Title: Links and Multiple Access
1Links and Multiple Access
- dedicated links
- point-to-point (single wire, e.g. PPP, SLIP)
- shared links
- broadcast (shared wire or medium e.g, Ethernet,
802.11b, etc.) - two or more simultaneous transmissions by nodes
interference, only one node can send successfully
at a time - multiple access protocol
- distributed algorithm that determines how
stations share channel, i.e., determine when
station can transmit - Ideal multiple access protocol
- When only one node wants to transmit, it can send
at full rate R. - When M nodes want to transmit, each can send at
average rate R/M - Fully decentralized
- no special node to coordinate transmissions
- no synchronization of clocks, slots
- Simple
2Media Access Control Protocols
- Three broad classes
- Channel Partitioning
- divide channel into smaller pieces (time slots
TDMA, frequency FDMA, code CDMA) - allocate piece to node for exclusive use
- Random Access
- allow collisions (how to detect collisions?)
- recover from collisions (how to recover from
collisions?) - ALOHA / slotted ALOHA, CSMA, CSMA/CD, and CSMA/CA
- Taking turns
- tightly coordinate shared access to avoid
collisions
3Code Partitioning (CDMA)
- CDMA (Code Division Multiple Access)
- unique code assigned to each user i.e., code
set partitioning - used mostly in wireless broadcast channels
(cellular, satellite, etc) - all users share same frequency, but each user has
own chipping sequence (i.e., code) to encode
data - encoded signal (original data) X (chipping
sequence) - decoding inner-product of encoded signal and
chipping sequence - allows multiple users to coexist and transmit
simultaneously with minimal interference (if
codes are orthogonal)
4CDMA Encode/Decode
5CDMA two-sender interference
6Random AccessPure (unslotted) ALOHA
- unslotted Aloha simpler, no synchronization
- pkt needs transmission
- send without awaiting for beginning of slot
- collision probability increases
- pkt sent at t0 collide with other pkts sent in
t0-1, t01
7Pure Aloha (cont.)
- P(success by given node) P(node transmits) .
- P(no
other node transmits in t0-1,t0 . - P(no
other node transmits in t0,t0 1 - p .
(1-p)N-1 . (1-p)N-1 - P(success by any of N nodes) N p . (1-p)N-1.
(1-p)N-1 -
choosing optimum p as N -gt infty ... -
1/(2e) .18
Even worse !
8Random Access Slotted ALOHA
- Operation
- when node obtains fresh frame, it transmits in
next slot - no collision, node can send new frame in next
slot - if collision, node retransmits frame in each
subsequent slot with prob. p until success
- Assumptions
- all frames same size
- time is divided into equal size slots, time to
transmit 1 frame - nodes start to transmit frames only at beginning
of slots - nodes are synchronized
- if 2 or more nodes transmit in slot, all nodes
detect collision
9Slotted ALOHA
Success (S), Collision (C), Empty (E) slots
- Pros
- single active node can continuously transmit at
full rate of channel - highly decentralized only slots in nodes need to
be in sync - simple
- Cons
- collisions, wasting slots
- idle slots
- clock synchronization
10Slotted Aloha efficiency
Efficiency is the long-run fraction of successful
slots when there are many nodes, each with many
frames to send
- Suppose N nodes with many frames to send, each
transmits in slot with probability p - prob that node 1 has success in a slot
p(1-p)N-1 - prob that any node has a success Np(1-p)N-1
- For max efficiency with N nodes, find p that
maximizes Np(1-p)N-1 - For many nodes, take limit of Np(1-p)N-1 as N
goes to infinity, gives 1/e .37
At best channel used for useful transmissions
37 of time!
11CSMA (Carrier Sense Multiple Access)
- CSMA listen before transmit
- Persistent (1-persistent)
- Most aggressive
- If channel sensed busy, defer transmission until
the end of transmission - If channel sensed idle, transmit entire frame
- Non-Persistent
- Less aggressive
- If channel sensed idle, transmit
- If channel sensed busy, wait a random period and
then retry the sense/transmit process - Human analogy dont interrupt others!
- P-Persistent
- Applies to slotted channels
- If channel sensed idle, transmit with probability
P
12CSMA collisions
spatial layout of nodes
collisions can occur propagation delay means
two nodes may not hear each others transmission
collision entire packet transmission time wasted
note role of distance and propagation delay in
determining collision prob.
