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Layer 2 Technology
Andres, Wen-Yuan Liao
Department of Computer Science and Engineering
De Lin Institute of Technology
http://www.cse.dlit.edu.tw/~andres
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Basics of Token Ring
Overview of Token Ring and its variants
Token Ring frame format
Token Ring MAC
Token Ring signaling
Token Ring media and physical topologies
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TR frame format
TokensTokens are 3 bytes in length a start delimiter, an access
control byte, and an end delimiter
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TR frame format
Access Control ByteThe priority and reservation field,
and a token and monitor bitThe token bit : data/command
frameA monitor bit determines whether
a frame is continuously circling the ring
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TR frame format
Data/Command Frames Data frames: data information Command frames: control
informationFrame control byte indicates
Data or control informationThe type of control information
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Token Ring MAC
Move a token around the network
Possession of the token grants the right to transmit data
Hold the token for a maximum period of time
Busy token/Free token
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Token Ring MAC
No collisions
DeterministicCalculate the maximum time that
will pass before any end station will be able to transmit
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TR MAC - Priority System
The priority field and the reservation field.
Only stations with higher or equal priority can seize that token
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TR MAC - Priority System
Only stations with higher priority can reserve the token for the next network pass
Stations that raise a token's priority level must reinstate the previous priority
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TR MAC - ManagementDetect and compensate for network faults
Active monitorRemove continuously circulating frames from th
e ring
Active MSAUs (multi-station access units) Check for problems, and to selectively remove
stations
Beaconing Detects and tries to repair network faults
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When a station detects a serious problem with the network (e.g. a cable break) it sends a beacon frame
The beacon frame defines a failure domain Include the station that is reporting the failure, it
s nearest active upstream neighbor (NAUN), and everything in between
Beaconing initiates a process called autoreconfigurationNodes within the failure domain automatically p
erform diagnostics
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Token Ring signaling
Manchester encoding 0 : a high-to-low transition1: a low-to-high transition
Differential 1: no polarity change at the start
of the bit time 0: a polarity change at the start of
the bit time
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TR media and physical topologies
Stations are directly connected to MSAUs, and can be wired together to form one large ringPatch cables connect MSAUs to other MSAUs that are adjacent to itLobe cables connect MSAUs to stationsMSAUs include bypass relays for removing stations from the ring
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Basics of Fiber Distributed Data Interface (FDDI)
Overview of FDDI and its variants FDDI format FDDI MAC FDDI signaling FDDI media
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Overview of FDDIANSI X3T9.5 standards committee Backbone technology Media Access Control (MAC)defines how the medium is accessed
Physical Layer Protocol (PHY) defines data encoding/decoding procedures
Physical Layer Medium (PMD) defines the characteristics of the transmission
medium
Station Management (SMT) defines the FDDI station configuration
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FDDI format
Frame status – Determine if an error occurred
and if the frame was recognized and copied by a receiving station
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FDDI MAC
Move a token around the network
Possession of the token grants the right
A maximum period of token holding time
No collisions
A new token can be released when the frame transmission has finished
Guaranteed their turn to transmit
Reliable
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FDDI MAC - Synchronous
Synchronous bandwidth is allocated to those stations requiring continuous transmission capabilityVoice and video informationThe remaining bandwidth is used for
asynchronous transmissions
The FDDI SMT specification defines a distributed bidding scheme to allocate FDDI bandwidth
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FDDI MAC - Asynchronous
Be allocated using an 8-level priority schemeEach station is assigned an
asynchronous priority level
FDDI also permits extended dialoguesStations may temporarily use all
asynchronous bandwidth
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FDDI MAC - Asynchronous
The FDDI priority mechanism can lock out stations that cannot use synchronous bandwidth, and that have too low an asynchronous priority
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FDDI media
100 Mbps, token-passing, dual-ring LAN that uses a fiber-optic transmission mediumAdvantages security reliability speed
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Two specified types of fiber: single-mode (also mono-mode); and multi-mode
Modes can be thought of as bundles of light rays entering the fiber at a particular angle
FDDI media
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Single-mode fiber
One mode of light to propagate through the fiber
Higher bandwidth
Greater cable run distances
Inter-building connectivity
Laser
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Multi-mode fiberMultiple modes of light to propagate Multiple modes of light propagating through fiber may travel different distances, depending on their entry anglesIntra-building connectivityUses LEDs as the light-generating devices
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Dual Rings FDDI
