Media Access Protocols

1
Media Access Protocols

• Organizational Communications and Technologies
• Prithvi N. Rao
• H. John Heinz III School of Public Policy and
  Management
• Carnegie Mellon University

2
Readings

• High Speed and Wireless LANs(Stallings and van
  Slyke) Chapters 4, 7, 9

3
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

4
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

5
LAN Communication Protocols

• Committee formed in 1980
• IEEE 802 LAN specificaiton

6
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
Broadband Technology Advisory Group
802.7
802.8
Optical Fiber Technology Group
7
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

8
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

9
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

10
Media Access Protocols Token Passing Active
Monitor
Standby monitor
node4
token
node1
Active Monitor
Standby monitor
Token Ring
node3
node2
Standby monitor
11
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

12
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

13
Ring Token Format
SD
AC
ED
One Byte
One Byte
One Byte
14
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

15
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

16
Ring Data Frame

• Frame Check Sequence (FCS) field holds a CRC
  validation number
• Frame Status (FS) field contains address
  recognized

17
Ring Data Frame
da
sa
info
fcs
ed
fs
sd
ac
fc
1B
4 B
1B
1B
1B
1B
2 6 B
2 6 B
Variable
18
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

19
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

20
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

21
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

22
Ethernet Frame Format
Start delimiter
Source address
Data field
pad
pcs
ed
preamble
Dest address
Type Or length
7B
1B
6B
6B
4B
23
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

24
Ethernet Frame Format
Ethernet Frame Format
Start delimiter
Source address
Data field
pad
pcs
ed
preamble
Dest address
Type Or length
7B
1B
6B
6B
4B
100 B
LLC Data
0
100
1500
25
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

26
LLC Format
Variable length
1 byte
1 byte
1 or 2 bytes
Control Field
LLC Data
DSAP
SSAP
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
27
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

28
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

29
Media Access Polling
workstation
workstation
workstation
workstation
Central Unit Server
Reservation or Round-robin Polling
30
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

31
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

32
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

33
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)
34
Service Access Points
Upper Layers
sap
sap
Transport Protocols
sap
sap
Network Protocols
sap
sap
Logical Link Control
Media Access Control
Physical
35
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

36
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

37
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

38
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