Download - CS Sublayer
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Convergence Sublayer & Common
Part Sublayer Service Specific
Convergence Sublayer
MAC Common PartSublayer
Network Entry and
Initialization
167677 Timo Salminen
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MAC Convergence Sublayer
Functions:
Classification &
possible processing ofhigher-layer PDUs
Delivery to properMAC SAP
Receives CS PDUsfrom peer
Two sublayers
specified: ATM and
packet
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ATM Convergence Sublayer
ATM cells mapped to MAC frames
differentiates Virtual Path switched / Virtual
Channel switched ATM connections assigns channel ID (CID)
can perform Payload Header Suppression (PHS)
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PHS in ATM CS
12/8 bit VPI
(VP connections) or28/24 bit VPI+VCI
(VC connections)mapped to 16 bit CID
ATM header thrown
away, except for payloadtype indicator (PTI) and
cell loss priority field(CLP)
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ATM CS - features
Several ATM cells with same connection ID may
be packed to save bandwidth
Convergence sublayer supports common channel
signalling (CCS)
CCS mechanism in ATM uses separate connection forsignaling messages
every 802.16 station needs channel id for CCS signalling to MAC mapping left to vendors to
implement
associated signalling, proxy signalling or virtual
UNI not supported
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Packet convergence sublayer
used for all packet-based protocols,
such as IP and IEEE 802.3 (Ethernet)
similar functions as ATM convergencesublayer, including PHS
PHSI identifies the rules used forsuppression -> higher-layer PDU can be
rebuilt at other end
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Classification of Data
Both layers perform classification of data from
higher layers
Data associated with connection QoS made possible
Classifier: a set of matching criteria applied to
each packet
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Classifiers
Several classifiers can point to same service flow
Multiple classifiers applied according to their priority
Can use various parameters (source address, destinationaddress, IP headers ...)
Classifiers also define use of PHS
Can be added by network management or dynamically
Give connection ID as output
Behaviour on data that doesn't match any classifiers is not
standardized
Possibly default CID used or packet discarded
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Classification
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Payload Header Suppression
Repetitive portion of header suppressed at sender
and restoring it at receiver
Repetitive portion replaced with index (PHSI) Suppression mask (PHSM): which bytes to
suppress
constant portions suppressed changing portions not suppressed (such as IP
sequence numbers)
Suppression valid -option (PHSV): is header
verified before suppression
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More on PHS
BS assigns all PHSI values
Either sender or receiver specifies suppression
field and suppression size (PHSF and PHSS) PHS intended for unicast (not defined for
multicast)
PHS rules generated by higher layer serviceentity
Higher layer is also responsible for constant
(suppressed) parts staying constant
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PHS Illustration
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PHS Operation
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CS operation summary
Classifier rules determine service flow, CID and
PHS Rule
If PHS used, sender uses suppression mask andsupresses chosen bytes, then prefixes frame with PHSIndex
Data sent to MAC SAP
Receiver determines associated CID
Packet reassembled using PHS Index
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MAC Common Part Sublayer
Defines multiple-access
mechanism
On downlink only basestation transmitting
no need for coordination
base station broadcasts,
stations retain onlymessages addressed tothem
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MAC CPS cont'd On uplink user stations have to share
Three basic principles used to determine right to transmit:unsolicited bandwidth grants, polling and contentionprocedures
Sharing dynamic and with on-demand basis
Sharing can depend on service classes (continuing rights totransmit / transmit right only on request)
Connection-oriented MAC Connections associated with preprovisioned service flows after
SS registration
QoS provided for service flows
Connection adding, modification and deletion can be static ord namic
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Duplexing
Duplexing possible in frequency or time domain
in frequency domain duplexing (FDD) uplink anddownlink on different frequencies
Adjusting width of frequency band devoted to links isdifficult -> static division between uplink and downlink
fixed duration frames -> allows different modulation types
in time-division duplexing (TDD) time divided intouplink and downlink periods
dynamic division between uplink and downlink
division can be controlled by higher layers
fixed duration frames
frame contains one downlink and one uplink subframe
integer number of slots / frame -> easy bandwidth
partitioning
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Addressing and Connections
Each SS has universal 48bit MAC address
Connections identified by 16-bit CID
used to distinguish between multiple uplink channelsassociated with the same downlink channel
also used with connectionless traffic (used as pointerto destination and context information)
