802.16 English

7/28/2019 802.16 English

1/44

WMAN

IEEE 802.16

7/28/2019 802.16 English

2/44

An introduction to 802.16The IEEE 802.16 Working Group on Broadband

Wireless Access Standards develops standards forbroadband Wireless Metropolitan Area Networks. IEEE802.16 is a part of the IEEE 802 LAN/MAN Standards

Committee, the international forum for wirelessnetworks standardization.
http://ieee802.org/http://ieee802.org/http://ieee802.org/http://ieee802.org/http://ieee802.org/http://ieee802.org/http://ieee802.org/

7/28/2019 802.16 English

3/44

The IEEE 802.16 Task Group d developed,with the reference IEEE PAR 802.16-REVd, arevision of IEEE 802.16 ("Air Interface for

Fixed Broadband Wireless Access Systems")as modified by IEEE 802.16a e 802.16c. Thecomplete standard has been approved on

24th June 2004 and then published as IEEEStandard 802.16-2004, replacing 802.16-2001, 802.16c-2002, and 802.16a-2003.

An introduction to 802.16
http://standards.ieee.org/board/nes/projects/802-16-REVd.pdfhttp://ieee802.org/16/arc/802-16list2/msg01651.htmlhttp://ieee802.org/16/arc/802-16list2/msg01651.htmlhttp://standards.ieee.org/board/nes/projects/802-16-REVd.pdf

7/28/2019 802.16 English

4/44

The base components of a WiMAX system (WiMAX is the commercial name adopted bythe WiMAX Forum, a consortium of device

manufacturers and network providers thatsupports the development of the IEEE 802.16standards family) are:

subscriber station(SS)

base station(BS)


7/28/2019 802.16 English

5/44


A BS and one or more SS compose a cell witha point-multipoint structure (P2MP).

The BS controls the access to the transmissive

medium, allocating resources to the SS,respecting QoS restraints and regulatingnetwork access with dedicated securitymechanisms.

7/28/2019 802.16 English

6/44

An introduction to 802.16 An 802.16 system often uses a fixed antenna at the SS site ;

that antenna is typically installed on a roof.

The standard optionally supports adaptive antenna systems(AAS) and subchannel techniques to improve the link quality,e.g. for indoor installations.

The 802.16e subcommittee, apart from studying mobilityextensions, has the purpose of handling power control forbattery operated terminals.

A BS uses a sectorized and/or directional antenna or anomnidirectional one.

The fixed SS typically use directive antennas, while mobile orportable ones have omni antennas.

7/28/2019 802.16 English

7/44

An introduction to 802.16 A group of BS can be configured to build a wireless

cellular network. Using OFDM we can have a cell radius up to 45 km, but

of course that requires a favourable radio environment,

free from interferences, and we can obtain only thelowest speeds ; typical cell dimensions have a cellradius of less then 7 km.

The 802.16 standard can manage P2P or meshgeometries, using couples of directive antennas. Thatcan be used to extend network reach beyond thatobtainable with P2MP.

7/28/2019 802.16 English

8/44

An introduction to 802.16 WiMAX supports TDD and FDD modes, as well as

various channel bandwidths. The OFDM PHY ( also called WirelessMAN-OFDM) is

used for frequencies between 2 and 11 GHz.

802.16 MAC has many interesting characteristics thatallow for an efficient control of BS and SS access ; thetiming is based on frames divided into slots ; the size offrames and slots is changing every frame , undercontrol of the allocation logic of the BS, that insuresthat the QoS of active links is always respected.

7/28/2019 802.16 English

9/44

An introduction to 802.16 The MAC level delivers a connection oriented service

to higher stack levels; every connection has its QoSthat is guaranteed and controlled over time by theMAC itself. Each SS can anyway ask the BS todynamically change the QoS without dropping theconnection. MAC level QoS service can have 4variations:

1. Constant data rate

2. real time polling3. non real time polling

4. best effort

7/28/2019 802.16 English

10/44

An introduction to 802.16 Amedia access control packet data unit(MPDU) is

transmitted in PHY level slots.

An MPDU encapsulates media access control servicedata units(MSDU); those are the packets exchangedbetween MAC level and higher level ones.

MPDUs are exchanged between MAC level and lowerlevels (PHY); an MSDU can be fragmented in moreMPDUs, or many MSDU can be aggregated in a single

MPDU, as well as some MSDU fragments can bepacked in an MPDU.

