Wireless Mesh Networks

Wireless Mesh Networks
Anatolij Zubow Wireless Metropolitan Area Network (WMAN)

Contents IEEE 802.16 family of standards Protocol layering
TDD frame structure MAC PDU structure Dynamic QoS management OFDM PHY layer 2

IEEE The standard IEEE defines the air interface, including the MAC layer and multiple PHY layer options, for fixed Broadband Wireless Access (BWA) systems to be used in a Wireless Metropolitan Area Network (WMAN) for residential and enterprise use. IEEE is also often referred to as WiMax. The WiMax Forum strives to ensure interoperability between different implementations - a difficult task due to the large number of options in the standard. IEEE cannot be used in a mobile environment. For this purpose, IEEE e is being developed. This standard will compete with the IEEE standard (still in early phase). 3

IEEE standardization The first version of the IEEE standard was completed in It defined a single carrier (SC) physical layer for line-of-sight (LOS) transmission in the GHz range. IEEE a defined three physical layer options (SC, OFDM, and OFDMA) for the 2-11 GHz range. IEEE c contained upgrades for the GHz range. IEEE d contained upgrades for the 2-11 GHz range. In 2004, the original standard, 16a, 16c and 16d were combined into the massive IEEE standard. 4

Uplink / downlink separation
IEEE offers both TDD (Time Division Duplexing) and FDD (Frequency Division Duplexing) alternatives. Wireless devices should avoid transmitting and receiving at the same time, since duplex filters increase the cost: TDD: this problem is automatically avoided FDD: IEEE offers semi-duplex operation as an option in Subscriber Stations. (Note that expensive duplex filters are also the reason why IEEE WLAN technology is based on CSMA/CA instead of CSMA/CD.) 5

Uplink / downlink separation (2)
Frame n-1 Frame n Frame n+1 … … TDD Adaptive … … Frequency 1 FDD … … Frequency 2 Semi- duplex FDD … … Downlink … … Uplink 6

IEEE PHY IEEE specifies three PHY options for the 2-11 GHz band, all supporting both TDD and FDD: WirelessMAN-SCa (single carrier option), intended for a line-of-sight (LOS) radio environment where multipath propagation is not a problem WirelessMAN-OFDM with 256 subcarriers (mandatory for license-exempt bands) will be the most popular option in the near future WirelessMAN-OFDMA with 2048 subcarriers separates users in the uplink in frequency domain (complex technology). 7

IEEE 802.16 basic architecture
BS SS Subscriber line replacement Fixed network AP Point-to-multipoint transmission AP SS WLAN BS = Base Station SS = Subscriber Station SS 8

IEEE 802.16 protocol layering
ATM transport IP transport Like IEEE , IEEE specifies the Medium Access Control (MAC) and PHY layers of the wireless transmission system. The IEEE MAC layer consists of three sublayers. Service Specific Convergence Sublayer (CS) MAC MAC Common Part Sublayer (MAC CPS) Privacy sublayer Physical Layer (PHY) 9

IEEE 802.16 protocol layering (2)
ATM transport IP transport CS adapts higher layer protocols to MAC CPS. Service Specific Convergence Sublayer (CS) CS maps data (ATM cells or IP packets) to a certain unidirectional connection identified by the Connection Identifier (CID) and associated with a certain QoS. MAC MAC Common Part Sublayer (MAC CPS) Privacy sublayer May also offer payload header suppression. Physical Layer (PHY) 10

ATM transport IP transport MAC CPS provides the core MAC functionality: System access Bandwidth allocation Connection control Service Specific Convergence Sublayer (CS) MAC MAC Common Part Sublayer (MAC CPS) Note: QoS control is applied dynamically to every connection individually. Privacy sublayer Physical Layer (PHY) 11

ATM transport IP transport Service Specific Convergence Sublayer (CS) MAC MAC Common Part Sublayer (MAC CPS) The privacy sublayer provides authentication, key management and encryption. Privacy sublayer Physical Layer (PHY) 12

