media access protocols organizational communications and technologies prithvi n. rao h. john heinz...
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Media Access Protocols
Organizational Communications and Technologies
Prithvi N. RaoH. John Heinz III School of Public
Policy and ManagementCarnegie Mellon University
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Readings
High Speed and Wireless LANs(Stallings and van Slyke)
Chapters 4, 7, 9
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Objectives Be familiar with the IEEE LAN standards
Recognize various methods of media access
Discuss token passing media access method
Describe broadcast media access
Recognize how information moves between protocol layers
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LAN Communication Protocols Required to define how workstations on LAN
interact
Protocols specify rules for successful data movement
Communication protocols define media access methods, addressing and routing information
Data link to network and transport layers
IEEE developed protocols for LAN Equipment and software
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LAN Communication Protocols Committee formed in 1980
IEEE 802 LAN specificaiton
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LAN Communication Protocols
802.1
802.2
802.3
802.4
802.5
802.6
802.7
802.8
Defines reference model, internetworking, etc.
Defines Link Control Information
Defines CSMA/CD
Defines Token Passing Bus access method
Defines Token Passing Ring access method
Defines Metropolitan Area Networks
Broadband Technology Advisory Group
Optical Fiber Technology Group
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Media Access Methods Three methods of media access are:
Token passing or the ticket system Broadcast Polling
Each is intended to gain access to the media
Fundamentally there is a message exchange
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Media Access Methods: Token Passing Deterministic access method
Node may only transmit when it has token Token is generated and travels throughout network Each node gets chance to transmit Node waiting to transmit grabs token and transmits Token is passed to next node if nothing to transmit
Node seizing token changes token bit Expands token to add frame to the end Token is transmitted over network to hardware of
destination node
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Media Access Methods: Token Passing Destination node does not remove the frame
Copies the token Token circles (with data) till it reaches source Source reconstructs token with new data frame Token is released with available bit set to available
status
Token Ring Summary Specification developed and supported by IBM Data rate is 4 or 16 Mbits per second Uses shielded or unshielded twisted pair Built in by-pass capability and a max of 260 stations on
ring
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Media Access Protocols: Token Passing Active Monitor
node1node3
node2
node4
Token Ring
token
ActiveMonitor
Standby monitor
Standbymonitor
Standby monitor
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Media Access Methods: Token Passing
All nodes are one of two types Active monitor – responsible for the high integrity of
the token Standby monitors – waiting to become an active
monitor
Active monitor is in charge of token Releases token and manages token passing protocol Active monitor is first station on the ring Standby monitors bid to become active monitors by
sending a claim token command Node with highest hardware address becomes new
active monitor
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Ring Token Format Two formats
Ring token Ring or data frame
Ring token or empty frame controls access to ring
Three byte packet Start delimiter (sd), access control (ac), ending
delimiter (ed)
Senders expand ring token by setting token bit in control byte and adding fields onto frame
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Ring Token Format
SD AC ED
One Byte One Byte One Byte
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Ring Data Frame Format Expanded version of Ring Token with token bit
set in access control field and data appended onto end of ring token
AC field is modified with token bit set to 1 indicating that it is a frame
SD, AC and ED are all present
Frame control (FC) field determines type of Frame Media Control (MC) ring data or Logical Link Control (LLC) user data
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Ring Data Frame Format Address fields contain six byte source
Contains destination hardware or MAC addresses of transmitting and receiving nodes
Route information field contains ring or bridge number
Data field length determined by token holding timer
Longer timer means more data can be carried in data field before it expires
4 k for 4 mega-bit-per-second (MBPS) 17.8 k for a 16MBPS
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Ring Data Frame Frame Check Sequence (FCS) field holds a CRC
validation number
Frame Status (FS) field contains address recognized
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Ring Data Frame
sd ac fc da sa info fcs ed fs
1B 1B 1B 2 – 6 B 2 – 6 B Variable 4 B 1B 1B
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Token Ring Equipment NICs and UDC connector from IBM
Multi Access Units Logical ring using a physical star topology Active MAU has own power supply Passive MAU draws power from ring
MAU enables construction of hub and spoke cable plant
Maintain ring logically with MAU Communicate with nodes arranged in a star
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Media Access Protocols: Broadcast CSMA/CD