ARQ can be used to ask for retransmission ofunfragmented MSDUs or for MSDU fragments.

7/28/2019 802.16 English

11/44


The MAC has a privacysublevel that runsauthentication, key

exchange and MPDUciphering.

802.16 protocol levels

are shown in thepicture.

7/28/2019 802.16 English

12/44

An introduction to 802.16 Using various coding options and a very flexible

PHY level modulation, added to slot and frameallocations dynamically adjustable, withpowerful QoS mechanisms, it is possible with

802.16 to deliver voice and broadband dataservices in cells with a huge choice of size,propagation environment and users type.

Various convergence sublevels, layered overthe MAC level, implement in a transparent wayEthernet,ATM,TDM and IP services.

7/28/2019 802.16 English

13/44


WiMAX defines a set of interoperable system profiles for themost common radio frequencies, both licensed and unlicensed; this permits 802.16 compatible device to be used in variousradio world configurations.

System profiles are built out of one of the MAC base profiles(P2MP or mesh) combined with one out of the 6 PHY profilesshown in the table above. Essentially we have 3 channelwidths (1,75 3,5 and 7 MHz) in the 3,5 GHz ETSI band, 2channel widths (3 and 5,5 MHz) for MMDS and a channel widthof 10 MHz for unlicensed bands.

7/28/2019 802.16 English

14/44


In the picture we can see the transmission chain for 802.16WirelessMAN OFDM ; this solution has been selected oversingle-carrier (SC) and CDMA because it has bettercharacteristics in NLOS applications, drastically simplifying theequalizator needed for operation in environments that showrelevant multipath propagation fenomena.

7/28/2019 802.16 English

15/44


Reed-Solomon and convolutional coding are FECtechniques that are used in the WirelessMAN OFDMPHY.

In the table we show the code rateobtained byapplying both codes according to the modulationtype.

7/28/2019 802.16 English

16/44


In WirelessMAN OFDM FFT size has been fixed to 256; in such a block 55subcarriers ( 28 low and 27 high ones) are reserved for guard band and 8are reserved for pilot.

Having 192 useful subcarriers, the system nevertheless uses variousoversampling ratios, 8/7 and 7/6, to maximize efficiency even if it remainswithin the spectral masks required by the various national norms.

The guard time interval is quite wide compared to the duration of an OFDMsymbol; we can have a maximum of 25% and a minimum of 3% infavourable conditions.

7/28/2019 802.16 English

17/44


In the table we displayed RMS delay spead values for an omni antenna in the various 802.16achannel models. The cell radius has been set to 7 km. It is important to note the 4 terrain

types:

1. Terrain type A: maximumpath loss, hilly area with density of trees medium to high

2. Terrain type B:path lossintermediate

3. Terrain type C: low loss, land mostly flat with low density of trees

The worst case, 5,24 microseconds, can be tolerated with the guard time option whenusing 10 MHz channels; in a mobile environment with omni antennas in the SS a 10

microseconds delay spread can be compensated with guard time for a 5 MHz bandwidth.

7/28/2019 802.16 English

18/44


In the table we notice various data rates obtainable using the allowed

combinations of bandwidth and modulation type and code rate, using a1/32 guard time; we also considered PHYoverheadbut not MAC andpreamble ones.

Optionally we can also use uplink subchanneling; this is quite useful for

limited power transmitters ( e.g.: a notebook) in indoor or mobileenvironments. With a subchanneling factor of 1/16 we can have a link gainof 12 dB. We define therefore 16 sets of 12 subcarriers that can be whollyassigned or in 1,2,4,8 subsets to an SS uplink.

7/28/2019 802.16 English

19/44

An introduction to 802.16 To handle variations over time of channel characteristics, 802.16 allows an

optional more frequent rate of preambles; in the uplink path we can insert

with programmable frequency some short preambles,that are called mid-ambles for this special usage. The available options are:

1. Only preamble

2. Mid-amble every L=8 symbols



In the downlink optionally we can have a short preamble at the start of

every frame. The BS scheduler must in fact guarantee a maximummandatory interval before sending the informations needed for a channelestimation.

A maxumum SS speed of 150 km/h can be managed with L better that 10,if we use appropriate interpolation schemas for channel characteristics

estimation.

