Overview Yesterday we looked at Ethernet: CSMA/CD network Today we will look at wireless networks: CSMA/CA

Challenges Limited wireless transmission range Broadcast nature of the wireless medium Hidden terminal problem Packet losses due to transmission errors Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions (security hazard) Nitin Vaidya @ UIUC

EM Spectrum    Pravin Bhagwat @ AT&T Labs ISM band 902 – 928 Mhz 2.4 – 2.4835 Ghz 5.725 – 5.785 Ghz ISM band AM radio S/W radio FM radio TV TV cellular LF HF VHF UHF SHF EHF MF   30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz 10km 1km 100m 10m 1m 10cm 1cm 100mm 3GHz X rays Gamma rays  infrared visible UV 1 kHz 1 MHz 1 GHz 1 THz 1 PHz 1 EHz Propagation characteristics are different in each frequency band Pravin Bhagwat @ AT&T Labs

Unlicensed Radio Spectrum  33cm 12cm 5cm 26 Mhz 83.5 Mhz 125 Mhz 902 Mhz 2.4 Ghz 5.725 Ghz 928 Mhz 2.4835 Ghz 5.785 Ghz cordless phones baby monitors Wireless LANs 802.11 Bluetooth Microwave oven 802.11a Pravin Bhagwat @ AT&T Labs

Spread Spectrum: resilient transmission Idea spread signal over wider frequency band than required originally designed to thwart jamming Frequency Hopping transmit over random sequence of frequencies sender and receiver share… pseudorandom number generator seed 802.11 uses 79 x 1MHz-wide frequency bands

Spread Spectrum (cont) Direct Sequence for each bit, send XOR of that bit and n random bits random sequence known to both sender and receiver called n-bit chipping code 802.11 defines an 11-bit chipping code 1 Data stream: 1010 1 Random sequence: 0100101101011001 1 XOR of the two: 1011101110101001

Collisions Avoidance Similar to Ethernet Problem: hidden and exposed nodes

Hidden Terminal Problem Node B can communicate with A and C both A and C cannot hear each other When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B A B C Nitin Vaidya @ UIUC

Solution: MACAW Sender transmits RequestToSend (RTS) frame Receiver replies with ClearToSend (CTS) frame Neighbors… see CTS: keep quiet see RTS but not CTS: ok to transmit Receive sends ACK when has frame neighbors silent until see ACK Collisions no collisions detection known when don’t receive CTS exponential backoff

RTS/CTS Handshake Nitin Vaidya @ UIUC Sender sends Ready-to-Send (RTS) Receiver responds with Clear-to-Send (CTS) RTS and CTS announce the duration of the transfer Nodes overhearing RTS/CTS keep quiet for that duration RTS/CTS used in IEEE 802.11 C CTS (10) 10 RTS (10) RTS/CTS serves two purposes quicker retransmissions if there is a collision because there is no collision detection avoids collisions caused by hidden terminals A B D Nitin Vaidya @ UIUC 10

Supporting Mobility Case 1: ad hoc networking Case 2: access points (AP) tethered each mobile node associates with an AP Distribution system AP-1 AP-3 AP-2 F A B G H C E D

Mobility (cont) Scanning (selecting an AP) When node sends Probe frame all AP’s w/in reach reply with ProbeResponse frame node selects one AP; sends it AssociateRequest frame AP replies with AssociationResponse frame new AP informs old AP via tethered network When active: when join or move passive: AP periodically sends Beacon frame

Shared Access Networks The next technology that we will look at tries to guarantee who gets access to the network Token Ring ATM Fibre channel Myrinet

Token Ring Overview Examples 16Mbps IEEE 802.5 (based on earlier IBM ring) 100Mbps Fiber Distributed Data Interface (FDDI) 4B/5B encoding

Token Ring (cont) Idea Frame Format Frames flow in one direction: upstream to downstream special bit pattern (token) rotates around ring must capture token before transmitting release token after done transmitting immediate release delayed release remove your frame when it comes back around stations get round-robin service Frame Format 8 8 48 48 32 8 24 Start of Dest Src End of Control Body CRC Status frame addr addr frame