many higher-layer sessions may share same CID(with same service parameters)
3 management connections in each direction
established automatically
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MAC Data Frame
MAC PDU consists of header, payload (optional) and CRC(optional)
Payload may have subheaders and several MAC PDUs (orPDU fragments)
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MAC Header Types
Two header formats defined, indicated by Header Type (HT) generic (HT 0): used for higher layer data and MAC management messages
bandwidth request (HT 1): no payload, identified by header
EC: Encryption Control, EKS: Encryption Sequence, HCS: HeaderCheck Sequence, LEN: Length, Type: payload type (which
subheaders present), BR: Bandwidth Request
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Features
Payload can be encrypted
BS responsible for refreshing keying material periodically
Use of CRC depends on connection ID
CRC calculated after encryption on header + payload
Multiple frames may be concatenated into singletransmission
may join all types: user data, bandwidth request frames andmanagement messages
One frame may be fragmented into several frames
efficient use of bandwidth relative to QoS
sequence numbers
uses Fragmentation subheader
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More Features
Several frames may be packed into single frame Packing subheaders may be used to mark beginning of
each frame -> allows retransmission of lost fragments
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MAC Management Messages
Handle ranging, registration, privacy and describingdownlink and uplink
link describing:
BS transmits channel uplink and downlink descriptor
messages (UCD and DCD) at periodic intervals UCD and DCD contain burst profile: info on modulation,
error-correction, preamble length, etc.
uplink and downlink map messages (UL-MAP, DL-MAP)define burst start times and allocate access to
corresponding link channel
ranging: subscriber stations trasmit ranging requestsat initialization and then periodically
determines power and burst profile changes
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Management Connections
3 management connections correspond to 3
different QoS levels of management traffic
basic connection: short delay
primary management connection: longer, more delay-
tolerant messages
secondary management connection: delay-tolerant,
standards-based messages (DHCP, SNMP etc.)
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Network Entry and Initialization
(subscriber station) 10 phases
1) Scan for downlink channel and synch with BS
first tries the channel used previously
on failure scans all possible channels
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Network entry cont'd
2) Get link parameters
BS transmits channel descriptor messages and link mapmessages periodically (addressed to all)
SS obtains downlink synch on reception of DL-MAP and staysin synch as long as it gets periodical DCD and DL-MAPmessages
SS builds set of usable uplink channel IDs from UCD-messages and checks channel parameters
when suitable uplink channel is found, time synchronization is
obtained from next DL-MAP
before transmitting SS has to wait for bandwidth allocationmap for chosen channel
uplink parameters are valid as long as SS gets periodical UCD
and UL-MAP messages
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Network Entry cont'd
3) perform ranging
SS needs to acquire correct timing offset to align it'stransmissions to the mini-slot boundaries
SS checks UL-MAP message for initial maintenance
interval interval is speficied by BS and is large enough to account
for delays
during the next initial maintenace interval SS sends
ranging request message connection ID is set to 0, since SS doesn't yet have ID
first message sent with minimum power level
if not successful, message is resent with more power until
successful
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Network entry - ranging
on successful reception BS sends back a ranging
response message message contains basic and primary mgt. CIDs, FR power
level adjustment, offset frequency adjustment and timingoffset
SS can then adjust signal accordingly initial ranging must be done at least once and SS tries
ranging on all suitable uplink channels
periodic ranging allows adjustments later on
ranging request transmitted with basic CID and containscurrent power level, time and frequency offset
response message contains required fine-tunings
each SS granted periodic ranging opportunity (period
depending on BS)
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Network entry cont'd
4) Inform BS of SS's basic capabilities BS responds with intersection of capabilities
5) Perform authorization and key exhange
each SS contains 48 bit IEEE-style MAC address andX.509 digital certificate
6) Registration
SS sends registration request BS responds with registration response, which
includes secondary management connection ID
SS is now authorized to forward traffic to network
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Network Entry cont'd
7) IP connectivity established on secondary
management connection
standard DHCP used to obtain IP address and
configuration
8) Current time and date required
necessary for timestamping
9) Exchanging operational parameters
10) Setting up preprovisioned service flows (if
mentioned in SS's service contract)
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Sources
T. Cooklev, Wireless Communications Standards,
A Study of 802.11, 802.15, and 802.16, IEEE
Press, 2004.
