CS Sublayer

8/8/2019 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

i


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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

N k 'd


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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

N k E 'd


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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

N t k t i


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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)

N t k t t'd


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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

S h d li i


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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

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