CSS432 Link Level Protocols (Shared Access Networks) Textbook Ch2.6 - 2.7

1. CSS432 Link Level Protocols (Shared Access Networks)Textbook Ch2.6 - 2.7 Prof. Athirai Irissappane http://courses.washington.edu/css432/athirai/ athirai@uw.edu CSS 432 1

2. Ethernet • local area networking (LAN) technology of last 20 years. • Uses CSMA/CD technology • Carrier Sense Multiple Access with Collision Detection. • Carrier sense means that all nodes can distinguish between an idle and a busy link. • Collision detection means that a node listens as it transmits and can therefore detect when a frame it is transmitting has collided with a frame transmitted by another node. Ethernet transceiver (sends and receives signal) and adaptor CSS 432 2

3. Ethernet Ethernet Hub Ethernet repeater (l = 500 * 4 seg + 500 = 2500 m) Ethernet segment is implemented on a coaxial cable of up to 500 m Bandwidth: 10Mbps (10Base2=Thin Coax 200m, 10B5=Yellow Thick Coax 500m, 10BT=Twisted pair 100m), 100Mbps(10BaseT), 1Gbps Broadcasts Data, Drawback: Contention for the same Ethernet link Multiple Ethernet segments joined by repeaters. A repeater forwards digital signals (only 4 allowed) Hub: multiway repeater,repeats data it hears on one port to all others CSS 432

4. Access Protocol for Ethernet Used by layer 3 IP: 0x0800 ARP: 0x0806 IPv6: 0x86DD • Multiple access network, access control to the shared Ethernet link • Media Access Control (MAC). implemented in Hardware on the network adaptor. • Frame format (Similar to HDLC framing protocol) • Preamble (64bit): allows the receiver to synchronize with the signal (sequence of alternating 0s and 1s). • Host and Destination Address (48bit each). • Packet type (16bit): demux key to identify the higher level protocol where message should be delivered • Data (up to 1500 bytes) • Minimally a frame must contain at least 46 bytes of data. • Frame must be long enough to detect collision. • CRC (32bit) bytes Inter-frame gap 8 6 6 2 46 ~ 1500 4 Next frame Src Dest 12 Preamble Type Body CRC addr addr Min: 64bytes (512bits) ~ Max: 1518bytes

5. Ethernet (MAC) Address • Each host on an Ethernet has a unique Address. • The (unicast) address belongs to the adaptor, not the host. • MAC Address • sequence of six numbers separated by colons • each number corresponds to 1 byte of the 6 byte (48 bit) address and is given by a pair of hexadecimal digits, one for each of the 4-bit nibbles in the byte • Leading 0s are dropped. • E.g., 8:0:2b:e4:b1:2 = 00001000 00000000 00101011 11100100 10110001 00000010 • address consisting of all 1s a broadcast address. • All adaptors pass frames addressed to the broadcast address up to the host. • an address with first bit set to 1 but is not the broadcast address is called a multicast address. • host can program its adaptor to accept multicast addresses. CSS 432 5

6. Ethernet Transmit Algorithm • If line is idle… • Adapter sends frame immediately • Upper bound message size of 1500 bytes: MTU (Maximum Transmission Unit) • Must wait 96bits between back-to-back frames • If line is busy… • Adapter waits until idle and transmit immediately • Called 1-persistent • Transmit a packet with probability 1. • (special case of p-persistent: transmitting a packet with P percent, where 0 < p ≤ 1) • Collision of frames • 2 or more two (or more) adaptors to begin transmitting at the same time CSS 432 6

7. Transmit Algorithm (cont) 10Mbps means 10bits/usec. 96bits needs 9.6usec • If collision… • Ethernet supports collision detection, each sender is able to determine that a collision is in progress • Send a 32-bit jamming sequence, and then stop transmitting frame (64bit preamble + 32bit jam = 96bits) • Minimum frame is 64 bytes (header + 46 bytes data + 4 bytes CRC) = 512bits • The farther the nodes, the longer the time it takes for a frame sent by one to reach the other, and the network is vulnerable to collision during this time 1010101… 64bits 1010.. 32bits CSS 432 7

8. Collisions 500m x 5 = 2500m (with 4 repeaters) latency A sends a frame at time t A B Time t A B Arrives at B at time t + td Ethernet Time t + td (td = 25.6us: approx. 0.01us/m) Token Ring A B B sends frame at t + td A collision occurred 30% Network load Jam seq A B Jamming seq arrives at t + 2td Time t + 2td (2td = 51.2us) CSS 432 8

9. Collisions • RTT = 51.2 us for 2*2500=5000m • Transmit Time >= RTT • Size/Bandwidth >=RTT • Size >= RTT * Bandwidth • Size >= 51.2us * 10Mbps • Size >=512 bits; min bits to be transmitted = 512 bits • If RTT > 51.2 us , i.e., cable length > 2500m, min number of bits > 512 for 10Mbps ethernet; Limited Cable Length!

