LLC optionally retransmits unsuccessful frames

Generic MACGeneric MAC Frame Format Frame Format

IEEE 802.11 Architecture and IEEE 802.11 Architecture and ServicesServicesIn 1990, the IEEE 802 Committee

formed a new working group, IEEE 802.11, specifically devoted to wireless LANs.

The initial interest was in developing a wireless LAN operating in the ISM (industrial, scientific, and medical) band.

IEEE 802.11 StandardsIEEE 802.11 Standards

IEEE 802.11 ArchitectureIEEE 802.11 Architecture

IEEE 802.11 ArchitectureIEEE 802.11 ArchitectureBSS(Basic service set)DS(Distribution system)AP(Access Point)Integrating the IEEE 802.11

architecture with a traditional wired LAN, a PORTAL is used.

IBSS(Independent BSS)An ESS(Extended service set) consists

of two or more basic service sets interconnected by a DS.

IEEE 802.11 ServicesIEEE 802.11 Services

IEEE 802.11 MEDIUM ACCESS IEEE 802.11 MEDIUM ACCESS CONTROLCONTROLThe IEEE 802.11 MAC layer covers

three functional areas: ◦ Reliable data delivery◦ Medium access control and ◦ Security.

Reliable Data DeliveryReliable Data Delivery IEEE 802.11 includes a frame exchange

protocol. When a station receives a data frame from

another station, it returns an acknowledgment (ACK) frame to the source station.

If the source does not receive an ACK within a short period of time, either because its data frame was damaged or because the returning ACK was damaged, the source retransmits the frame.

Thus basic data transfer in IEEE 802.11 involves an exchange of two frames.

Reliable Data DeliveryReliable Data Delivery

To enhance reliability a four frame exchange may be used, RTS from source to destination, destination sends CTS and after receiving CTS, source sends data frame and destination responds with an ACK.

Medium Access ControlMedium Access ControlThe 802.11 working group considered

two types of proposals for MAC algorithm,

Distributed Access Protocol( like Ethernet)

Centralized Access ProtocolFinally ended up with a MAC algorithm

called DFWMAC(Distributed Foundation Wireless MAC)

Distributed Coordination Distributed Coordination FunctionFunctionUses CSMA algorithm without

CD(CD not possible in wireless).DCF includes a set of delays to

give priorities.Single delay known as IFS(Inter

frame Space).IFS

SIFS: Short IFS(short) PIFS: Point Coordination IFS(medium) DIFS: Distributed Coordination IFS(longer)

Distributed Coordination Distributed Coordination FunctionFunctionSIFS has the highest priority to

determine transmission opportunity.

SIFS is used in ACK: Each frame is acknowledged after

SIFS by the recipient. When source receives ACK it immediately sends next frame in sequence.

CTS: Poll response

Point Coordination Point Coordination FunctionFunction

Point Coordination Point Coordination FunctionFunctionPCF is an alternative access method

implemented on top of the DCF, the operation consists of polling by the centralized polling master (point coordinator).

PC makes use of PIFS when issuing polls(PIFS<DIFS), so PC will seize the medium.

Time sensitive traffic is controlled by Point Coordinator by issuing polls to all stations in Round Robin mode.

Point Coordination Point Coordination FunctionFunctionWhen poll is issued , the polled station may

respond using SIFS, if PC receives a response, it issues another poll using PIFS.

If this continues asynchronous data would be delayed because of continuous polling seizes medium, to prevent this a super frame is used.

During the first part of this interval, the PC issues polls in RR fashion to all stations configured for polling. Then PC idles for the remainder of the super frame , allowing a contention period for asynchronous access.

IEEE 802.11 Physical IEEE 802.11 Physical LayerLayerIssued in four stages.First part, IEEE 802.11includes the

MAC layer and three physical layer specifications, two in the 2.4-GHz band (ISM) and one in the infrared, all operating at 1 and 2 Mbps.

IEEE 802.11a operates in the 5-GHz band at data rates up to 54 Mbps.