ATM transport IP transport Service Specific Convergence Sublayer (CS) IEEE offers three PHY options for the 2-11 GHz band: WirelessMAN-SCa WirelessMAN-OFDM WirelessMAN-OFDMA MAC MAC Common Part Sublayer (MAC CPS) Privacy sublayer Physical Layer (PHY) 13

OFDM Frame Format (TDD)
The following slides present the overall IEEE frame structure for TDD. It is assumed that the PHY option is WirelessMAN-OFDM, since this presumably will be the most popular PHY option. The general frame structure is applicable also to other PHY options, but the details may be different. Frame n-1 Frame n Frame n+1 Frame n+2 Frame length 0.5, 1 or 2 ms 14

OFDM Frame Format (TDD) (2)
Frame n-1 Frame n Frame n+1 Frame n+2 DL subframe UL subframe DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 UL PHY burst k … TDM signal in downlink For initial ranging For BW requests TDMA bursts from different subscriber stations (each with its own preamble) Adaptive 15

DL subframe structure … … … UL PHY burst k DL PHY PDU Contention
slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n The DL subframe starts with a preamble (necessary for frame synchronization and equalization) and the Frame Control Header (FCH) that contains the location and burst profile of the first DL burst following the FCH. The FCH is one OFDM symbol long and is transmitted using BPSK modulation. 16

DL subframe structure (2)
UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n The first burst in downlink contains the downlink and uplink maps (DL MAP & UL MAP) and downlink and uplink channel descriptors (DCD & UCD). These are all contained in the first MAC PDU of this burst. The burst may contain additional MAC PDUs. 17

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n DL MAP UL MAP DCD UCD The downlink map (DL MAP) indicates the starting times of the downlink bursts. 18

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n DL MAP UL MAP DCD UCD The uplink map (UL MAP) indicates the starting times of the uplink bursts. 19

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n DL MAP UL MAP DCD UCD The downlink channel descriptor (DCD) describes the downlink burst profile (i.e., modulation and coding combination) for each downlink burst. 20

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n DL MAP UL MAP DCD UCD The uplink channel descriptor (UCD) describes the uplink burst profile (i.e., modulation and coding combination) and preamble length for each UL burst. 21

Modulation and coding combinations
BPSK QPSK 16-QAM 64-QAM Coding rate 1/2 3/4 2/3 Info bits / subcarrier 0.5 1 1.5 2 3 4 4.5 Info bits / symbol 88 184 280 376 568 760 856 Peak data rate (Mbit/s) 1.89 3.95 6.00 8.06 12.18 16.30 18.36 Depends on chosen bandwidth (here 5 MHz is assumed) 22

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n BPSK … … 64 QAM Downlink bursts are transmitted in order of decreasing robustness. For example, with the use of a single FEC type with fixed parameters, data begins with BPSK modulation, followed by QPSK, 16-QAM, and 64-QAM. 23

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Sorry, I cannot decode … Preamble FCH DL burst 1 DL burst n BPSK … … 64 QAM A subscriber station (SS) listens to all bursts it is capable of receiving (this includes bursts with profiles of equal or greater robustness than has been negotiated with the base station at connection setup time). 24

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n A subscriber station (SS) does not know which DL burst(s) contain(s) information sended to it, since the Connection ID (CID) is located in the MAC header, not in the DL PHY PDU header. 25

UL PHY burst k DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 … … … Preamble FCH DL burst 1 DL burst n … MAC PDU 1 MAC PDU k pad IEEE offers concatenation of several MAC PDUs within a single transmission burst. 26

UL subframe structure … DL PHY PDU Contention slot A Contention slot B
UL PHY burst 1 UL PHY burst k … The uplink subframe starts with a contention slot that offers subscriber stations the opportunity for sending initial ranging messages to the base station (corresponding to RACH operation in GSM). A second contention slot offers subscriber stations the opportunity for sending bandwidth request messages to the base station. 27