10. Quiz: First to solve get 0.02 points extra added to discussion score • The speed of transmission: 10.24 ns/m, i.e, time/distance • Min frame size = 512 bits • Calculate the maximum length of the ethernet cable for a 100Mbps network • The higher network bandwidth, the more sensitive the NICs should be to detect a collision. CSS 432

11. Collisions • On collision detection • adapter stops transmission • Sends jamming sequence (total of 96 bits) • delays and tries again. • (Exponential Backoff) • Doubles delay interval between retransmission • 1st time: waits for 0 or 51.2us (selected at random) • 2nd time: 0, 51.2, 102.4us or 153.6us (random) • 3rd time: waits for k * 51.2, k = 0…23 – 1 (random) • nth time: wait for k x 51.2us, for randomly selected k=0..2n – 1 • give up after several tries (usually 16) • Wait 96 bit time ,i.e., 9.6us for 10Mbps; 960ns for 100Mbps before transmitting back to back frames • Recovery time in case of collisions, clock recovery

12. Wireless Network • No physical medium for transmission • Transmission through electromagnetic signals • Radio, microwave, infrared • Wireless links (WIFI, Bluetooth, FM radio) all share the same medium (air/free space) • Share medium efficiently without interference • Dividing the medium using frequency and space dimensions • Allocations of bands (frequency) ranges determined by government agencies such as FCC in USA • Bands for government use, AM radio, FM radio, televisions, satellite communications, cell phones, • Specific frequencies within these bands are then allocated to individual organizations for use within certain geographical areas. • Several bands set aside for which a license is not needed CSS 432 12

13. Wireless Network • Devices that use license-exempt frequencies are still subject to certain restrictions • Limitation on transmission power: limits the range of signal, making it less likely to interfere with another signal • For example, a cordless phone might have a range of about 100 feet. • Baby monitors, home security systems, even WIFI.. • Use Spread Spectrum Technique: • Sharing spectrum without interference • While using the same constant frequency for transmission results in interference from other signals, easy to intercept (not secure) CSS 432 13

14. Wireless Network • Spread Spectrum technique: spread the signal over a wider frequency band • Frequency hopping: Transmit signal over a random set of frequencies • Transmit at one frequency, then a second, then a third… • Sequence of frequencies is not truly random, instead computed algorithmically by a pseudorandom number generator • Receiver uses the same algorithm • Hop frequencies in sync with the transmitter to correctly receive the frame • Directsequence: Represents each bit in the frame by multiple bits in the transmitted signal. • For each bit, the sender sends the XOR of that bit and n random bits • random bit generator known to both the sender and the receiver. • The transmitted values, known as an n-bit chipping code, spread the signal across a frequency band that is n times wider CSS 432 14

15. Wireless Network • A second spread spectrum technique called Directsequence • Represents each bit in the frame by multiple bits in the transmitted signal. • For each bit the sender wants to transmit • It actually sends the exclusive OR of that bit and n random bits • The sequence of random bits is generated by a pseudorandom number generator known to both the sender and the receiver. • The transmitted values, known as an n-bit chipping code, spread the signal across a frequency band that is n times wider 4-bit chipping code CSS 432 15

16. Wireless Network • CDMA: Code Division Multiple Access (Cellular-3G) • Multiplexing technique different from Time/Frequency Division multiplexing • Senders send information, share bandwidth, transmit at different frequencies; • Direct Sequence Spread spectrum technique • Differentiated using chipping codes (random number) • Each cell phone sends data with a different (but pre-assigned chipping code.) • A base station distinguish many cell phones using their unique chipping code. CSS 432 16

17. Wireless Network • Topology • Wireless Network with Base Station • Mesh or Ad-hoc network • Wireless Network with Base Station • Two end-points are usually different kinds of nodes • One end-point usually has no mobility, but has wired connection to the Internet (known as base station) • The node at the other end of the link is often mobile • Wireless communication supports point-to-multipoint communication • Communication between non-base (client) nodes is routed via the base station • Three levels of mobility for clients • No mobility: the receiver must be in a fix location to receive a directional transmission from the base station (initial version of WiMAX) • Mobility is within the range of a base (Bluetooth) • Mobility between bases (Cell phones and Wi-Fi) • Mesh or Ad-hoc network (no base station) • Nodes are peers • Messages may be forwarded via a chain of peer nodes CSS 432 17

