Shared Access Networks

1. Shared Access Networks Outline Ethernet Wireless

2. Ethernet Overview • History • developed by Xerox PARC in mid-1970s • roots in Aloha packet-radio network • standardized by Xerox, DEC, and Intel in 1978 • similar to IEEE 802.3 standard • CSMA/CD • carrier sense • multiple access • collision detection • Frame Format

4. Ethernet (cont) • Addresses • unique, 48-bit unicast address assigned to each adapter • example: 8:0:e4:b1:2 • broadcast: all 1s • multicast: first bit is 1 • Bandwidth: 10Mbps, 100Mbps, 1Gbps, 10Gbps • Length: 2500m (500m segments with 4 repeaters) • Problem: Distributed algorithm that provides fair access

5. Transmit Algorithm • If line is idle… • send immediately • upper bound message size of 1500 bytes • must wait 9.6us between back-to-back frames • If line is busy… • wait until idle and transmit immediately • called 1-persistent (special case of p-persistent)

6. Algorithm (cont) • If collision… • jam for 32 bits, then stop transmitting frame • minimum frame is 64 bytes (header + 46 bytes of data) • RTT required for detecting collision • RTT for 2.5Km max length Ethernet is 51.2 us, equivalent to 512 b (64B) @10 Mbps (must accommodate slowest version) • delay and try again • 1st time: 0 or 51.2us • 2nd time: 0, 51.2, or 102.4us • 3rd time51.2, 102.4, or 153.6us • nth time: k x 51.2us, for randomly selected k=0..2n - 1 • give up after several tries (usually 16) • exponential backoff

7. Collisions

8. Wireless Networks

9. IEEE 802.11 (WiFi) Protocol Stack

10. The 802.11 Physical Layer (Earlier Versions) • Infrared: • 1 or 2 Mbps, good indoor isolation (do not penetrate walls), poor outdoor performance (sunlight interferes), rarely used • Frequency Hopping Spread Spectrum (FHSS): • 79 1MHz channels, at 2.4 GHz ISM band, both sender and receiver frequency jumps according to a pseudorandom number generator with the same seed. • Advantage: secure, resistant to fading and interference. • Disadvantage: low bandwidth • Direct Sequence Spread Spectrum (DSSS)/CDMA • Uses 11chip Baker sequence, modulation is PSK at 1 MHz, each baud could encode 1 or 2 bits, giving data rates at 1 or 2 Mbps.

11. The 802.11 Physical Layer • High Rate DSSS (HR-DSSS, 802.11b) • 11 Mbps at 2.4 GHz ISM band, can adapt to lower rates to cope with high load/noise but rarely needed, uses Walsh/Hadamard sequence. • Orthogonal Frequency Division Multiplexing (OFDM, 802.11a): • 54 Mpbs at 5 GHz ISM band, signal is split into 52 frequency bands (48 data plus 4 synchronization). • Better resistance to fading and interference, better spectral efficiency, but the range is 7 times shorter than 802.11b. • OFDM (802.11g): • A compromise between 802.11a and b, 54 Mbps at 2.4 GHz.

12. The 802.11 Physical Layer(Later Versions) • 802.11n: Uses 4*4 MIMO, 40 MHz channel @ 2.4/5GHz ~200 Mbps. • 802.11ac: Use 8*8 MIMO, 160 MHz channel @ 5GHz ~Gbps • 802.11ad: Directional antenna, 2 GHz channel @ 60 GHz 7 Gbps short range (1-10m , high attenuation)

13. MAC Collision: Hidden and Exposed Station Problem (a) The hidden station problem, (b) The exposed station problem.

14. 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

15. The 802.11 MAC Sublayer • 802.11 supports two modes: • DCF: distributed coordination function, ad hoc networking • PCF: point coordination function, base station based, optional • DCF: uses CSMA/CA (collision avoidance). Two methods of operation: • Physical channel sensing: sender senses the channel, transmit if idle but stop sensing during the transmission, not transmit if busy, exponential backoff if collision. • Virtual channel sensing: MACAW like RTS/CTS scheme

16. Supporting Mobility • Case 1: ad hoc networking • Case 2: access points (AP) • tethered • each mobile node associates with an AP

17. Mobility (cont) • Scanning (selecting an AP) • 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

18. The 802.16 (WiMax) Protocol Stack

19. The 802.16 Physical Layer The 802.16 transmission environment. • Unlike wireless LAN, directional antenna is used, users are divided into sections that do not interfere with each other. • Since signal falls off sharply in the millimeter bands, different modulation schemes are used at different distances. For example, for a 25 MHz channel, closer users get 150 Mbps, distant ones only get 50 Mbps.

20. The 802.16 MAC Sublayer Protocol: Class-Based • Constant bit rate service: for uncompressed voice, channels are statically allocated. • Real-time variable bit rate service: for compressed multimedia, base station polls the uses at fixed intervals asking how much bandwidth is needed. • Non-real-time variable bit rate service: for large file transfers, the base station polling is not at fixed rapid interval, and the users can ask for channels. If the user does not respond to the polling k times in a row, it is put into a multicast polling group, where anyone can respond to a poll, contention for the service is possible but bandwidth waste is reduced. • Best efforts service: for everything else, no polling, users contend for service by making the channels in the upstream map, if successful, will be noted in the next downstream map, if unsuccessful, exponential backoff. • Bandwidth can be allocated per station or per connection.

21. 802.15.1(Bluetooth) • The basic unit of Bluetooth is a piconet, which consists of a master node and up to seven active slave nodes within a distance of 10 meters. Multiple piconets are connected through a bridge node forming a scatternet. • In addition to the slave nodes, each piconet can have up to 255 parked nodes, which the master node puts to sleep to save power. In the parked state, a node can only respond to an activation or beacon signal from the master node. • The master assigns time slot in a centralized TDM scheme, all communication is between master and slave, no slave-slave communication. • The master-slave design makes possible for dumb and inexpensive slaves. The target price for Bluetooth cards is under $5.

22. Bluetooth Architecture Two piconets can be connected to form a scatternet.

23. Bluetooth Physical Layer • Low power radio (2.5 mW) operates at 2.4 GHz ISM band, 10 m range, interferes with 802.11. • In comparison, WiFi requries 100s mW, 4G cellphone a little less. • The band is divided into 79 1MHz channels, FSK is used, 1 bit per symbol, so the raw data rate per channel is 1 Mbps, but much of that is used for overhead. • FHSS is used, with hopping frequency of 1600 hops/sec, and a dwell time of 625 us, the master node dictates the hoping sequence. • Time slots are 625 us long. The master’s transmission starts in even slots, the slaves’ in odd ones, so they get half bandwidth each.

24. 4G Cellphone • Speed: 100 Mbps for high mobility communications (cars) 1 Gbps for low mobility communications. • All-IP protocol, support IPv6 • Physical layer: OFDM and MIMO