Media Access Protocols
Organizational Communications and
Technologies
Prithvi N. Rao
H. John Heinz III School of Public
Policy and Management
Carnegie Mellon University
Readings
High Speed and Wireless LANs(Stallings
and van Slyke) Chapters 4, 7, 9
Objectives
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Be familiar with the IEEE LAN standards
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Recognize various methods of media access
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Discuss token passing media access method
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Describe broadcast media access

Recognize how information moves between protocol
layers
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

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Data link to network and transport layers
IEEE developed protocols for LAN

Equipment and software
LAN Communication Protocols

Committee formed in 1980

IEEE 802 LAN specificaiton
LAN Communication Protocols
802.1
Defines reference model, internetworking, etc.
802.2
Defines Link Control Information
802.3
Defines CSMA/CD
802.4
Defines Token Passing Bus access method
802.5
Defines Token Passing Ring access method
802.6
Defines Metropolitan Area Networks
802.7
Broadband Technology Advisory Group
802.8
Optical Fiber Technology Group
Media Access Methods

Three methods of media access are:
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Token passing or the ticket system
Broadcast
Polling
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Each is intended to gain access to the media

Fundamentally there is a message exchange
Media Access Methods: Token
Passing

Deterministic access method

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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
Media Access Methods: Token
Passing

Destination node does not remove the frame

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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

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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
Media Access Protocols: Token
Passing Active Monitor
Standby monitor
token
Active
Monitor
node1
node4
Token Ring
node2
Standby monitor
node3
Standby
monitor
Media Access Methods: Token
Passing

All nodes are one of two types


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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

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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
Ring Token Format

Two formats

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Ring token or empty frame controls access to ring

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Ring token
Ring or data frame
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
Ring Token Format
SD
One Byte
AC
One Byte
ED
One Byte
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

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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
Ring Data Frame Format
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Address fields contain six byte source

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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
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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
Ring Data Frame

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Frame Check Sequence (FCS) field holds a CRC
validation number
Frame Status (FS) field contains address recognized
Ring Data Frame
sd
ac
1B
1B
fc
1B
da
2–6B
sa
2–6B
info
fcs
Variable
4B
ed
1B
fs
1B
Token Ring Equipment
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NICs and UDC connector from IBM
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Multi Access Units
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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
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Maintain ring logically with MAU
Communicate with nodes arranged in a star
Media Access Protocols:
Broadcast
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CSMA/CD
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Carrier Sense Multiple Access with Collision Detection
Non-deterministic
Originally designed with satellite for the media
Originated from the University of Hawaii
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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
CSMA/CD
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Uses bus topology to build multiple access channel
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Transfers at rate of 10 Mbits per second
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Uses thicknet, thinnet or unshielded twisted pair wire
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IEEE 802.3
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Ethernet –II specification
CSMA/CD Implementation

Ethernet
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DEC
Intel
Xerox
Assigned number in the framing field excluded other
vendors not part of consortium from implementing
Ethernet with native upper level protocols
Ethernet Frame Format
preamble
7B
Start
delimiter
1B
Dest
address
6B
Source
address
6B
Type
Or
length
Data
field
pad
pcs
4B
ed
IEEE 802.3 Specification

IEEE developed specification
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Permitted vendors to use CSMA/CD with variety of upper
level protocols
Result was 802.3 specification
IEEE 802.3 specification

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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
Ethernet Frame Format
preamble
7B
Start
delimiter
1B
Dest
address
6B
Source
address
Type
Or
length
Data
field
pad
6B
pcs
4B
100 B
LLC Data
0
100
1500
ed
Ethernet Frame Format

Difference between Ethernet and 802.3 specification
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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
LLC Format
1 byte
DSAP
1 byte
SSAP
1 or 2 bytes
Control
Field
Variable length
LLC Data
DSAP – Destination Service Access Point
SSAP – Source Service Access Point
Control Field – Used for connection oriented service to accomplish error detection and
Flow control
LLC Data – Upper layer headers and application data
Media Access Polling
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Token passing and broadcast media access methods
rely on network to permit access to media
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Rules are built into the protocol
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Token passing is more equitable
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Each node has equal chance to use media
Rely on bursty nature of network usage
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Permits access to media whenever node wishes to transmit
Media Access Polling
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Polling is combination of design of two methods
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Polling protocol defines central intelligent device
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Pre-determined order
Node requesting network service accomplished when node is
queried or polled
Node begins to use network
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Node completes transmission
Central device can interrupt node in favor of higher priority
node
Media Access Polling
workstation
workstation
workstation
Central Unit Server
Reservation or Round-robin Polling
workstation
Media Access Polling
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Polling not as popular as token passing or broadcast
methods
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AKA round robin used in mainframe or mini-computer mulltiuser networks
Polling could come back into vogue
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High-speed multi-channel media getting popular
Data Link Layer Sub Layers
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Subdivided into two parts
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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
Data Link Layer Sub Layers

LLC sub-layer indicates service access point (sac)
used to move transmission unit up to next level

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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
Data Link Layer Sub Layers
Network
Network
Logical Link Control
Logical Link Control
Media Access Control
Media Access Control
Physical
Physical
Media
Logical Link Control (LLC)
Service Access Points
Upper Layers
sap
sap
Transport Protocols
sap
sap
Network Protocols
sap
sap
Logical Link Control
Media Access Control
Physical
Service Access Points
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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
LLC

LLC provides header

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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

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Well Known Ports
Sockets
Named Pipes
Summary

Three prevalent forms of media access

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

Token passing
Broadcast
Polling
Token passing defined in IEEE 802.3

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Several implementations of non-deterministic Ethernet
protocol
Ethernet LANs provide access to the LAN on a first-comefirst serve or most persistent basis
Summary
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Polling is controlled by central monitor

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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