CSC4220/6220 Computer Networks Dr. WenZhan Song Associate Professor, Computer Science

Course roadmap Introduction Application Layer: WWW, FTP, email, DNS, multimedia Transport Layer: reliable end-end data transfer principles, UDP, TCP Network Layer: IP addressing, routing and other issues Data Link Layer: framing, error control, flow control Medium Access Control (MAC) Layer: multiple-access, channel allocation Physical Layer: wired, wireless, satellite Other Topics: network security, social issues, hot topics, research directions

Data Link Layer Road Map Data link layer design issues Framing Error Control Reliable data transfer and flow control Example data link protocols HDLC and PPP Multiple Access Protocols Static channel allocation Dynamic channel allocation LAN technologies and their MAC protocols Ethernet WiFi and WiMax

Multiple Access Links and Protocols Two types of “links”: point-to-point PPP for dial-up access point-to-point link between switch and host broadcast (shared wire or medium) traditional Ethernet upstream HFC 802.11 wireless LAN

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination

Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. Simple

MAC Protocols: a taxonomy Static Channel Allocation divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Dynamic Channel Allocation channel not divided, allow collisions “recover” from collisions

Static Channel Allocation: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

Static Channel Allocation: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle time frequency bands

Dynamic Channel Allocation ALOHA, slotted ALOHA CSMA, CSMA/CD, CSMA/CA – in future slides Collision free protocols Bit-map protocol Binary countdown Limited contention protocols Adaptive tree walk Wavelength division multiple access protocol Not currently used in major system yet

Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately, if collision, then retransmit with p, or waits for another frame time with 1-p collision probability increases: frame sent at t0 collides with other frames sent in [t0-1,t0+1]

Pure Aloha efficiency P(success by given node) = P(node transmits) . P(no other node transmits in [t0-1,t0] . P(no other node transmits in [t0,1+t0] = p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1) … choosing optimum p and then letting N -> infty ... = 1/(2e) = .18 Even worse !

Slotted ALOHA Assumptions all frames same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation when node obtains fresh frame, it transmits in next slot no collision, node can send new frame in next slot if collision, node retransmits frame in each subsequent slot with prob. p until success

Slotted ALOHA C – collision E – empty S - successful single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple collisions, wasting slots idle slots (even with data) nodes may be able to detect collision in less than time to transmit packet clock synchronization

Slotted Aloha efficiency Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1 For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37 Suppose N nodes with many frames to send, each transmits in slot with probability p prob that node 1 has success in a slot = p(1-p)N-1 prob that an arbitrary node has a success = Np(1-p)N-1 At best: channel used for useful transmissions 37% of time!

Throughput versus offered traffic for ALOHA systems.

CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: may transmit entire frame If channel sensed busy, defer transmission Human analogy: don’t interrupt others!

Nonpersistent CSMA Station wishing to transmit listens and obeys following: If medium is idle, transmit; otherwise, go to 2 If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1 Consequences: Random delays reduces probability of collisions Consider two stations become ready to transmit at same time While another transmission is in progress If both stations delay same time before retrying, both will attempt to transmit at same time Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting

1-persistent CSMA Station wishing to transmit listens and obeys following:  If medium idle, transmit; otherwise, go to step 2 If medium busy, listen until idle; then transmit immediately Consequences: avoid idle channel time 1-persistent stations selfish If two or more stations waiting, collision guaranteed Gets sorted out after collision

P-persistent CSMA Station wishing to transmit listens and obeys following:  If medium idle, transmit with probability p, and delay one time unit with probability (1 – p) If medium busy, listen until idle and repeat step 1 If transmission is delayed one time unit, repeat step 1 Consequences: Compromise that attempts to reduce collisions Like nonpersistent And reduce idle time Like 1-persistent What is an effective value of p? depend on load

Persistent and Nonpersistent CSMA

Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols.

Collision-Free Protocols The basic bit-map protocol. Contention period, consists N slots Station j want to transmit frame, insert a 1 bit into slot j Each station has complete knowledge after listen contention period Transmit frame in numerical order Never has collisions Overhead is 1 bit per station

Collision-Free Protocols (2) The binary countdown protocol Station want to transmit, broadcasts its address as a binary string, starting with the highest bit Bits in each address position from different stations are Boolean ORed together The result address is the winner of the bidding, it may now transmit a frame Higher address has a higher priority (good or bad)

Limited-contention protocols Adaptive tree walk protocol

Wavelength Division Multiple Access Protocols

Wireless LAN Protocols (a) Hidden station problem (b) Exposed station problem

MACA protocol (Multiple Access with Collision Avoidance ) (a) A sending a RTS (Request to Send) to B. (b) B responding with a CTS (Clear to Send) to A.

Wireless LAN Protocols Multiple Access with Collision Avoidance (MACA) The sender stimulate the receiver into outputting a short frame, so stations nearby can detect this transmission and avoid transmitting for the duration of the upcoming data frame Multiple Access with Collision Avoidance for Wireless (MACAW) Introduce ACK to enable retransmit Add carrier sense to avoid conflict with RTS Run backoff algorithm for each pair, instead of each station Exchange congestion info between neighbors

Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code Time Division, Frequency Division Random partitioning (dynamic), ALOHA, S-ALOHA CSMA carrier sensing: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet Collision-free and limited contention protocols WDMA MACA, MACAW – wireless CSMA/CA used in 802.11

Reading Assignment Chapter 5.3 and 5.5 Chapter 6.3