Ethernet Frame Structure Sending adapter encapsulates IP datagram

Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: • 7 bytes with pattern 1010 followed by one byte with pattern 10101011 • used to synchronize receiver, sender clock rates

Ethernet Frame Structure (more) • Addresses: 6 bytes – if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net -layer protocol – otherwise, adapter discards frame • Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and Apple. Talk) • CRC: checked at receiver, if error is detected, the frame is simply dropped

Unreliable, connectionless service • Connectionless: No handshaking between sending and receiving adapter. • Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter – stream of datagrams passed to network layer can have gaps – gaps will be filled if app is using TCP – otherwise, app will see the gaps

Ethernet uses CSMA/CD • No slots • adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense • transmitting adapter aborts when it senses that another adapter is transmitting, that is, • Before attempting a retransmission, adapter waits a random time, that is, random access

Ethernet CSMA/CD algorithm 1. Adaptor gets 4. If adapter detects datagram from and another transmission creates frame while transmitting, aborts and sends jam 2. If adapter senses signal channel idle, it starts to transmit frame. If it 5. After aborting, senses channel busy, adapter enters waits until channel idle exponential backoff: and then transmits after the mth collision, adapter chooses a K 3. If adapter transmits at random from entire frame without m-1}. {0, 1, 2, …, 2 detecting another

Ethernet’s CSMA/CD (more) Jam Signal: make sure all Exponential Backoff: other transmitters are • Goal: adapt aware of collision; 48 retransmission bits; attempts to estimated Bit time: . 1 microsec for current load 10 Mbps Ethernet ; – heavy load: random wait will be longer for K=1023, wait time is about 50 msec • first collision: choose K from {0, 1}; delay is See/interact with Java applet on AWL Web site: K x 512 bit highly recommended ! transmission times • after second collision: choose K from {0, 1, 2, 3}…

CSMA/CD efficiency • Tprop = max prop between 2 nodes in LAN • ttrans = time to transmit max-size frame • Efficiency goes to 1 as tprop goes to 0 • Goes to 1 as ttrans goes to infinity • Much better than ALOHA, but still decentralized, simple, and cheap

• Ethernet Technologies: 10 Mbps; 2: 10 Base 2 under 200 meters max cable length • thin coaxial cable in a bus topology • repeaters used to connect up to multiple segments

10 Base. T and 100 Base. T • 10/100 Mbps rate; latter called “fast ethernet” • T stands for Twisted Pair • Nodes connect to a hub: “star topology”; 100 m max nodes distance between nodes and hub • Hubs are essentially physical-layer repeaters: – bits coming in one link go out all other

Manchester encoding • Used in 10 Base. T, 10 Base 2 • Each bit has a transition • Allows clocks in sending and receiving nodes to synchronize to each other – no need for a centralized, global clock among nodes!

Gbit Ethernet • use standard Ethernet frame format • allows for point-to-point links and shared broadcast channels • in shared mode, CSMA/CD is used; short distances between nodes to be efficient • uses hubs, called here “Buffered Distributors” • Full-Duplex at 1 Gbps for point-to-point links • 10 Gbps now !

Chapter 5 outline • • • 5. 6 Hubs, bridges, 5. 1 Introduction and switches and services • 5. 7 Wireless links 5. 2 Error detection and LANs and correction • 5. 8 PPP 5. 3 Multiple access • 5. 9 ATM protocols • 5. 10 Frame Relay 5. 4 LAN addresses and ARP 5. 5 Ethernet

Multiple Access Links and Protocols Two types of “links”: • point-to-point – PPP for dial-up access – point-to-point link between Ethernet 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 – only one node can send successfully at a 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! • what to look for in multiple access protocols:

Ideal Mulitple 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 Three broad classes: • Channel Partitioning – divide channel into smaller “pieces” (time slots, frequency, code) – allocate piece to node for exclusive use • Random Access – channel not divided, allow collisions – “recover” from collisions • “Taking turns” – tightly coordinate shared access to avoid collisions

Channel Partitioning MAC protocols: 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

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

Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) • unique “code” assigned to each user; i. e. , code set partitioning • used mostly in wireless broadcast channels (cellular, satellite, etc) • all users share same frequency, but each user has own “chipping” sequence (i. e. , code) to encode data • encoded signal = (original data) X (chipping sequence) • decoding: inner-product of encoded signal and chipping sequence

