LAN Technologies: Token Rings and Wireless LANs, Lecture notes of Local Area Network (LAN)

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Lectures 14: LAN Wireless LANs technologies: Token Rings,

Eytan Modiano

• Token rings were developed by IBM in early 1980 – IEEE Standard 802.5^ Token rings ’ s
• Token: a bit sequence – Token circulates around the ring Busy token: 01111111 Free token: 01111110
• When a node wants to transmit – – – – Wait for free tokenRemove token from ring (replace with busy token)Transmit messageWhen done transmitting, replace free token on ring
• Token ring is basically a polling system^ –^ Nodes must buffer 1 bit of data so that a free token can be changed to a^ busy token Token does the polling

Token Ring

(release after transmission)^ PACKET TRANSMISSION

• • When not transmitting their own packets nodes relay whatever they receiveAfter receiving an idle token a node can start sending a new packet (discard incoming bits)
• After a node sends a packet and the idle token, it sends idle fill until: – – The packet followed by idle, orbusy token, returns around the ring

BT^ BT Packet^ Transmitted bitsPacket ITBT New packetIdle fill Packet returnIT^ Idle fill BTBTPacketPacket Received bits

<-one time unit-> BT

PACKET TRANSMISSION (release after reception)

• In many implementations (including IEEE802.5, but not including FDDI), a node waits to check its packet return before sending the idle token. This increases packet transmission time by one round trip delay.

BT^ BT PacketPacket Idle fillIdle fill^ ITBT New packet (^) Idle fill BTPacket returnIdle fill^ IT Idle fill^ Idle fillBT^ BT

• Gated system with limited service - each node is limited to^ Throughput analysis (non-exhaustive)
• sending one packet at a timeSuppose each node transmits one packet and then releases the^ –^ When system is heavily loaded nodes are always busy and have a^ packet to send
• token to the next nodeThe amount of time to transmit N packets^ –^ Vi^ = propagation and transmission time for token between two nodes^ (transmission time is usually negligible) T λN = NE[X] + V< NE[X]/(NE[X] + NE[V]) = 1/(1+E[V]/E[X]) 1 + V 2 +…+ VN = NE[X] + NE[V]
• Compare to CSMA/CD, but notice that V is the delay between two nodes and not the maximum delay on the fiber

(token release after reception)^ Throughput analysis

• • • Nodes release token only after it has returned to itAgain assume each node sends one packet at a timeTotal time to send ONE packet
• T = E[X] + V 1 + V 2 +…+ Vm + Vi
• T = E[X] + (m+1)E[V] => λ < E[X]/T = 1/(1+(m+1)E[V]/E[X]) M nodes on the ring^ Time to send token to next node

Token ring issues

• • Fairness: Can a node hold the token for a long timeToken failures: Tokens can be created or destroyed by noise – Solution: maximum token hold time
  • Distributed solution: Nodes are allowed to recognize the loss of a token and create a new token Collision occurs when two or more nodes create a new token at the same time => need collision resolution algorithms
• • Node failures: Since each node must relay all incoming data, the failure of a single node will disrupt the operation of the ringToken ring standard: IEEE 802.

• Fiber distributed data interface (FDDI) is a 100 Mbps Fiber Optic Token Ring network standard^ Token Ring Example: FDDI
• FDDI uses two counter-rotating rings – – Single faults can be isolated by switching from one ring to the other on each side of fault (loop back)Only one ring used under normal operation (one direction)
• • Token release after transmissionLimit on token hold time
• Upper-bound on time between token visits at a node – – Support for guaranteed delaysImposes a limit on the size of a ring (distance between nodes, number of nodes)
• FDDI designed to be a metro or campus area network technology

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Eytan Modiano Slide 13

Wireless Networking Technologies

SHORT <^ RANGE^ ZigBee

LONG LOW < DATA RATE > HIGH^ Wireless Personal^ Area Network

Wireless Local Area Network Bluetooth

802.11b^ 802.16 ( 802.11a/g/nwimax)^ Wireless Metropolitan^ Area Network WiMedia

• Standards typically define the Medium Access Control (MAC) and the Physical layers

Most slides on wireless MAC borrowed with permission from Prof. Gil^ applicationLink length Peripheral^ devices^ 10 meters Zussman of Columbia UniversityLAN^100 meters^ Access^10 km

Data rate 2.1 Mbps^ Bluetooth 54 Mbps^ WiFi^ (802.11) 70 Mbps^ WiMax^ (802.16)

• • Nodes are scattered in a geographic areaNeed to somehow coordinate the access to channel^ Medium Access Control (MAC)
• Centralized^ –^ Transmission time, power, rate, etc.
• Distributed^ – –^ Managed by an Access Point/Base StationRandom access (Aloha, CSMA, Ethernet)
• Requirements^ –^ Scheduled access
  • Throughput, delay, fairness, energy efficiency

(^23) 5 6

4 (^7 )

1

• Ad Hoc mode^ Ad Hoc and Infrastructure Modes

• – The stations communicate with one anotherNot connected to a larger network

• Infrastructure mode – – An Access Point connects Stations to a wired networkOverlapping Access Points

• connected to each otherAllows Stations to roam between Access Points

Carrier Sense Multiple Access \ Collision Avoidance^ Medium Access Control - CSMA\CA

• • • Station wishing to transmit a Data packet senses the mediumIf it is idle for a given period - TransmitsACK packet is sent by the receiving station
• Collision assumed if sending station doesn – Data is retransmitted after a random time ’ t get ACK

CSMA

Station A Station B Data^ Ack A^ B

• Hidden Node interfere with its transmissions – Solution: Busy tone - A node that a station does not hear but can multiple access (Tobagi, 1975)

• Enhancement: – – – ABA  B Request to Send (RTS)A Clear to Send (CTS)B Data

• Neighboring nodes will keep quiet for the duration of the transfer^ – –^ BNetwork allocation vector (NAV) - specifies duration of transfer^ ^ A ACK

Hidden Node Problem

CA^ A^ B C

Station A Station B Station C RTS CTS DataDon’t Transmit! Ack

• A station that sensed the medium busy or did not receive an ACK will try to retransmit

• • The back-off interval is uniformly distributed within the CWThe window is doubled every time there is a need to retransmit – – Upper limit on CWCount down back-off interval when channel idle

• – Stop counting when busy (resume when idle again)Transmit when back-off interval reaches 0

Contention Window

Station A Station B Station C Don^ Data’t Transmit! Ack Contention Window (CW)

B=10 Channel (^9) idle (^8 7 6 5) Channel busy (^4) Channel (^) idle (^3 2 1 0) Data ACK