Wireless Networks Physical and Link Layers Wired Typically

Wireless Networks: Physical and Link Layers Wired • Typically point-topoint connections • Interference effects are not significant • Not power constrained Wireless • Typically broadcasted • Interference not only from other hosts, but other devices/physical world phenomenon • Power constrained

Adhoc vs. Wireless LANs Adhoc • Peer-to-peer networking • Short range (10 s of meters) • One device in multiple networks • Little supervision/management Wireless LANs • Substitute for wired LANs • Longer range (100 s of meters) • Typically connected to a wired backplane • Device typically in only 1 network. • Needs management

Bluetooth: A Case Study for Adhoc Networks From J. Haartsen, The Bluetooth Radio System. IEEE Personal Communications, Feb 200.

Overview • Wireless personal area adhoc networking • Uses 2. 45 GHz spectrum (open to public) • Several miniature networks (called Piconets) can co-exist. • A host can reside in multiple piconets • Each piconet channel has 1 master and up to 7 slaves • Unreliable (and shared) medium • Limited power

Multiplexing the bandwidth • If you do not reserve the slots when someone should transmit, then there would be a lot of collisions/contention. • How do you allocate the slots to different hosts? • In the 2. 45 GHz range, we are allowed 2400 -2483 MHz, and we need to find out what frequency to use at each instance of time. • Bluetooth uses 79 frequencies at 1 MHz spacing.

The Multiplexing Problem frequency (how to divide resource among multiple channels? ) time

Frequency-Division Multiplexing frequency user 1 user 2 user 3 user 4 guard-band time

Time Division Multiplexing frequency user 1 user 2 user 3 user 4 user 1 user 2 guard-band time

Frequency-Time Division Multiplexing frequency time-slot (usually of the same size) time

• Bluetooth uses frequency-time multiplexing (frequency hopping) • You do not want to perform multiplexing statically (since you do not know what hosts are present, and who will transmit) • Dynamically determine multiplexing. • However, if everything is dynamic then we need an extensive protocol to figure our who transmits when • Bluetooth uses a frequency hopping pattern wherein the identity of the master is used to determine the (sequence of) frequencies that should be used at each time slot.

Frequency Hopping • Use a well defined hopping pattern sequence for each piconet.

Hop Selection Logic Master identity chooses sequence Clock chooses index (phase) in sequence Offset established at connection time

Connection Establishment • How do units find each other and establish connections? • No common control unit! • A unit wakes up to listen (scan) for its id for around 10 ms. • Wake up hop sequence is 32 hops (cyclic) and unique for each device. • The burden is on paging unit to ensure the appropriate unit is woken up.

• It knows id of dest, and its wakeup 32 hop (unique) sequence • It transmits the dest id repeatedly at different frequencies in the sequence every 1. 25 ms (2*625 us) • It transmits two dest id codes and listens twice for a response.

Polling Device • • • In 10 ms (sleep period) 16 frequencies visited (half sequence) If polling device does not receive response after the “sleep period”, will repeat on the hop carriers of the remaining 16 in the sequence. Maximum delay is thus twice the sleeping period When dest. receives page, it returns back a msg with its identity. Paging unit then sends the dest its identity and clock, and the two now establish a piconet (pager becomes master, and dest a slave).

Medium Access • A piconet channel is defined by id and system clock of Master • All other units are slaves • When a piconet is established, slaves add offset to their native clocks to sync with Master • Different channels have different Masters (and different hopping patterns) • Wired solutions for media access control (e. g. CSMA) do not suffice.

The Hidden Terminal Problem A B C • A sends to B, C cannot receive A • C wants to send to B • If use CSMA/CD: • C senses a “free” medium, thus C sends to A • Collision at B, but A cannot detect the collision • Therefore, A is “hidden” for C

The Exposed Terminal Problem A B C D • B sends to A, C wants to send to D • If use CSMA/CD • C senses an “in-use” medium, thus C waits • But A is outside the radio range of C, therefore waiting is not necessary • Therefore, C is “exposed” to B

• Master completely controls access control, making access contention free. • Time slots are alternately used for Master and Slave transmissions. • The master decides for each slave->master slot which slave should get it. • Only the slave addressed in the preceding master>slave slot is allowed to transmit in this slave>master slot. • If the master has no information to send, it has to poll the slave explicitly with a short poll packet.

Master-controlled Media Access

Packet Structure (in bits)

Acks/Retransmissions A bit in header is used to indicate whether previous packet was received correctly (or if a re-transmission is needed)