Timed Token Algorithm Token Holding Time (THT) upper limit on how long a station can hold the token Token Rotation Time (TRT) how long it takes the token to traverse the ring. TRT <= ActiveNodes x THT + RingLatency Target Token Rotation Time (TTRT) agreed-upon upper bound on TRT

Algorithm (cont) Each node measures TRT between successive tokens if measured-TRT > TTRT: token is late so don’t send if measured-TRT < TTRT: token is early so OK to send Two classes of traffic synchronous: can always send asynchronous: can send only if token is early Worse case: 2xTTRT between seeing token

Token Maintenance Lost Token Generating a Token (and agreeing on TTRT) no token when initializing ring bit error corrupts token pattern node holding token crashes Generating a Token (and agreeing on TTRT) execute when join ring or suspect a failure send a claim frame that includes the node’s TTRT bid when receive claim frame, update the bid and forward if your claim frame makes it all the way around the ring: your bid was the lowest everyone knows TTRT you insert new token

Maintenance (cont) Monitoring for a Valid Token should periodically see valid transmission (frame or token) maximum gap = ring latency + max frame < = 2.5ms set timer at 2.5ms and send claim frame if it fires

FDDI (Fiber Distributed Data Interface) The late-80’s version of token ring (100Mbps, fiber-based) Dual-ring (two fibers): 2nd ring used for fault recovery Can handle single point failures

ATM Technology (courtesy: ATM Forum) Negotiated Service Contract Connection Oriented - virtual circuit End-to-End Quality of Service Cell Switching 53 Byte Cell 48 Byte Payload, 5 Byte Header Fixed Size Header contains virtual circuit information Payload can be voice, video or other data types ATM Technology is based on powerful, yet flexible concepts. 1. When information needs to be communicated, the sender NEGOTIATES a “requested path” with the network for a connection to the destination. When setting up this connection, the sender specifies the type, speed and other attributes of the call, which determine the end-to-end quality of service. An analogy for this negotiation of qualities would be similar to determining a method of delivery using US mail. One can choose to send 1st class, overnight, 2 day delivery, etc. and can ask for certified mail. 2. Another key concept is that ATM is a switched based technology. By providing connectivity through a switch (instead of a shared bus) several benefits are provided: * Dedicated bandwidth per connection * Higher aggregate bandwidth * Well-defined connection procedures * Flexible access speeds Using ATM, information to be sent is segmented into fixed length cell, transported to and re-assembled at the destination. The ATM cell has a fixed length of 53 bytes. The fixed length allows the information to be transported in a predictable manner. This predictability accommodates different traffic types on the same network. The cell is broken into two main sections, the header and the payload. The payload (48 bytes) is the portion which carries the actual information-either voice, data, or video. The header (5 bytes) contains information used for routing. 3

ATM System Architecture End Station Switch End Station Voice Voice AAL ATM PHY PHY ATM PHY PHY ATM AAL Data Data Video Video Cells Adaptation Layer (AAL): Inserts/extracts information into 48 byte payload ATM Layer: Adds/removes 5 byte header to payload Physical Layer: Converts to appropriate electrical or optical format ATM is a layered architecture allowing multiple services like voice, data and video, to be mixed over the network. Three lower level layers have been defined to implement the features of ATM. The Adaptation Layer assures the appropriate service characteristics and divides all types of data into the 48 byte payload that will make up the ATM cell. The ATM Layer takes the data to be sent and adds the 5 byte header information that assures the cell is sent on the right connection. The Physical Layer defines the electrical characteristics and network interfaces. This layer puts the “bits” on the transport media. ATM is not tied to a specific type of physical transport. 4

Fibre Channel Connect servers, workstations, disk storage etc. Optical or electrical media 133 Mbps to 1062 Mbps 10 km point-to-point links or loop or connect to a switch IP, SCSI etc. http://hsi.web.cern.ch/HSI/fcs/spec/overview.htm

Myrinet 2 GB full duplex high speed network interface http://www.myri.com/myrinet/performance/index.html http://www.conservativecomputer.com/myrinet/perf.html

Myrinet Few sec latency