UL subframe structure (2)
DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 UL PHY burst k … The usage of bandwidth request messages in this contention slot (and response messages in downlink bursts) offers a mechanism for achieving extremely flexible and dynamical operation of IEEE systems. Bandwidth (corresponding to a certain modulation and coding combination) can be adaptively adjusted for each burst to/from each subscriber station on a per-frame basis. 28

Example: Efficiency vs. robustness trade-off
Large distance => high attenuation => low bit rate SS 64 QAM BS SS 16 QAM SS QPSK 29

UL subframe structure (3)
DL PHY PDU Contention slot A Contention slot B UL PHY burst 1 UL PHY burst k … UL PHY burst = UL PHY PDU … Preamble MAC PDU 1 MAC PDU k pad Preamble in each uplink burst. IEEE offers concatenation of several MAC PDUs within a single transmission burst also in uplink. 30

MAC PDU structure 6 bytes 0 - 2041 bytes 4 bytes MAC Header
MAC Payload CRC-32 Two MAC header formats: 1. Generic MAC header (HT=0) 2. Bandwidth request header (HT=1) MAC payload contains management message or user data For error control No MAC payload, no CRC 31

Generic MAC header Length of MAC PDU in bytes (incl. header)
Connection ID (CID) is in MAC header! 32

OFDMA Frame Format (TDD)
OFDMA is a multi-user version of the popular OFDM digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the next figure. This allows simultaneous low data rate transmission from several users. 33

OFDMA Frame Format (TDD) and Access
34

QoS in 802.16 - Four service classes
The IEEE MAC layer defines four service classes: Unsolicited Grant Service (UGS) Real-time Polling Service (rtPS) Non-real-time Polling Service (nrtPS) Best Effort (BE) service The scheduling algorithms needed for implementing the three first types of services are implemented in the BS (while allocating uplink bandwidth to each SS) and are not defined in the standard. Each SS negotiates its service policies with the BS at the connection setup time. QoS increases 35

Unsolicited grant service (UGS)
UGS offers fixed size grants on a real-time periodic basis, which eliminates the overhead and latency of SS requests and assures that grants are available to meet the flow’s real-time needs. The BS provides fixed size bursts in the uplink at periodic intervals for the service flow. The burst size and other parameters are negotiated at connection setup. Typical UGS applications: E1/T1 links (containing e.g. delay-sensitive speech signals), VoIP (without silence suppression). UGS rtPS nrtPS BE 36

Real-time Polling Service (rtPS)
The Real-time Polling Service (rtPS) is designed to support real-time service flows that generate variable size data packets on a periodic basis, such as VoIP (with silence suppression) or streaming video. This service offers real-time, periodic, unicast request opportunities, which meet the flow’s real-time needs and allow the SS to specify the size of the desired uplink transmission burst. This service requires more request overhead than UGS, but supports variable grant sizes for optimum data transport efficiency. UGS rtPS nrtPS BE 37

Non-real-time Polling Service (nrtPS)
The Non-real-time Polling Service (nrtPS) is designed to support non-real-time service flows that require variable size bursts in the uplink on a regular (but not strictly periodic) basis. Subscriber stations contend for bandwidth (for uplink transmission) during contention request opportunities. The availability of such opportunities is guaranteed at regular intervals (on the order of one second or less) irrespective of network load. UGS rtPS nrtPS BE 38

Best Effort (BE) service
The Best Effort service is intended to be used for best effort traffic where no throughput or delay guarantees are provided. Subscriber stations contend for bandwidth (for uplink transmission) during contention request opportunities. The availability of such opportunities depends on network load and is not guaranteed (in contrast to nrtPS). UGS rtPS nrtPS BE 39

Radio Link Control in IEEE 802.16
The main task of Radio Link Control (RLC) in IEEE systems is to provide dynamic changing of UL and DL burst profiles on a per-connection and per-frame basis, depending on radio channel characteristics and QoS requirements. As an example, RLC provides signaling for initial access (ranging) and bandwidth allocation in the downlink direction: Ranging request (RNG-REQ) from SS to BS Ranging response (RNG-RSP) from BS to SS Bandwidth requests (DBPC-REQ) from SS to BS Bandwidth confirmation (DBPC-RSP) from BS to SS 40