13CSMA/CD
- CSMA/CD carrier sensing, deferral as in CSMA
- collision detection
- collisions detected within short time
- colliding transmissions aborted, reducing channel
wastage - easy in wired LANs measure signal strengths,
compare transmitted, received signals - difficult in wireless LANs receiver shut off
while transmitting - human analogy the polite conversationalist
14IEEE 802.11 CSMA/CA
- avoid collisions 2 nodes transmitting at same
time - 802.11 CSMA - sense before transmitting
- dont collide with ongoing transmission by other
node - 802.11 no collision detection!
- difficult to receive (sense collisions) when
transmitting due to weak received signals
(fading) - cant sense all collisions in any case hidden
terminal, fading - goal avoid collisions CSMA/C(ollision)A(voidance
)
distributed inter frame space
short inter frame space
15Wireless LANs Issues (CSMA)
- The range of a single radio may not cover the
entire system - Hidden station problem (A-gtB, C-gtB since C does
not hear A, collision) - Exposed station problem (B-gtA, C hears B, C wont
send to D, reduced efficiency)
16Multiple Access with Collision Avoidance
- IDEA having a short frame transmitted from both
sender and receiver before the actual long data
transfer - A sends a short RTS (30 bytes) to B with length
of L - B responds with a CTS to A, whoever hears CTS
shall remain silent for the duration of L - A sends data (length L) to B
- Avoid data collision with small reservation frames
17WLAN MACDistributed Coordination Function
- Distributed Control, Ethernet-like CSMA
- CSMA/CA (collision avoidance)
- Physical channel sensing
- Sense channel, transmit entire frame, retry if
necessary - Virtual channel sensing (MACAW)
- Add ACK frame
Short frame (30B) Contains data length
data length copied from RTS
Network Allocation Vector (quiet time)
18Fragmentation for Throughputfragment burst
- Unreliable ISM bands
- Error rate p 10-4,
- success rate for full Ethernet frame (12,144 bit)
lt30, (1-p)n - Error rate p 10-6, 1 will be damaged.
19WLAN MACPoint Coordination Function (PCF)
- Central Control
- Base polls other stations
- Broadcast a beacon frame periodically (10ms to
100ms) with system parameters (hopping sequence,
dwell time, clock synchronization) - Base determines the transmission priority
- QoS guarantee
- Can Coexist with DCF
RTS/CTS/ACK
Fragment burst
Short InterFrame Spacing
20Quiz
- If a node is transmitting a packet of size K, and
transmission bandwidth is R. Given the
propagation speed is Sp, and the length of the
shared link is L, answer the following question - under what condition will a transmitting node be
able to apply collision detection during its
transmission?
21Quiz
- Which of the following are correct
- A. Channel partitioning MAC protocols share
channel efficiently at high load - B. Channel partitioning MAC protocols share
channel efficiently at low load - C. Random access MAC protocols share channel
efficiently at high load - D. Random access MAC protocols share channel
efficiently at low load
22Taking Turns MAC protocols
- channel partitioning MAC protocols
- share channel efficiently and fairly at high load
- inefficient at low load delay in channel access,
1/N bandwidth allocated even if only 1 active
node! - Random access MAC protocols
- efficient at low load single node can fully
utilize channel - high load collision overhead
- taking turns protocols
- look for best of both worlds!
23Taking Turns MAC protocols
- Polling (e.g. PCF)
- master node invites slave nodes to transmit in
turn - concerns
- polling overhead
- latency
- single point of failure (master)
- Token passing
- control token passed from one node to next
sequentially. - token message
- concerns
- token overhead
- latency
- single point of failure (token)
-
- Bit Map Reservation
- Station reserves contention slots in advance
- concerns
- polling overhead
- latency
24Taking Turns MAC protocols
- Token passing
- control token passed from one node to next
sequentially. - token message
- concerns
- token overhead
- latency
- single point of failure (token)
-
25 Summary of MAC protocols
- What do you do with a shared media?
- Channel Partitioning, by time, frequency or code
- Time Division,Code Division, Frequency Division
- Random partitioning (dynamic),
- MA ALOHA
- MA/CD -- S-ALOHA
- CSMA
- persistent, non-persistent, p-persistent
- CSMA/CD used in Ethernet
- CSMA/CA used in WiFi
- Taking Turns
- polling from a central site, token passing