18. Ad-hoc Network Base Stations Wireless Network CSS 432

19. Wireless Network • Wireless technologies differ in a variety of dimensions • How much bandwidth they provide • How far apart the communication nodes can be • Four prominent wireless technologies • Bluetooth • Wi-Fi (more formally known as 802.11) • 3G cellular wireless CSS 432 19

20. IEEE 802.11 (WIFI) • Wireless Link Technology • Like its Ethernet and token ring siblings, 802.11 is designed for use in a limited geographical area (homes, office buildings, campuses) • Primary challenge is to mediate access to a shared communication medium – in this case, signals propagating through space • Operates in the license exempt band 2.4 GHz or 5 GHz CSS 432

21. Collision • Ethernet: • Every node receives every other node’s transmissions • Every node can transmit and receive at the same time • WIFI • Cannot transmit and receive at the same time on the same frequency, power generated by transmitter is much higher than receiver; WIFI runs at half-duplex • Every node cannot received transmissions from the other node because of the WIFI range, distance between the nodes, blockage between the nodes CSS 432

22. Collision Avoidance • Hidden Nodes: • A and C want to exchange frames with B but do not know about each other. A and C are said to hidden nodes • Hidden Node Problem: Signals from A and C can collide at B • Exposed Node Problem: • B sends signal to A; C becomes aware and stops sending to D thinking it will interfere with A • MACA Multiple Access with Collision Avoidance Alg. • Sender and receiver exchange control frames (Request to SendRTS, Clear to SendCTS) with each other before the sender actually transmits any data. • Any node that sees the CTS frame is close to the receiver, therefore cannot transmit for the period of time it takes to send a frame of the specified length • Any node that sees the RTS but not CTS is free to transmit • Receiver sends an ACK to the sender after successfully receiving a frame All nodes must wait for this ACK before trying to transmit CSS 432

23. Distribution System • 802.11 uses Base Station topology currently • Nodes are free to move around • Access Points / Base stations (routers in WIFI) • some nodes are allowed to roam • some are connected to a wired network infrastructure • they are called Access Points (AP) and they are connected to each other by a so-called distribution system • Extend the WIFI range: Main base station connects to internet, others physically connect to each other • Each of these regions is analogous to a cell in a cellular phone system with the APs playing the same role as a base station CSS 432

24. Distribution System • Each nodes associates itself with one access point • For node A to communicate with node E, A first sends a frame to its AP-1 which forwards the frame across the distribution system to AP-3, which finally transmits the frame to E • The technique for selecting an AP is called scanning • The node sends a Probe frame • All APs within reach reply with a Probe Response frame • The node selects one of the access points and sends that AP an Association Request frame • The AP replies with an Association Response frame CSS 432

25. Bluetooth (802.15.1) • Used for very short range communication between mobile phones, PDAs, notebook computers and other personal or peripheral devices • Operates in the license-exempt band at 2.45 GHz • Has a range of only 10 m • Communication devices belong to one person or group, Bluetooth is categorized as Personal Area Network (PAN) • Version 2.0 provides speeds up to 2.1 Mbps • Power consumption is low active slave parked slave Request : : 7 Reponses master 255 active slave parked slave TDM o e o e o e o e CSS 432

26. Bluetooth (802.15.1) • Bluetooth network configuration is called a piconet • Consists of a master device and up to seven slave devices • Any communication is between the master and a slave • The devices can switch roles, by agreement • The slaves do not communicate directly with each other • A slave can be parked: set to an inactive, low-power state (255 parked slaves allowed in a piconet) • Synchronous Time division multiplexing: Master transmits in odd time slot, slave in the even time slot. Slave transmits as a response to request in the previous master slot (avoids contentions between slaves) active slave parked slave Request : : 7 Reponses master 255 active slave parked slave TDM o e o e o e o e CSS 432

27. Cell Phone Technology • Wireless Technology • Use Licensed Spectrum, owned by cellular phone operators (AT & T, Verizon, T-Mobile) – costly • Topology: Relies on a wired network of base stations • Geographic area served by a base station is a cell • Base station can server one or more cells • Cells overlap • A phone is in control of only one base station at a time • When phones moves to an area of overlap, the base station senses the weakening of signal from phone and gives control to another base station which receives the strongest signal. • If the phone is involved in a call, this transfer is called handoff Call Handoff Base satations 1 P 2 3 P CSS 432

28. Reviews • Ethernet: k-persistent, exponential back off, and the relationship between the minimum frame size and collisions. • Exercises in Chapter 2 • Ex. 42 (Ethernet) • Ex. 46 (Ethernet) • Ex. 53 (Wi-Fi) • Ex. 54 (Wi-Fi) CSS 432 28