Transcript No Slide Title

Southern Methodist University Fall 2003
EETS 8316/NTU CC745-N
Wireless Networks
Lecture 8: Mobile Data, Part III
Instructor: Jila Seraj
email: [email protected]
http://www.engr.smu.edu/~jseraj/
tel: 214-505-6303
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Session Outline
Review of last week
Wireless LAN
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Announcements
Answer to homework #1 is on the web
Homework #2 is on the web.
—Deadline for in-campus students October 24
—Deadline for distant students November 7
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Review, GPRS - Network Architecture
Internet or other
networks
HLR
SGSN
MSC/
VLR
GGSN
Gateway GSN = packet switch
interworks with other networks
SGSN
Serving GPRS support node
= packet switch with mobility
management capabilities
BSC/PCU
GPRS makes use of existing
GSM base stations
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Review, GPRS , Cont...
GSM Release’97 introduced general packet
radio service (GPRS) for bursty data
Make use of existing GSM network
equipment and functions
In Contrast to CDPD, it is integrated into
GSM, i.e. dedicated Control channel and
data channel.
Requires two new network element, GGSN
and SGSN
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Review, GPRS , Cont...
GGSN = Gateway GPRS Support Node
— External interfaces
— Routing
GPRS register maintains GPRS subscriber
data and routing information. Normally it is
integrated in GSM HLR
PCU (Packet Control Until) is collocated
with BSC.
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Review, GPRS , Cont...
Three class of mobile terminals
—Class A: Operates GPRS and Circuit
switched service simultaneously
—Class B: Monitors the Control channels of
GPRS and GSM simultaneously but can
operate one set of services at a time
—Class C: Only CS or GPRS capable.
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Review, GPRS , Cont...
For mobility management a new concept is
defined, Routing Area
RAI = MCC +MNC + LAC + RAC
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Review, GPRS Interfaces
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Review, GPRS – Data Connection
GPRS data connection starts with Attach
and ends with Detach.
Attach is the phase when the mobile
informs the network of its intention to
create a data connection
At conclusion of Attach, SGSN is ready to
set up data services on behalf of the mobile
user.
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Review, GPRS – Data Connection, Cont…
Detach is the phase when mobile
terminates the connection.
GPRS requires subscription
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Review, GPRS Attach Scenario
BTS
BSS
SGSN
HLR
IMSI, P_TMSI+OLD RAI…
Update Location
Insert Subs. Data
Insert Data Ack
Update Location
GPRS Attach Accepted
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Review, GPRS – Mobile Attach Scenario
Mobile sends Attach message. This
message contains P-TMSI or TMSI. It also
contains NSAPI (Network Service Point
Identifier)
SGSN contacts HLR to verify if the user is
permitted to use the service
After authentication, SGSN send back
Attach Accepted together with a TLLI
(Temporary Logical Link Identity)
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Review, GPRS – Mobile Attach Scenario
A database in SGSN is now populated with
mobile identity and TLLI. TLLI is used by
logical link controller in the SGSN.
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Review, GPRS – Setting Up Packet Data
After attach the mobile is known by SGSN and
have an identity there, but it is not known to the
external network.
First it needs to create an identity for itself by
performing a procedure called PDP Context
Activation. PDP is Packet Data Protocol, which
could be IP or x.25 protocol.
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Review, PDP Context Activation
BTS
BSS
SGSN
Activate PDP Context
Create PDP
Context Request
NASPI, PDP type
PDP, QoS,APN
Activate PDP Context Accepted
PDP Type, PDP Address, QoS
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GGSN
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Create PDP
Context Response
PDP Address, QoS
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Review, PDP Context Activation, Cont..
Mobile requests PDP Context Activation
Based on the information provided, SGSN
determines which GGSN to connect to. The
GGSN should be capable to support the
PDP requested by mobile
GGSN updates its data base and assign a
TID to the mobile and SGSN
SGSN updates its data base with the GGSN
address and TID. It then send PDP Context
Activation Accepted message to mobile
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Review, Actually Sending Data
After PDP Context Activation the mobile is
known to the external packet network (PDN)
When SGSN receives data from mobile, it
looks up its database and relate the TLLI to
NSAPI.
SGSN and SNDPC pad the IP packet and
replace the destination address with GGSN
IP address and sets GTP header to TID
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Review, Actually Sending Data, Cont…
Packets are then sent to GGSN with SGSN
as sender
At GGSN, the additional information is
removed to get the original packet . The
packet can now be routed to its intended
destination.
