LAN StandardsCE 536 - Networking and Microcomputer Systems
Day 5: Network Standards and Access Methods
LAN Standards
Standards Organizations
LAN Access Control Methods
The Ethernet (IEEE 802.3) Access Method
The Token Ring (IEEE 802.5) Access Method
Fiber-Distributed Data Interface
LAN Standards
Assure Compatibility
Multivendor Interoperability
Break down proprietary barriers
Reduce Costs
Commodity pricing
Standards Organizations
ISO - International Standards Organization - world body for standards - released the OSI (Open systems Interconnection) model in 1983
IEC - International Electrotechnical Commission - part of ISO responsible for telecommunications (voice and data)
ANSI - American National Standards Institute - U.S. accredited body to the IEC
IEEE - Institute of Electrical and Electronic Engineers - an ANSI accredited standards development organization
ITU-TSS - International Telecommunications Union Telecommunications Standardization Sector - formerly CCITT (Consultative Committee in International Telegraphy and Telephony) - devises standards for inter-country communications - (e.g., V.34 for modems)
LAN Access Control Methods
Orderly way of gaining access to the medium
Data Link Layer of Open Systems Interconnection Model
Random Control - any station can transmit without specific permission of a controlling entity (e.g. Ethernet, AppleTalk)
Distributed Control - control is embedded in the medium as a token passed from station to station; if the token is free, a station can place data on the line (e.g., Token Ring)
Probabilistic Access - there is always a probability that a given station can transmit data, but no guarantee (Ethernet)
Deterministic Access - each station is assured eventual access to the medium
Ethernet (IEEE 802.3)
Token Ring (IEEE 802.5)
The Ethernet (IEEE 802.3) Access Method - (10Base2 and 10BaseT exist only within the IEEE standard)
CSMA/CD - Carrier-Sense Multiple Access/Collision Detection
Carrier-Sense - station checks line for presence of signal voltage; if none, waits 9.6 
sec. and then transmits
1 sec. = 1 microsecond = 0.000001 second
1 msec. = 1 millisecond = 0.001 second
Multiple Access - any station can transmit if the line is clear
Transmissions received by all stations
Address filtering used to determine whether to pay attention to the transmission
Collision Detection - collision occurs when two or more stations attempt to transmit simultaneously
All stations ignore the garbled result
Caused by propagation delay in cable
slot time - maximum time to get from one end of the cable to the other and back again - 512 bits or 51.2 sec. (0.0000512 seconds)
Transmitters resort to a backoff algorithm
Each generates a random number to start a timer
Maximum backoff delay: 52.4 msec. (0.0524 sec)
Local Collisions - cable appears clear, but a packet is coming from the other end
caused by excessive cable lengths
Late Collisions - error in transmission occurring after first 60 bytes have been transferred
defective cabling
topology-rule violation
electromagnetic interference
Up to 15% collisions is considered normal
Frames - packets of information
Length - station must be transmitting long enough to detect collisions - 1 slot time (51.2 sec or 512 bits = 64 bytes)
Runts - frames that are too short - bad network card
Jabbers - frames that are too long (more than 1518 bytes) - bad network card
IEEE 802.3 Frame
Preamble (7 bytes) - 0101...
SFD - Start Frame Delimiter - 10101011
Destination address (6 bytes) - e.g., 00:00:C0:9B:C5:4C
Source address (6 bytes)
Length (2 bytes) - length (in bytes) of data field
Data (46-1500 bytes)
Logical Link Control - data field can contain higher layer protocols such as IPX (Internetwork Packet Exchange) for Novell NetWare or TCP/IP
FCS - Frame Check Sequence (4 bytes) - error checking
Advantages of Ethernet (IEEE 802.3)
Simple
Readily Expandable
Easy to Troubleshoot (10BaseT only)
Cable faults are responsible for 80% of LAN problems
Low Cost
High Speed
Compatibility
Disadvantages of Ethernet (IEEE 802.3)
No inherent redundancy
Performance deteriorates rapidly under heavy load
Break into smaller pieces via a Bridge
Probabilistic access means no guarantee of access to the medium
The Token Ring (IEEE 802.5) Access Method
Token Ring Architecture
Physically wired as a star with hubs
simplified installation
simplified troubleshooting (with the right software!)
simplified maintenance
Logically a ring
Redundancy built in with an alternate path between hubs
Token Passing (IEEE 802.5)
Any station wishing to transmit data watches for a message on the LAN called a token
If the token is free, the station takes it and begins to send data
A token containing data is called a frame
The frame passes through each station until it gets to its destination
The destination station reads the data, sets bits within the frame indicating reception, and continues passing the frame
When the frame gets back to its originator, the station places the (empty) token back into the ring
Called deterministic access as every station is guaranteed the chance to transmit within a given time
Monitors - built into every token ring network interface card
Active Monitor - the first station powered up on the ring
Issues tokens - one token per ring
Looks for next token to pass through it within 10 msec. (0.01 second) of the last one
Watches for error conditions (e.g., a frame going twice)
Purges ring and restarts if token is missing or in error
Provides master timing to synchronize other stations
Sends active monitor present frame every 7 seconds
All other stations respond by sending a standby monitor present frame
This ring poll lets each station identify its next active upstream neighbor
Standby Monitor - all other stations on the ring
Steps in if active monitor stops working
After no token for 2.6 seconds and no active monitor present frame for 15 seconds
Usually the first downstream neighbor of the old active monitor becomes the new active monitor
Token Passing Frames
Similar in function to Ethernet frames, but with more fields
Frame Status field indicates if data was copied or not
Logical Link Control (i.e., IPX, TCP/IP) can be imbedded in the data field
FDDI (Fiber-Distributed Data Interface)
Fiber-optic LAN
Multimode Fiber - allows light to take multiple paths down the fiber - good up to 2km between stations
Singlemode Fiber - allows only one path for light to travel - good up to 60km between stations
Ring Topology
Up to 100km circumference
Operating speed of 100 Mbit/sec
Supports IEEE 802.2 Logical Link Control (as do IEEE 802.3 and IEEE 802.5)
Anything running above this level does not need to worry about the underlying media
Primary uses:
Backbone network interconnecting LANs
Data center network connecting high-speed devices
LAN connecting high-end workstations
Assignment for Class 7 ...
Relationship to the OSI Model (pp. 5-12)
Chapter 9 - skim
Chapter 10