IEEE 802.16-2001, IEEE Standard for Local and
Metropolitan area networks Part 16: Air
Interface for Fixed Broadband Wireless Access
Systems
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MAC layer: Channel access and QoSSome enhancements
Antti [email protected]
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Some words used
SS = Subscriber station
BS = Base station
NLOS = Non-line of sight
PHY = physical layer
BW = bandwidth
DL = downlink UL = uplink
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QoS principles
Packets are associated with a service flow, which is thecentral concept of the MAC protocol
Service flow = an unidirectional flow of packets with aparticular QoS
The QoS parameters of a flow can be specified by giving aservice class name or explicitly
When data comes to mac layer, the convergence sublayer
gives it an connection ID (CID)
The service flow is mapped to this ID
Service class is optional; it may be implemented in BS
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Service class
Service class serves two purposes:
The burden of configuring service flows can be movedfrom provisioning server to BS
Higher-layer protocols can create service flows bytheir service class names
Service class = identifier for set of QoS parameter
set values
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Service flows
Service flows are identified by Service Flow Identifier (SFID),active ones have also Connection Identifier (CID)
Service flow has parameters like bandwidth, latency, jitter andother QoS-related variables
Service flow QoS parameters may:
include a reference to service class
override service class QoS
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Authorization
Two types: dynamic and static
On static, all services are defined in initial configuration of SS
On dynamic, the module communicates with a server that
gives information on authorization
Policy server
may provide the authorization module with advance notice
of upcoming admission and activation requests will specify the proper authorization action to be taken on
those requests
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Service flow types
Three types of flows: two-stage activation model
Provisioned:Flow is provisioned but not immediatelyactivated. BS assigns a SFID but does not reserve resources.Provisioned flow can be later activated by SS or BS.
Admitted: The resources are reserved but the flow isn'tactive yet.
Active: The flow is requesting and being granted bandwidth.
f f
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Modifying service flows
Service flows can be created, changed or deleted.
BS and SS can initiate a creation of dynamic flow
This is done by series of MAC management messages
Protocols are defined for modifying and deleting serviceflows: when a flow is deleted, all its resources are released
If a basic, primary or secondary management service flow
of an SS is deleted, the SS will have ro re-register itself
C i
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Connections
The MAC is connection-oriented
> All communications are in context of a connection
after installation & registration of SS, connectionswill be associated with service flows
one connection, one flow
When needed by customer, new connections willbe created or existing connections deleted.
Q S d b d idth
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QoS and bandwidth
SS requests uplink bandwidth separately for eachconnection
different connections, different bandwidth needs
BS grants bandwidth for entire SS
requests are assigned to ID, not to SS
SS can have several connections
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Scheduling services
Four types, everyone has some QoS parameters associated
UGS, Unsolicited Grant Service
real-time service flows, fixed packet size (VoIP)
rtPS, real-time Polling Service
real-time data, variable packet size (MPEG)
nrtPS, non-real-time Polling Service
non-real-time data, variable packet size (FTP)
BE, best-effort
best effort traffic
C lli i
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Collisions
SSs need to send a request for bandwidth to BS
BS grants some minislots from uplink channel for request intervalsto avoid collisions
> however, collisions may happen on these intervals
backoff counter is needed
If SS receives data grant from BS, the transmission was succesful
If not, SS will increase its backoff window by factor of two
this continues until a grant is received or maximum value ofbackuff window is achieved
S it
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Security
The MAC security sublayer has two componentprotocols
encapsulation protocol for data encryption
defines cryptocraphic suites i.e. pairings of data encryptionand authentication algorithms
the rules for applying those algorithms to a MAC payload
privacy key management (PKM)
describes how the BS distributes keys to client SS
Enhancements for 2 11GH
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Enhancements for 2-11GHz
Frequencies of 2-11GHz make the NLOS linkspossible
This freq. range requires a different physical layer
and some changes to MAC layer
Enhancements needed by point-to-multipoint andlicence-exempt bands