Carrier Sense Multiple Access with Collision Detection Non-deterministic Originally designed with satellite for the media
Originated from the University of Hawaii ALOHA net Used as an old naval intelligence satellite for UH
campuses Listen to satellite channel for carrier Specification enhanced to add collision detection Workstations wait for arbitrary time elapse before
transmit
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CSMA/CD Uses bus topology to build multiple access channel
Transfers at rate of 10 Mbits per second
Uses thicknet, thinnet or unshielded twisted pair wire
IEEE 802.3
Ethernet –II specification
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CSMA/CD Implementation Ethernet
DEC Intel Xerox
Assigned number in the framing field excluded other vendors not part of consortium from implementing Ethernet with native upper level protocols
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Ethernet Frame Format
preamble
Startdelimiter
Destaddress
Sourceaddress
TypeOr length
Datafield pad pcs ed
7B 1B 6B 6B 4B
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IEEE 802.3 Specification IEEE developed specification
Permitted vendors to use CSMA/CD with variety of upper level protocols
Result was 802.3 specification
IEEE 802.3 specification 10Base5, 10 MBPS thicknet max length 500 m per
segment 10Base-2, 10 MBPS thinnet max length 185 meters 10Base-T, UTP specification, 10 MBPS
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Ethernet Frame FormatEthernet Frame Format
preamble
Startdelimiter
Destaddress
Sourceaddress
TypeOr length
Datafield pad pcs ed
7B 1B 6B 6B 4B
100 B
0 100 1500
LLC Data
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Ethernet Frame Format Difference between Ethernet and 802.3
specification Lack of type field in 802.3 and replacement of length
field Field specifies the length in data field containing the
802.2 Logical Link Control (LLC) header LLC header specifies upper level protocol or type
that created frame
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LLC Format
DSAP SSAP ControlField
LLC Data
DSAP – Destination Service Access Point
SSAP – Source Service Access Point
Control Field – Used for connection oriented service to accomplish error detection andFlow control
LLC Data – Upper layer headers and application data
1 byte 1 byte 1 or 2 bytes Variable length
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Media Access Polling Token passing and broadcast media access
methods rely on network to permit access to media
Rules are built into the protocol
Token passing is more equitable Each node has equal chance to use media
Rely on bursty nature of network usage Permits access to media whenever node wishes to
transmit
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Media Access Polling Polling is combination of design of two methods
Polling protocol defines central intelligent device Pre-determined order Node requesting network service accomplished when
node is queried or polled
Node begins to use network Node completes transmission Central device can interrupt node in favor of higher
priority node
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Media Access Polling
workstation workstation workstation workstation
Central Unit Server
Reservation or Round-robin Polling
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Media Access Polling Polling not as popular as token passing or
broadcast methods AKA round robin used in mainframe or mini-computer
mullti-user networks
Polling could come back into vogue High-speed multi-channel media getting popular
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Data Link Layer Sub Layers Subdivided into two parts
Media Access Control (MAC) Logical Link Control (LLC)
Important to understand LLC before moving up the OSI model
Key is that each layer must know what to do with data it is handed
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Data Link Layer Sub Layers LLC sub-layer indicates service access point
(sac) used to move transmission unit up to next level
SAC is address of next protocol to which transmission should be sent
SAC are interface between layers N + 1 and N – 1 for layer N
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Data Link Layer Sub Layers
Media
Logical Link Control (LLC)
Network
Logical Link Control
Media Access Control
Physical
Network
Logical Link Control
Media Access Control
Physical
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Service Access Points
sap sap
sap sap
sap sap
Physical
Media Access Control
Upper Layers
Transport Protocols
Network Protocols
Logical Link Control
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Service Access Points Modular design of OSI model enables variety of
protocols to be available at each level
Given layer knows about layer to send transmission using the source and destination SAP header
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LLC LLC provides header
Indicates network layer protocol to which data should be sent (3)
SAP is identified to move to transport layer (4) Continues till destination is reached
SAPs are called by Well Known Ports Sockets Named Pipes
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Summary Three prevalent forms of media access
Token passing Broadcast Polling
Token passing defined in IEEE 802.3 Several implementations of non-deterministic
Ethernet protocol Ethernet LANs provide access to the LAN on a first-
come-first serve or most persistent basis
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Summary Polling is controlled by central monitor
Future LANs may benefit from this
Information moves between layers of communication architecture through addressing
Service access points are specific address locations that enable information to move through a layered architecture