7/28/2019 802.16 English

20/44

An introduction to 802.16 In 802.16 the PHY is in charge of radio channel framing, of

shared media access and of slot allocation. The MAC assignsMSDU into MPDU; this is slightly higher than what happenscommonly.

The radio transmission time is divided into frames; in an FDDsystem we have uplink frames( from SS to BS ) e downlinkframes( from BS to SS ) that are temporally aligned overseparate channels. In a TDD system every frame is split in adownlink and an uplink subframe.

In both TDD ed FDD modes the frame length can be variedunder BS scheduler control in each frame; in TDD mode thesplitting point between uplink and downlink changes too, and

the time can be allocated asymmetrically between the twodirections.

7/28/2019 802.16 English

21/44

An introduction to 802.16 The downlink frame includes a preamble, a DL_MAP, an

UL_MAP and a downlink slots set. The DL_MAP is a map of the slots in the downlink subframe.

The UL_MAP is a map of the slots in the uplink subframe.

With those 2 frame descriptors the BS allocates channel accessboth for down and uplink.

The SS uses the DL_MAP to identify MPDUs within the frame

and gets them sequentially, receiving only those whoseconnection ID is owned by the SS.

7/28/2019 802.16 English

22/44

An introduction to 802.16 The uplink frames structure is more complex because for the

best effort service and for the initial network connection weneed a contention based multiple access , to allow for multipleSS that want to access at the same time.

According to connections QoS, each connection can have a

guaranteed slot, can have a polled slot, or must be contendingfor a TDMA slot.

This last type happens only in slots reserved for this type ofoperation, divided among initial connection slots and

bandwidth request slots. Those slots are then divided intominislots.

The contending SS use an exponential truncated binary backoffalgorithmto decide which minislot they should use.

7/28/2019 802.16 English

23/44

An introduction to 802.16 The initial ranging contentionslot is used as part of the

network access procedure. An SS send a ranging request (RNG-REQ) in such a slot. This

packet has a long preamble, because the BS must find theexact timings used by the SS.

When the BS receives an RNG-REQ, it answers with a RNG-RSPranging responsethat sends back to the SS the timing andpower control informations.

The SS can then set its own timings according to the transittime and to the path loss; in this way its transmissions canreach the BS with the same alignment and level of the otherSS.

7/28/2019 802.16 English

24/44

An introduction to 802.16 The bandwith request contentionslot is used by the

SS to decide who can use the channel. The bandwidth requests are sent in this slot..

Evey time a request has been authorized by the BS,

the SS can transmit in the slots that the BS allocatedon purpose.

The BS decides the length of the contended slots too;

the optimal length is varied according to the numberof SS, number and QoS type of connections, currentnetwork activity and so on.

7/28/2019 802.16 English

25/44

An introduction to 802.16 In a 802.16 system MAC communicates using MAC protocol

data unit (MPDU) transported by PHY.

The MAC generic header (MGH) contains MPDU details, mainlyformed by:

1. Connection ID (CID) that identified the connection that ownsthe MPDU

2. Frame length

3. various bit that qualify CRC presence, subheaders, if thepayload has been ciphered using which key and so on

Optionally the frame can have a CCITT 32 bit CRC if needed.

7/28/2019 802.16 English

26/44

An introduction to 802.16 The payload can host both management and user

data. Some connection can have only management

purposes and therefore can carry only management

messages ; all the other channels are transportchannels that carry only user data.

The payload in a transport connection may contain awhole MSDU, MSDU fragments, MSDU aggregates,

MSDU fragment aggregates, bandwidth requests orARQ requests according to MAC rules for bandwidthrequests, fragmentation, packing and ARQ.

7/28/2019 802.16 English

27/44


Above you can see the frame formats for 802.16a

7/28/2019 802.16 English

28/44

An introduction to 802.16 To request variations to the guaranteed

specifications of a connection we send a 6byte bandwidth request header instead ofthe GMH

It is characterized by an header type bit(HT) set at 1 and by the encryption type bit(ET) set to 0. The 6 bit Type field can be 0to indicate an incremental request or 1 to

indicate and aggregate request.

The CID contains the connection identifierof the connection that has to be modified;it must not be the same one of the GMH,but it can be any one that belongs to the

SS. The BR field is the number of requested

uplink bytes.

The HCS is an 8 bit CRC for the previous 5bytes.