CDMA Encode/Decode

CDMA: two-sender interference

Random Access Protocols • When node has packet to send – transmit at full channel data rate R. – no a priori coordination among nodes • two or more transmitting nodes -> “collision”, • random access MAC protocol specifies: – how to detect collisions – how to recover from collisions (e. g. , via delayed retransmissions) • Examples of random access MAC

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 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

Slotted ALOHA Pros • single active node can continuously transmit at full rate of channel • highly decentralized: only slots in nodes need Cons • collisions, wasting slots • idle slots • nodes may be able to detect collision in less than time to

Slotted Aloha efficiency Efficiency is the long-run fraction of successful slots when there’s many nodes, each with many frames to send • Suppose N nodes with many frames to send, each transmits in slot with probability p • prob that 1 st node has success in a slot = p(1 -p)N -1 • 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 = At best: channel. 37 used for useful transmissions 37% of time!

Pure (unslotted) ALOHA • unslotted Aloha: simpler, no synchronization • when frame first arrives – transmit immediately • collision probability increases: – frame sent at t 0 collides with other frames sent in [t 0 -1, t 0+1]

Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [t 0 -1, t 0]. P(no other node transmits in [t 0, t 0+1] = p. (1 -p)N-1 = p. (1 -p)2(N-1) Even worse ! … choosing optimum p and then letting n -> infty. . . = 1/(2 e) =. 18

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

spatial layout of nodes CSMA collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability

CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA – collisions detected within short time – colliding transmissions aborted, reducing channel wastage • collision detection: – easy in wired LANs: measure signal strengths, compare transmitted, received signals – difficult in wireless LANs: receiver shut off while transmitting

CSMA/CD collision detection

Chapter 5 outline • • • 5. 6 Hubs, bridges, 5. 1 Introduction and switches and services • 5. 7 Wireless links 5. 2 Error detection and LANs and correction • 5. 8 PPP 5. 3 Multiple access • 5. 9 ATM protocols • 5. 10 Frame Relay 5. 4 LAN addresses and ARP 5. 5 Ethernet

IEEE 802. 11 Wireless LAN • 802. 11 a • 802. 11 b – 2. 4 -5 GHz unlicensed radio spectrum – up to 11 Mbps – direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code – 5 -6 GHz range – up to 54 Mbps • 802. 11 g – 2. 4 -5 GHz range – up to 54 Mbps • All use CSMA/CA for multiple access • All have basestation and ad-hoc network versions

Base station approach • Wireless host communicates with a base station – base station = access point (AP) • Basic Service Set (BSS) (a. k. a. “cell”) contains: – wireless hosts – access point (AP): base station • BSSs combined to form distribution system (DS)

Ad Hoc Network approach • No AP (i. e. , base station) • wireless hosts communicate with each other – to get packet from wireless host A to B may need to route through wireless hosts X, Y, Z • Applications: – “laptop” meeting in conference room, car – interconnection of “personal” devices – battlefield

IEEE 802. 11: multiple access • Collision if 2 or more nodes transmit at same time • CSMA makes sense: – get all the bandwidth if you’re the only one transmitting – shouldn’t cause a collision if you sense another transmission • Collision detection doesn’t work: hidden terminal problem

IEEE 802. 11 MAC Protocol: CSMA/CA 802. 11 CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff 802. 11 CSMA receiver - if received OK return ACK after SIFS

Collision avoidance mechanisms • Problem: – two nodes, hidden from each other, transmit complete frames to base station – wasted bandwidth for long duration ! • Solution: – small reservation packets – nodes track reservation interval with internal “network allocation vector” (NAV)

• Collision Avoidance: RTSCTS exchange sender transmits short RTS (request to send) packet: indicates duration of transmission • receiver replies with short CTS (clear to send) packet – notifying (possibly hidden) nodes • hidden nodes will not transmit for specified

• Collision Avoidance: RTSCTS exchange RTS and CTS short: – collisions less likely, of shorter duration – end result similar to collision detection • IEEE 802. 11 allows: – CSMA/CA: reservations – polling from AP

A word about Bluetooth • Low-power, small radius, wireless networking technology • Interference from wireless LANs, digital cordless phones, microwave ovens: – 10 -100 meters – frequency hopping helps • omnidirectional – not line-of-sight infrared • Interconnects gadgets • 2. 4 -2. 5 GHz • MAC protocol supports: – error correction – ARQ • Each node has a 12 -bit address