Physical Layer

• Radio Models: modulation, encoding, antenna, “capture”

• Propagation models: free space; fading

• Multiplexing (bandwidth sharing): FDMA, TDMA, CDMA

MAC Layer

• Media Access Control protocol: coordination and scheduling of transmissions among competing neighbors

• Goals: low latency, good channel utilization; best effort + real time support

• MAC layer clustering: aggregation of nodes in a cluster (= cell) for MAC enhancement; different from network layer clustering/partitioning such as used for routing.

Frequency Hopping (FH)

• Frequency spectrum sliced into frequency subbands (eg, 125 subbands in a 25 Mhz range)

• Time is subdivided into slots; each slot can carry several bits (slow FH)

• A typical packet covers several time slots

• A transmitter changes frequency slot by slot (frequency hopping) according to unique, predefined sequence; all users are clock and slot synchronized

• Ideally, unique sequences are “orthogonal” (ie, non overlapped); in practice, come conflicts may occur

MAC protocols reviewed

• CSMA (Packet Radio Net)

• MACA, MACA-BI

• IEEE 802.11

• PRMA

• MACA/PR

• Cluster TDMA

• Bluetooth

• HomeFR

Foils from Kurose-Ross re MAC layer

Multiple Access Links and Protocols

Three types of links:

(a) Point-to-point (single wire)

(b) Broadcast (shared wire or medium; eg, E-net, wireless, etc.)

(c) Switched (eg, switched E-net, ATM etc)

We start with Broadcast links. Main challenge:

Multiple Access Protocol

Multiple Access Control (MAC) Protocols

• MAC protocol: coordinates transmissions from different stations in order to minimize/avoid collisions

• (a) Channel Partitioning MAC protocols

• (b) Random Access MAC protocols

• (c) “Taking turns” MAC protocols

• Goal: efficient, fair, simple, decentralized

Channel Partitioning MAC protocols

• TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

• FDM (Frequency Division Multiplexing): frequency subdivided.

Channel Partitioning (CDMA)

• CDMA (Code Division Multiple Access): exploits spread spectrum (DS or FH) encoding scheme

• unique “code” assigned to each user; ie, code set partitioning• Used mostly in wireless broadcast channels (cellular, satellite,etc)• All users share the same frequency, but each user has own “chipping”

sequence (ie, code)• Chipping sequence like a mask: used to encode the signal• encoded signal = (original signal) X (chipping sequence)• decoding: innerproduct of encoded signal and chipping sequence (note,

the innerproduct is the sum of the component-by-component products)• To make CDMA work, chipping sequences must be chosen orthogonal

to eachother (ie, innerproduct = 0)

CDMA Encode/Decode

CDMA: two-sender interference

CDMA (cont)

CDMA Properties:

• protects users from interference and jamming (used in WW II)

• protects users from radio multipath fading

• allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

• requires “chip synch” acquisition before demodulation

• requires careful transmit power control to avoid “capture” by near stations in near-far situations

• FAA requires use of SS (with limits on tx power) in the Unlicensed Spectrum region (ISM), eg, .9 Ghz and 2.4 Ghz (WaveLANs)

• CDMA used in Qualcomm cellphones (channel efficiency improved by factor of 4 with respect to TDMA)

Random Access protocols

• A node transmits at random (ie, no a priory coordination among nodes) at full channel data rate R.

• If two or more nodes “collide”, they retransmit at random times

• The random access MAC protocol specifies how to detect collisions and how to recover from them (via delayed retransmissions, for example)

• Examples of random access MAC protocols:

(a) SLOTTED ALOHA

(b) ALOHA

(c) CSMA and CSMA/CD

Slotted Aloha

• Time is divided into equal size slots (= full packet size)

• a newly arriving station transmits a the beginning of the next slot

• if collision occurs (assume channel feedback, eg the receiver informs the source of a collision), the source retransmits the packet at each slot with probability P, until successful.

• Success (S), Collision (C), Empty (E) slots

• S-ALOHA is channel utilization efficient; it is fully decentralized.