7/28/2019 802.16 English

29/44

An introduction to 802.16 The generic MPDU have both transport and

management informations, according tothe connection CID of their header. In factevery generic MPDU starts with a GMH,whose structure is shown in the picture.

A GHM is identified by HT bit set to zero.

The EC bit specifies if the frame is

cpihered, while the CI bit specifies if at theend of the MPDU there is an optional CRC.

The encryption key sequence (EKS) bitsidentify the key that has been used toencrypt the frame; the privacy sublevel

allows the contemporary use of more thanone key, so the stream can go onundisturbed during the key changeoverprocedure; EKS distinguishes between oldand new keys.

7/28/2019 802.16 English

30/44

An introduction to 802.16 The 11 bit long LEN field specifies the number of

bits of the MPDU including header and CRC, limitingthe maximum frame length to 2047 byte.

The CID indicates the connection of the GMH; theHCS is an 8 bit CRC of the previous bytes.

The Type fields indicates in 6 bits what is embeddedin the payload:

1. bit 0=1 if there is a grant management subheader

2. bit 1=1 if there is a packing subheader

3. bit 2=1 if there is a fragmentation subheader

4. bit 3=1 if the 2 previous subheader have anextended length

5. bit 4=1 if there is an ARQ feedback

6. bit 5=1 if there is a mesh subheader

7/28/2019 802.16 English

31/44


The grant management subheader is a fast way to ask for uplinkbandwidth without sending a full MPDU.

Every connessione, identified by a 16 bit CID, has its own QoS class, thatwe will define later on. If the GMH CID indicates an unsolicited grantservice (UGS) channel the subheader format is the one described in thepicture above.

The slip indicator (SI) is set by the SS when the uplink buffer of a certainstream is full, due to a small slip between uplink and arrival speeds ; itrequires therefore a small increase in uplink capability.

The poll me (PI) asks the BS to send a bandwidth poll.

7/28/2019 802.16 English

32/44


If the CID in the GMH indicates a channel that is using other schedulingservices (rtPS, nrtPS, BE) the subheader format is the one in the above

picture.

The piggyback request is a 16 bit number that represents the number ofuplink bytes needed ; it explicitely asks for bandwidth to the BS.

The previous format is used only in UGS because in this case thebandwidth grants are regular and implicit ; the data flow in bothdirection is quasi synchronous and the 2 flow bits are enough tomaintain the synchronization.

7/28/2019 802.16 English

33/44


An MSDU can be divided into fragments that are indipendentlytransmitted, using a fragment subheader (FSH) like in the picture above.

The FSH describes an MSDU fragment.

The fragment control bits (FC) indicates (10) if it is the first fragment,the last one (01) or another in the middle (11) ; the 00 combinationindicates a non fragmented MSDU.

The fragment sequence number (FSN) increments by 1 for eachfragment so we can know the correct order for receiver reassembly.

7/28/2019 802.16 English

34/44


Many MSDU or many MSDU fragments can be aggregated in a singleMPDU; this is the MAC-level packet aggregation.

A bit in the GMH indicates the presence of the packing subheader, that isshown in the picture above.

An MPDU can have more than a packing subheader, that may delimit anMSDU or an MSDU fragment.

An MSDU can therefore be split into fragments or packed into packedframes ; this allows the BS to better use the available slots.

We can notice that the packing and fragmentation subheaders are verysimilar, this explains the FC value of 00.

7/28/2019 802.16 English

35/44


The ARQ is the tool the allow both link sides to ask for MSDUritransmission, usually due to a bad reception.

We define a system block size and we consider the MSDU as built out

from a set of such blocks, apart from the last one that can have asmaller size.

When an 802.16 system has ARQ support, the FSH is replaced by anextended FSH an the PSH by an extended PSH, as shown in the pictureabove.

We replace the FSN with a block sequence number BSN, to allow ARQoperation. When we send an ARQ request, the blocks that have to beresent are identified with their BSN.

In an extended FSH the BSN indicates the first block of the fragment;the fragments must have an integral number of blocks to correctly alignthem.

7/28/2019 802.16 English

36/44


Similarly to FSH, the PSHis extended so as to usea block number BSN with11 bits instead of the 3bits long FSN.

To ask a blockretransmission (NACK) orwhen we have a correctreception (ACK) we

include in the payload anARQ feedback.

The ACK maps are abitmap of succesfullyreceived blocks.

7/28/2019 802.16 English

37/44

An introduction to 802.16 The 802.16 MAC is connection oriented.

