Media Access Protocols

Organizational Communications and Technologies

Prithvi N. RaoH. John Heinz III School of Public

Policy and ManagementCarnegie Mellon University

Readings

High Speed and Wireless LANs(Stallings and van Slyke)

Chapters 4, 7, 9

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

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

LAN Communication Protocols Committee formed in 1980

IEEE 802 LAN specificaiton

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

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

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

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

Media Access Protocols: Token Passing Active Monitor

node1node3

node2

node4

Token Ring

token

ActiveMonitor

Standby monitor

Standbymonitor

Standby monitor

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

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

Ring Token Format

SD AC ED

One Byte One Byte 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

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 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

Ring Data Frame Frame Check Sequence (FCS) field holds a CRC

validation number

Frame Status (FS) field contains address recognized

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

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

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

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

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

Ethernet Frame Format

preamble

Startdelimiter

Destaddress

Sourceaddress

TypeOr length

Datafield pad pcs ed

7B 1B 6B 6B 4B

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

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

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

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

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

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

Media Access Polling

workstation workstation workstation workstation

Central Unit Server

Reservation or Round-robin Polling

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

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

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

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

Service Access Points

sap sap

sap sap

sap sap

Physical

Media Access Control

Upper Layers

Transport Protocols

Network Protocols

Logical Link Control

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

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

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

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