Multipath is significant
> losses over the wireless medium will increase
Mesh systems
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Mesh systems
Mesh systems are multipoint-to-multipointnetworks
Nodes are able to forward packets
Mesh terms
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Mesh terms
Downlink = traffic away from the mesh BS
Uplink = traffic towards the mesh BS
Neighbor = a direct link to another station Neighborhood = all neighbors of a node
Extended neighborhood = all neighbors of
neighbors of a node
The problem
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The problem
In mesh networks, the nodes are close to each other
they cannot transmit at the same time in the same channel
even the BS must coordinate the transmissions with other
stations
The main difference between mesh and a point-to-multipointnetworks is that the channel resources are shared between thesystems on demand basis
Two ways to assign channel resources
distributed scheduling: distributed decisions
centralized scheduling: the BS makes decisions
Coordinated distributed scheduling
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Coordinated distributed scheduling
All stations coordinate their transmissions in theextended neighborhood
Stations transmit periodically in the same channel
their own schedules and proposed schedulechanges
> scheduling does not rely on the BS
Uncoordinated distributed
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scheduling
Uncoordinated scheduling is used in ad-hoc typenetworks
Schedules are estabilished between two nodes
In uncoordinated scheduling, managementmessages can collide
In coordinated they are scheduled in control subframe
Centralized scheduling
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Centralized scheduling
Scheduling is performed by the BS
BS broadcasts the scheduling message to all itsneighbors and they rebroadcast it
BS gathers resource requests from all the SSswithin a certain treshold hop range and grantsresources for up- and downlink according to those
requests
Advanced antenna systems (AAS)
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Advanced antenna systems (AAS)
In 2-11 Ghz band, AASes can be used
In AAS, more than one antenna is used
more range and capacity
Spectral efficiency increases linearly with the number of antennas
Initially, only BS can have AAS
Migration from non-AAS to AAS system is easy and it does not
make an interrupt to services
The goal of 802.16 is to make possible systems that consist ofAAS-BS, some AAS-SSs and some non-AAS-SSs.
AAS frame structure
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AAS frame structure
framecontrol
downlink AAS downlink uplink AAS uplink
SS burst SS burst SS burst
AAS requirements for MAC layer
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AAS requirements for MAC layer
Control functionality
responsible for DL- and UL-map distribution andchannel description
Utility functionality
responsible for PHY-related information provided bythe MAC layer
Additional logical channels for AAS
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Additional logical channels for AAS Downlink synchronization channel
time and frequency synchronization
Downlink polling channel
needed if downlink traffic channels cannot be opened for SS in any other way
Downlink traffic channels
carries downlink user traffic as scheduled by BS
Uplink contention channel
for SS-initiated random access, including ranging and BW requests
Uplink traffic channels
carries uplink user traffic as scheduled by BS
Automatic repeat request (ARQ)
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Automatic repeat request (ARQ)
Multipath in 2-11GHz
> intersymbol interference
> reduced SNR
> increased BER & PER
For these issues, there is ARQ
ARQ is optional part of MAC layer an can be enabled on a per-
connection basis
parameters can be negotiated on connection start/change
no mixing of ARQ and non-ARQ traffic is allowed
ARQ in MAC
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ARQ in MAC
MAC frame consists of single header andfragmented payload(s)
When ARQ is used, MAC frame can contain ARQ
feedback payload preceeded by an appropriatesubheader
ARQ transmitter state machine
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ARQ transmitter state machine
not sent outstandingWaiting for
retransmission
done
discarded
ACK
transmit
Retransmit
timeout or NACK
ACK
fragment
lifetime
fragment
lifetime
ARQ discardsent andacknowledged
Dynamic frequency selection
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y a c eque cy se ect o
DFS is required when non-licenced bands are used
There are some primary users for some bands
wireless systems can communicate on these bands onlyif they can avoid the primary users with DFS
Dynamic frequency selection
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y q y
DFS provides following features
Requesting and reporting measurements
Probing channels for primary users
Detecting primary users
Ceasing operation on a channel if primary users arefound
Selecting and advertising new channel
DFS requires also monitoring by SS
Thank you!
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y