Each connection has QoS parameters, is handled by one out of 4scheduler services, can have its own security association, can be amanagement or transport connection. It is identified by an unique CID;the connections are unidirectionali, therefore in a bidirectionalconnection we have 2 different CIDs.

Some CID are reserved for special purposes, like initial ranging andmulticast polling and broadcast.

Every SS, when it joins an active networks, allocates a base connection,a primary connetion and a secondary management connection. The BSreserves an m number of CID for the base,primary and secondary

connections that it has been setup to handle. When all the management connections have been setup, an SS can

then create the transport connections. The service streams associatedto the transport connections can join multicast groups and listen themulticast polling channels that have been associated to those groups.

7/28/2019 802.16 English

38/44


In the picture there is alist of 802.16connection IDs.

7/28/2019 802.16 English

39/44

An introduction to 802.16 A device that wants to join a network must run a series of tasks.

First it has to look for a downlink signal from a BS and synchronize to it. Once is

is capable of receiving the downlink, it can use the descriptor of the channeluplink (UCD) to identify the transmission parameters and the timing of the initialranging uplink contended slot.

The SS can then perform the initial ranging, that warns the BS of the presenceof the SS and provides the SS with the basic and primary management

connections. Using management messages on the primary connection the basic features of

the system are negotiated. Registration takes place which leads to theactivation of the secondary management link.

Before being allowed to continue the SS must obtain permission to access thesystem and a security association for their secondary managementconnection,to do this it is used the privacy and key management (PKM) protocol.

On completion of this phase, as an example an IP connection can be enabledvia the secondary management channel, for example by using TFTP to finishthe configuration of the various parts.

7/28/2019 802.16 English

40/44

An introduction to 802.16 When a connection has been setup, it must be

refreshed with a periodic ranging.

The ranging operation is used in the control loopsthat synchronize the timings and power levels ofBS and SS.

When the channel conditions vary, or the distancebetween SS and BS changes ( even if this is morea task for 802.16e) we must quickly adjust the

operating parameters. We use the same messages of initial ranging, but

we use in the RNG-REQ the base connection CIDinstead of 0.

7/28/2019 802.16 English

41/44

An introduction to 802.16 The 802.16 security techniques are derived from

the Docsis ones. The MPDU encryption in transport connections and

in the secondary management one is done with

cipher block chaining (CBC) DES.

The PKM protocol, based on certificates, authorizesSS and transfers the keys between BS and SS.

Since the encryption is done on the payload of theMPDU, it hides all the details of fragmentation,

packaging, authorization requests and ARQ.

7/28/2019 802.16 English

42/44

An introduction to 802.16 The PKM protocol uses RSA asymmetrical key protocols and methods to

authenticate an SS to a BS.

The SS delivers its X.509 certificate to the BS, showing its identity andpublic key.

The BS sends back to the SS an authorization key encrypted with SSpublic keys, that decrypts it with its own private key.

The autorization key is then derived to have the key enciphering keys(KEK).

The authorization key is known both to BS and SS, and the two partiescompute the same KEK.

A temporal ciphering key (TEK) is sent from the BS to the SS, afterciphering it with EDE DES using a KEK.

The TEK is then used in DES-CBC mode to encrypt MPDU data.

7/28/2019 802.16 English

43/44

An introduction to 802.16 The convergence sublevels (CS) map 802.16 MAC services

that are connection oriented into a standard network

interface.

The 2 specified levels are the l'ATM CS and the Packet CS.

The ATM sublayer transports ATM connections.

The Packet sublayer is composed by 3 base modes:

1. IP Specific, transports IP native frames and supports IPv4,IPv6 ed IP mobile

2. IEEE Std 802.3, transports Ethernet 802.3 frames

3. IEEE Std 802.1Q-1998, transports VLAN according to802.1Q

7/28/2019 802.16 English

44/44

An introduction to 802.16 The task group 802.16e has been working to add mobile

support to 802.16 ; the amendment has been approved asIEEE Std 802.16e-2005

An 802.16e MSS is able to use broadband services, doinghandovers, using an omni antenna with power savingtechniques.

The 802.16e specification add further securityspecifications, like AES-CCM encryption and EAP

authentication.

Date post:	03-Apr-2018
Category:	Documents
Upload:	wasyhun-asefa
View:	231 times
Download:	0 times

802.16 English

Documents