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Wireless LANs
Wireless LANs are usually logical bus topology
(broadcast medium)
Why wireless LANs?
—Saves trouble of rewiring a building
—Portable computing devices (laptops, PDAs)
are more common
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MAC Protocols
MAC protocol is a sublayer in data link
layer
For LANs, data link layer = logical link
control (LLC) sublayer + MAC sublayer
network
LLC
data link
MAC
physical
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- defines how stations
access the shared
medium
#21
MAC Protocols, Cont..
—LLC sublayer builds on MAC sublayer to
provide medium-independent
communication service to higher layers
(makes MAC sublayer transparent)
—LLC can provide appearance of
connectionless or connection-oriented
service
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MAC Protocols, Cont..
• Connectionless service treats each
message independently. No connection
setup and no sequential order
• Connection-oriented service requires
connection setup and preserves
sequential order of messages
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MAC protocols, Token Passing
Token ring and token bus
—Every station connected to the bus is given
a token
—The token is passed according to order
—When a station has something to send, it
keeps the token until it is done, before
sending it to the next station.
It is fair and has no contention
The system encounters delays for sending
the token.
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MAC Protocols: Token Passing, Cont..
Token passing is another technique to
eliminate contention (collisions)
Token is short packet representing permission
to transmit
—Token is passed from station to station
according to an arranged order defining a
logical token ring topology
—A station with the token can transmit for a
limited time
—After transmission, token is sent to next
station in ring
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MAC Protocols: Polling
Objective to eliminate random contention
(collisions) which reduces throughput of
system
Polling is centralized control
—One station will periodically poll other
stations to see if they have data to transmit
—A polled station may transmit data,
otherwise controller will poll next station in a
list
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MAC Protocols: Polling
Polling involves exchange of control messages
between stations and controller
—Efficient only if
• roundtrip propagation delay is small
• overhead due to control messages is
small
• user population is not large and bursty
—As population increases with more bursty
users, performance of polling degrades
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MAC Protocols: Polling
Polling is used in wired network environments
but not popular in wireless networks
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Token passing, Cont..
Commonly used in wired LANs (IEEE 802.4
token bus and 802.5 token ring), token passing
has not found much adoption in wireless
networks
Overhead is increased to improve throughput
under heavy load
—Issue is efficiency
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MAC protocols: Aloha
Aloha
—Stations starts sending when they have
something to send
—Pure Aloha, no contention resolution, relies
on timed-out acks, max throughput 18%
—Slotted Aloha, no contention resolution,
relies on timed-out acks, only can start
sending in the beginning of a slot, max
through put 36%
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MAC Protocol: Pure ALOHA, Cont..
Throughput
—Assume infinite population of stations
generating frames at random times
—Each frame is transmitted in fixed time T
—Assume average number of transmission
attempts is S in any interval T
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MAC Protocol: Pure ALOHA, Cont..
Throughput
—Number of new transmission attempts in
any interval t has Poisson probability
distribution:
Pr(k transmissions in interval t ) = (St)ke- St /k!
—Let G = “offered load” = new transmissions
and retransmissions
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MAC Protocol: Pure ALOHA, Cont..
—In equilibrium, throughput (rate of
successfully transmitted frames) = rate of
new transmissions, S
S = GP0
where P0 = probability of successful
transmission (no collision)
—P0 depends on “vulnerable interval” for
frame, 2T
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MAC Protocol: Pure ALOHA, Cont..
frame A
frame B
frame C
-T
0
time
T
- transmission attempt at time 0
- collision if starts in interval (-T,0)
- collision if starts in interval (0,T)
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MAC Protocol: Pure ALOHA, Cont..
P0 = Pr(no other frame in 2T interval)
—Assume total number of frames in any
interval t is also Poisson distributed, with
average G:
Pr(k transmissions in t) = (Gt)ke-Gt/k!
then P0 = e-2G
—By substitution, throughput is
S = GP0 = Ge-2G
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MAC Protocol: Pure ALOHA, Cont..
—This is maximum at G = 0.5, where S = 1/2e
= 0.184 (frames per interval T)
• Pure ALOHA achieves low throughput
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MAC Protocol: Slotted ALOHA
Slotted ALOHA is a modification to increase
efficiency
—Time is divided into time slots =
transmission time of a frame, T
—All stations are synchronized (eg, by
periodic synchronization pulse)
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MAC Protocol: Slotted ALOHA
Slotted ALOHA is a modification to increase
efficiency
—Any station with data must wait until next
time slot to transmit
—Any time slot with two or more frames
results in a collision and loss of all frames –
retransmitted after a random time
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MAC Protocol: Slotted ALOHA, Cont..