Traffic on each ring travels in opposite directionsThe rings consist of two or more point-to-point connections between adjacent stationsThe primary ring: data transmissionThe secondary ring: back up
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DAS/SAS
Class A, or dual attachment stations (DAS)
Class B, or single-attachment stations (SAS), attach to one ring
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Ethernet and IEEE 802.3
Comparing Ethernet and IEEE 802.3
Ethernet family tree
Ethernet frame format
Ethernet MAC
Ethernet signaling
Ethernet 10BASE-T media and topologies
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Comparing Ethernet and IEEE 802.3
Most widely used local area network Carry sporadic, occasionally heavy traffic at high peak data rates Listen before talk/Listen while talkCollision/Back off algorithmBroadcast networks
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100BASE-TX/10BASE-T
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18 varieties of Ethernet
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Ethernet MAC
Listen-before-transmit mode
Listen-while-transmit mode
Ethernet is a connectionless network architecture and is referred to as a best-effort delivery system
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CollisionThe amplitude of the signal on the networking media will increase
When a collision occurs, each transmitting device will continue to transmit data for a short time
Backed off for a random period of time
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Ethernet signaling
Manchester encoding
10BASE-T 4 wires 1 pair of wires: transmitting
data1 pair of wires: receiving data
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Ethernet 10BASE-T media and topologies
An active hub connects the networking media as well as regenerates the signalA passive hub is a device used to connect networking media and does not regenerate a signal
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Start Topology
Advantages The easiest to design and installEase of maintenance Easy to modify and troubleshootGreater reliability
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Start Topology
Disadvantages Limit one device per run Single point of failure (hub)
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TIA/EIA-568-A specification
Maximum length of horizontal cabling : 90 mThe maximum length for patch cords at the telecommunications outlet/connector is 3 m the maximum length for patch cords/jumpers at the horizontal cross-connect is 6 m.
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TIA/EIA-568-A specification
If a signal travels beyond the specified maximum distance, there is no guarantee that when it reaches a NIC card, the NIC card will be able to read it.
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Layer 2 Devices
NICs
Bridges
Bridge Layer 2 operations
Switches
Switch Layer 2 operations
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Bridge Layer 2 operations
Upper-layer protocol transparency
Bridges filter network traffic by only looking at the MAC address, not protocols
Reduce large collision domains
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Bridge Layer 2 operations
Bridges work best where traffic is low from one segment of a network to other segments
When no idea, the source sends out a broadcast to all devices on a network.
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Switches
Alleviates congestion in Ethernet LANs by reducing traffic and increasing bandwidth
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Switch Layer 2 operations
LAN switches are considered multi-port bridges with no collision domain, because of microsegmentation
The frame is sent to the port of the receiving station prior to the entire frame entering the switch
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Switch Layer 2 operations
Virtual circuit: it exists only when needed, and is established within the switch.
Because switching is performed in hardware instead of in software, it is significantly faster.
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Effects of Layer 2 Devices on Data Flow
Ethernet LAN segmentation Bridge segmentation of a collision domain Switch segmentation of a collision domain Router segmentation of a collision domain Teaching topology segmentation by bridges, switches, and routers
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Ethernet LAN segmentation
Isolate traffic between segmentsCreate smaller collision domain Bridge/SwitchAct as a firewall for some
potentially damaging network errors
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Switch (to Bridge)
Switches are significantly faster (hardware)A 10 Mbps Ethernet LAN and a 100 Mbps Ethernet LAN can be connected by using a switch Higher port densities Cut-through switching Reduce collisions and increase bandwidth
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Router
Smaller collision domains and smaller broadcast domains
Perform bridging and switching functions
Best path selection
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Router
Connect different networking media, and different LAN technologies Connect LANs running different protocols (IP vs. IPX vs. AppleTalk) and can have serial connections to WANs
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Bridge
Bridges increase the latency (delay) in a network by 10-30%
A store-and-forward device
Compute the cyclic redundancy check (CRC) before forwarding
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Switch segmentation
The available bandwidth can reach close to 100% Ethernet networks perform best when kept under 30-40% of full capacity CSMA/CD
Micro-segments: create collision free domains
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Router segmentation
Operates at the network layer, and bases all of its forwarding decisions on the Layer 3 protocol address
Operate with a higher rate of latency
Determine the best path for forwarding them to their destinations
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Troubleshooting workstations
It is best to start troubleshooting at Layer 1
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Summary
Token ring
FDDI
EthernetBridgeSwitch