Slotted Aloha efficiency

If N stations have packets to send, and each transmits in each slot with probability P, the probability of successful transmission S is:

S = Prob (only one transmits) = N P (1-P)^(N-1)

Optimal value of P: P = 1/N

For example, if N=2, S= .5

For N very large one finds S= 1/e (approximately, .37)

Pure (unslotted) ALOHA

• Slotted ALOHA requires slot synchronization

• A simpler version, pure ALOHA, does not require slots

• A node transmits without awaiting for the beginning of a slot

• Collision probability increases (packet can collide with other packets which are transmitted within a window twice as large as in S-Aloha)

• Throughput is reduced by one half, ie S= 1/2e

CSMA (Carrier Sense Multiple Access)

• CSMA: listen before transmit. If channel is sensed busy, defer transmission

• Persistent CSMA: retry immediately when channel becomes idle (this may cause instability)

• Non persistent CSMA: retry after random interval

• Note: collisions may still exist, since two stations may sense the channel idle at the same time ( or better, within a “vulnerable” window = round trip delay)

• In case of collision, the entire pkt transmission time is wasted

CSMA collisions

CSMA/CD (Collision Detection)

• CSMA/CD: carrier sensing and deferral like in CSMA. But, collisions are detected within a few bit times.

• Transmission is then aborted, reducing the channel wastage considerably.

• Typically, persistent retransmission is implemented

• Collision detection is easy in wired LANs (eg, E-net): can measure signal strength on the line, or code violations, or compare tx and receive signals

• Collision detection cannot be done in wireless LANs (the receiver is shut off while transmitting, to avoid damaging it with excess power)

• CSMA/CD can approach channel utilization =1 in LANs (low ratio of propagation over packet transmission time)

(b) wireless LANs– mostly indoor

– base station ( like cellular); or ad hoc networking (mostly point to point)

– standards: IEEE802.11; HiperLAN (ETSI)

Wireless ground radio networks (cont’d)

Wireless LAN Configurations

BS

With or without control (base) station

Peer-to-peer NetworkingAd-hoc Networking

IEEE 802.11 Standard (Wireless LANs)

• Basic Service Set: single hop ad hoc network• Extended Service Set: multiple BSSs interconnected via wired

net• Physical Layer: DSSS, FHSS, IR (1 or 2 MBPS)

• MAC: several options:(a) DCF (Distr. Coord. Funct): CSMA or RTS-CTS-DATA; phy and

virtual carrier sense; pos ACK; p-persistent; binary backoff; priority access via staggered Inter Frame Spacing (IFS) eg., short IFS > high priority (ACKs, CTS)

(b) PCF (Point Coord.Funct): polling performed from Access Point (AP) ; connection oriented mode; PCF and DCF cycles are repeated (PCF repetition interval)

Voice support in IEEE 802.11 (Sobrinho, Krishnakumar Globcom 96)

• DCF mode, with CSMA• voice has priority over data (short IFS)• voice users tx staggered "black bursts", proportional to waiting

time (and speech bytes in buffer)• voice user who waited longest wins• pos ACK guarantees success (no hidden term.)

• voice connections tend to evenly spread out in time frame

Possible Improvement:• instead of pos ACK, neg ACK (less OH)• receiver "invites" the sender with neg ACK if did not receive

pkt after time out

PRMA (Packet Reservation Multiple Access)

• Developed at WINLAB,Rutgers (D. Goodman)• Cellular system for voice and data• Two separate carriers for upstream (terminal to

base station) & downstream (base station to terminal)

• A derivative of Reservation ALOHA (once you got a slot in the frame, you keep it until you clear your backlog)

PRMA (cont’d)

• Upstream channel: slotted time frame, with available and reserved time slots

• Successful terminal obtains reservation until it quits transmission

• Base station broadcasting feedback packet (success, collision, empty slot) at the end of each frame

• ALOHA contention with permission probability p (i.e. contending terminal transmits in available slot with prob. p)

PRMA protocol example

R11 R5 A R3 R1 R8 A R2

frame kcontending terminals: 6,4

transmitting

terminals 11 5 6,4 1 8 2

R11 R5 A R4 R1 A R6 R2

frame k+2contending terminals: 12

transmitting

terminals 11 5 12 24 6

- -

R11 R5 A A R1 R8 A R2

frame k+1contending terminals: 6,4

transmitting

terminals 11 5 1 24 6--

- -A: available slotRx: slot reserved for terminal x

Functional Integration

headset

cell phone

storage

palmtop

PDA

Network Topology – Piconet

master

slave 1

slave 2

slave 3• Piconets created ad-hoc• Master-Slave concept

• Piconets defined by itsfrequency hopping sequence

slave

master

master/slave

Multiple Piconets: A Scatternet

Topology

CP

CP

Connection PointPSTN

CP

CSMAA Node

TDMAI Node

TDMAI Node

TDMAI Node

CSMAA Node

CSMA & TDMA

A/I Node

Grandma’s3 cups flour1 cup sugar...

Fridge Pad

CSMAA Node

CSMAA Node

•It’s a circuit switched, isochronous network•It’s a packet switched, asynchronous network•It’s both - I nodes get priority on bandwidth