“Vulnerable interval” is reduced by factor of 2
to just T
frame A
frame B
time
-T
0
T
- transmission attempt at time 0
- collision if frame B was ready in
interval (-T,0)
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MAC Protocol: Slotted ALOHA, Cont..
Throughput
P0 = Pr(no frames ready in previous time slot)
= e-G
—Now throughput is
S = GP0 = Ge-G
—This is maximum at G = 1, where S = 1/e =
0.368 (frames per interval T)
• Slotted ALOHA doubles throughput of
pure ALOHA
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MAC Protocol: Slotted ALOHA, Cont..
Note that throughput is never very high
Also, at high loads, throughput goes to 0, a
general characteristic of networks with shared
resources
—Number of empty time slots and successful
slots decrease, number of collisions
increase
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MAC Protocol: Slotted ALOHA, Cont..
—Average number of retransmissions per
frame increases
—Average delay (from first transmission
attempt to successful transmission)
increases
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MAC Protocol: CSMA
Carrier Sense Multiple Access (CSMA)
Sense the presence of carrier, sense the
channel is free, send data, wait for Ack, resend if timed-out, if busy back off and try again.
Max throughput 60%
Many versions, most popular method in LAN.
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MAC Protocol: CSMA, Cont..
Family of CSMA protocols defined by rules for
backing off with varying degrees of persistence
—1-persistent CSMA: stations are most
persistent
—P-persistent CSMA: persistence increases
with value of p
—Non-persistent CSMA: stations are not that
persistent
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MAC Protocol: 1-persistent CSMA
Slotted or un-slotted versions
If channel is busy, station will transmit
immediately after channel becomes idle
If collision is detected, then back off and try
again after a random time
Propagation delay can effect performance –
station A takes longer to detect that station B is
transmitting
—Causes collisions to be more likely
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MAC Protocol: 1-persistent CSMA, Cont..
Even without propagation delays, collisions are
possible
—Stations A has the channel, stations B and C
are ready and will both transmit after station
A is done
Throughput analysis is complicated
—Carrier sensing improves throughput over
ALOHA
—Throughput goes to 0 under very high load
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MAC Protocol: P-persistent CSMA
If channel is idle, station will transmit with
probability p
Otherwise, goes to next time slot and senses if
channel is idle
If idle, transmits with probability p or otherwise,
goes to next time slot and repeats procedure
Performance depends on choice of p
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MAC Protocol: Non-persistent CSMA
If channel is idle, station will transmit
If channel is busy, station will wait for random
number of time slots before trying again - even
if channel is idle meanwhile
Helps avoid collisions right after an active time
slot
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MAC Protocol: MAC protocols, Cont..
Carrier Sense Multiple Access-Collision
Detection (CSMA-CD)
—Send when carrier is free.
—Listen to detect collision
—If collision is detected, back off and retry
—Second order of improvement to CSMA
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MAC Protocol: MAC protocols, Cont..
Carrier Sense Multiple Access-Collision
Detection (CSMA-CD)
—Not possible in wireless LAN environment,
the same frequency for sending and
receiving (unlike cellular)
—CSMA-CA is the method of choice
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MAC Protocol: CSMA/CD, Cont..
3 alternating states: (1) transmission (2)
contention (3) idle
time
frame
transmission
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frame
contention:
series of time
slots for collisions
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MAC Protocol: CSMA/CD, Cont..
Performance depends on time to detect
collision (assume transmissions can be
aborted immediately)
If D is worst-case propagation delay
between any two stations, then collision
detection time is 2D
A begins transmit
A detects collision after 2D
station A
signal
time
station B
B begins transmit just before signal reaches B
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MAC Protocol: CSMA/CD, Cont..
Assume
N = number of stations
2D = length of collision time slots
T = time to transmit frame
(T > 2D, otherwise collisions are not
detected)
P = probability a station will transmit in idle
time slot
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MAC Protocol: CSMA/CD, Cont..
After successful frame, there is contention
period of series of collision time slots (multiple
attempts) or idle (no attempts), ended by a
successful frame (exactly one attempt)
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MAC Protocol: CSMA/CD, Cont..
Find P1 = Pr(exactly 1 attempt in time slot) =
NP(1-P)N-1
Maximum when P = 1/N, then
N1
1
P1  1  
 N 
Mean length of contention period:
—Pr(j slots with collisions or idle followed by
one transmission) = (1 - P1)jP1
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MAC Protocol: CSMA/CD, Cont..
