LAN Standards

Standards Organizations

The Topology (Layout) of the LAN Wiring

• Star
• Ring
• Bus

• The Ethernet (IEEE 802.3) Access Method
• The Token Ring (IEEE 802.5) Access Method

LAN Standards

• Assure compatibility
• Multivendor interoperability
• Break down proprietary barriers
• Reduce costs
• Commodity pricing

• 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 intercountry communications - (e.g., V.34 for modems)

• Star
• Point-to-Point links from a central control unit
• Becoming the predominant form of LAN wiring
• Can be insecure
• Simple to diagnose and resolve problems
• Wiring intensive to install

• Ring
• Stations connected in series
• Unidirectional
• May utilize a backup ring
• Very good reliability
• Relatively easy to manage

• Bus

• Transmissions received by all; only addressed unit accepts
• Can be insecure
• Easy to wire segments
• Difficult to diagnose problems
• Can degenerate over time
• No provision for an alternate path

• 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 (Token Ring)
• Ethernet (IEEE 802.3)
• Token Ring (IEEE 802.5)

• CSMA/CD - Carrier-Sense Multiple Access/Collision Detection

• Carrier-Sense - station checks line for presence of signal voltage; if none, waits 9.6 m sec. and then transmits
• 1 m 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 m 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 m 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
• 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

• 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

• 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

• 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. 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

• 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