LAN Protocol Architecture
LAN Protocol Architecture
• Lower layers of OSI model
• IEEE 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 OSI
IEEE 802 Protocol Layers
• Physical 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
• LLC is concerned with transmission of linklevel PDUs between two stations
• LLC 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 Services
• The operation and format of this standard is based on
HDLC.
• 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 Protocol
• Is 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 Control
• Key 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 Techniques
• Round 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 Format
• MAC layer receives data from LLC layer
• The 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 MAC Frame Format