—Mean length of contention period is

1 P1
 j(1 P1) P1  P (slots)
j 1
1
j
—Maximum utilization is
T
frame time
=
1 P1
frame time + contention period
T
2D
P1
—Note utilization decreases for large D or
small T
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MAC Protocol: CSMA-CA
Carrier Sense Multiple Access-Collision
Avoidance (CSMA-CA)
—When node A has something to send to
node B, it send Request-To-Send (RTS)
packet to B with the amount of data to be
sent
—B responds with Clear-To-Send (CTS)
packet with time of transmission and
amount of transmission back to node A
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MAC Protocol: CSMA-CA
Carrier Sense Multiple Access-Collision
Avoidance (CSMA-CA)
—When a node has something to send, it
should also checks CTS before start
transmitting.
—Improves CSMA performance
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What Is Hidden Node?
A
B
C
A can hear B
C can hear B
A can not hear C
C can not hear A sending data
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MAC Frame Format
2
6
6
6
2
2
0-2304
4
Frame Duration Address 1 Address 2 Sequence Address 4 Frame CRC
Control
Control
Body
Protocol
Version
2
Type Sub type To From
DS DS
2
4
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1
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Last
Retry Power
Fragment
Mgt
1
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1
2
EP RSVD
1
1
#60
Frame type and subtypes
Three type of frames
— Management
— Control
— Asynchronous data
Each type has subtypes
Control
— RTS
— CTS
— ACK
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Frame type and subtypes, Cont..
Management
— Association request/ response
— Re-association request/ response
— Probe request/ response
— privacy request/ response
— Beacon (Time stamp, beacon interval, TDIM period,
TDIM count, channels sync info, ESS ID, TIM
broadcast indicator)
— TIM (Traffic Indication Map) indicates traffic to a
dozing node
— dissociation
— Authentication
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Power Management
AP knows the power management of each
node
AP buffers packets to the sleeping nodes
AP send Traffic Delivery Information Message
(TDIM) that contains the list of nodes that will
receive data in that frame, how much data and
when.
The node is awake when it is sending data,
receiving data or listening to TDIM.
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Authentication
Three levels of authentication
—Open: AP does not challenge the identity of
the node.
—Password: upon association, the AP
demands a password from the node.
—Public Key: Each node has a public key.
Upon association, the AP sends an
encrypted message using the nodes public
key. The node needs to respond correctly
using it private key.
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Mobility Management
Access point connects other access points
via backbone network
Backbone Network
Access Point
Access Point
Access Point
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Mobility Management, Cont..
A node can associate when it enters the
coverage area of an AP
It shall re-associate when it handoffs to another
AP.
AP bridge function keeps track of all nodes
associated with it.
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Access Point Functions
Access point has three components
—Wireless LAN interface to communicate with
nodes in its service area
—Wireline interface card to connect to the
backbone network
—MAC layer bridge to filter traffic between
sub-networks. This function is essential to
use the radio links efficiently
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Bridge Functions
Listen to all packets being sent.
Find out which nodes are in which sub-network
by analyzing the source address. Store that
data in a routing table.
If a packet is addressed to a known node, only
repeat the data on that sub-network, otherwise
repeat it on all networks.
Age the entries after a timer value has expired
since the last communication
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Bridge Functions, Cont..
If the timer is too long, we might send data to a
node that might have left the sub-network or is
turned off or even gone to coverage area of
another access point.
If the timer is too short, we remove the user too
early and repeat the packet destined to it in all
sub-networks.
Other functions of a bridge, buffering for speed
conversion, changing frame format between
LANs.
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Routing
Building routing tables can be done as
—Source tree, keeps track where other nodes
are and the best way of reaching them. When
sending a packet the route is also determined.
It must be done in each node and is heavy.
—Spanning tree, is built iteratively, each bridge
advertises it identity and all other bridges it
knows and how many hops it takes to get there.
Then each bridge follows a specific algorithm to
calculate how get to each bridge with least hop.
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IEEE 802.11 WLAN
1997 IEEE 802.11 working group developed
standard for inter-working wireless LAN
products for 1 and 2 Mbps data rates in 2.4
GHz ISM (industrial, scientific, and medical)
band (2400-2483 MHz)
Required that mobile station should
communicate with any wired or mobile station
transparently (802.11 should appear like any
other 802 LAN above MAC layer), so 802.11
MAC layer attempts to hide nature of wireless
layer (eg, responsible for data retransmission)
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802.11 WLAN, Cont..
1999 IEEE 802.11a amendment for 5 GHz
band operation and 802.11b amendment to
support up to 11 Mbps data rate at 24 GHz
MAC sublayer uses CSMA/CA (carrier sense
multiple access with collision avoidance) - very
similar to CSMA/CD except collisions are
detected by ACKs after entire packets are
transmitted
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802.11 WLAN, Cont..
—Station will listen to channel if ready to
transmit
—If channel is idle, begins to transmit
—If channel is busy, will wait until channel is
free and transmit after a random time (to
reduce collisions)
—In case of collisions, stations will try again
following a random exponential back off
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#73
802.11 WLAN, Cont..
Random exponential back off:
—Stations keep track of contention window
parameter, CW
• Initially CW is a minimum value
—When station wants to transmit, it chooses a
random (uniformly likely) value between 0
and CW
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#74
802.11 WLAN, Cont..
—Waits for chosen number of time slots
before transmitting
—After each collision, CW is doubled
(exponential increase)
collisions
Example
X
X
X
tries in one
of 2 slots
tries in one
of 4 slots
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
tries in one
of 8 slots
#75
802.11 WLAN, Cont..
Other MAC sublayer functions:
—Optional “point coordination function”:
centralized contention-free multiple access
for time-sensitive data (a centralized polling
mechanism)
—“Association” and “re-association”
processes to dynamically establish
connections between mobile stations and
fixed access points
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#76
802.11 WLAN, Cont..
—Optional encryption for security
—Power management to allow mobile stations
to power down (sleep) without losing data
(eg, access point will buffer packets for
sleeping stations until requested)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#77
HIPERLAN
WLANs have small share of LAN market now
—Higher costs per station
—Standards are recent (HIPERLAN, IEEE
802.11)
—Rapid growth is projected
1995 ETSI technical group RES 10 (Radio
Equipment and Systems) developed
HIPERLAN/1 wireless LAN standards using 5
channels in 5.15-5.3 GHz frequency range
— Technical group BRAN (Broadband Radio Access
Network) is standardizing HIPERLAN/2 for wireless
ATM
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#78
HIPERLAN, Cont..
HIPERLANs with same radio frequencies
might overlap
—Stations have unique node identifiers (NID)
—Stations belonging to same HIPERLAN
share a common HIPERLAN identifier (HID)
—Stations of different HIPERLANs using
same frequencies cause interference and
reduce data transmission capacity of each
HIPERLAN
—Packets with different HIDs are rejected to
avoid confusion of data
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#79
HIPERLAN, Cont..
Data link layer = logical link control (LLC)
sublayer + MAC sublayer + channel access
control (CAC) sublayer
network
LLC
data link
MAC
CAC
physical
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#80
HIPERLAN, Cont..
MAC sublayer:
—Keeps track of HIPERLAN addresses (HID
+ NID) in overlapping HIPERLANs
—Provides lookup service between network
names and HIDs
—Converts IEEE-style MAC addresses to
HIPERLAN addresses
—Provides encryption of data for security
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#81
HIPERLAN, Cont..
MAC sublayer:
—Provides “multi hop routing” – certain
stations can perform store-and-forwarding of
frames
—Recognizes user priority indication (for timesensitive frames)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#82
HIPERLAN, Cont..
CAC sublayer:
—Non-preemptive priority multiple access
(NPMA) gives high priority traffic preference
over low priority
—Stations gain access to channel through
channel access cycles consisting of 4
phases:
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#83
HIPERLAN, Cont..
CAC sublayer:
1. Priority phase: if station has data with
priority N, will wait for N-1 priority slots and
transmit “priority assertion” burst in Nth slot
• If it hears another station of higher
priority, it will give up on this channel
access cycle
• Winning stations of same priority go into
next phase
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#84
HIPERLAN, Cont..
2. Elimination phase: each station will transmit
a burst of random length (geometrically
distributed number of time slots) and see if
channel is idle
• If channel is idle, it will go to next phase
• If busy, it will give up on this access cycle
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#85
HIPERLAN, Cont..
3. Yield phase: surviving stations will listen to
channel for a random number of time slots
(geometrically distributed)
• If it hears another station transmitting, it
will give up on this access cycle
• If channel is idle, it will begin to transmit
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#86
HIPERLAN, Cont..
4. Transmission phase: winning station will
transmit
• CAC is designed to give each station (of same
priority) equal chance to access the channel
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#87