Initial access (initial ranging)
RNG-REQ RNG-RSP DBPC-REQ DBPC-RSP During initial access, the SS sends a ranging request message in the contention slot reserved for this purpose, among others indicating which kind of DL burst profile should be used. Note: There is the possibility of collision since other subscriber stations also send ranging request messages in this contention slot. Contention slot A Contention slot B UL PHY burst 1 UL PHY burst 2 … … UL traffic 41

Initial access (initial ranging)
RNG-REQ RNG-RSP DBPC-REQ DBPC-RSP In response to the RNG-REQ message, the BS returns a ranging response message in a DL burst with a sufficiently robust burst profile. This message includes the timing advance value for correct alignment of bursts in UL, as well as UL power control information. DL PHY burst … … Preamble FCS DL burst 1 DL burst k DL burst n Timing Advance (TA) ist ein Wert der zur Synchronisation zwischen Uplink und Downlink verwendet wird. 42

DL burst profile change
RNG-REQ RNG-RSP DBPC-REQ DBPC-RSP The SS continuously measures the radio channel quality. If there is a need for change in DL burst profile, the SS sends a DL burst profile change request message in the contention slot reserved for this purpose, indicating the desired new DL burst profile. Contention slot A Contention slot B UL PHY burst 1 UL PHY burst 2 … … UL traffic 43

DL burst profile change
RNG-REQ RNG-RSP DBPC-REQ DBPC-RSP In response to the DBPC-REQ message, the BS returns a DL burst profile change response message confirming the new burst profile. This is done in a DL burst with the old burst profile (when changing to a less robust DL burst profile) or using the new burst profile (when changing to a more robust DL burst profile). DL PHY burst … … Preamble FCS DL burst 1 DL burst k DL burst n 44

Transition to a new DL burst profile
BS SS Channel measurement indicates that different DL burst profile should be used DL burst profile n is used DBPC-REQ DBPC-RSP DL data is sent using new burst profile k DL burst profile k is used 45

UL burst profile change
RNG-REQ RNG-RSP DBPC-REQ DBPC-RSP DCD The BS measures the UL signal quality and may request a change in UL burst profile as indicated in the downlink channel descriptor (DCD) within the first DL burst of the DL PHY burst. SS: Read DCD and change UL burst profile accordingly DL PHY burst … … Preamble FCS DL burst 1 DL burst k DL burst n 46

Dynamic QoS management in practice
The request-response mechanism described on the previous slides is designed to be scalable, efficient, and self-correcting. While extensive bandwidth allocation and QoS mechanisms are specified in the IEEE standard, the details of scheduling and reservation management have not been standardized and thus provide an important mechanism for vendors to differentiate their equipment. (There is a similar situation regarding standardization of a transmission system in general: the transmitted signal is standardized in detail, whereas receivers can process the received signal as they like, using innovative technology.) 47

Three types of management connections
When a subscriber station acesses the network, three types of management connections are established between the SS and the BS (before transport connections can be established): Basic management connection for exchange of short, delay-critical MAC management messages Primary management connection for exchange of longer, more delay tolerant MAC management messages Secondary management connection for exchange of delay tolerant IP-based messages, such as used during DHCP transactions. 48

Connection establishment
Channel acquisition: The MAC protocol includes an initialization procedure designed to eliminate the need for manual configuration. Upon installation, the SS scans for a suitable BS downlink signal. The SS synchronizes to this signal and reads the downlink channel descriptor (DCD) and uplink channel descriptor (UCD) information in the first DL burst of the DL PHY PDU, in order to learn the modulation and coding schemes used on the carrier. Initial ranging and negotiation of SS capabilities (1): Upon learning what parameters to use for its initial ranging signal in UL, the SS looks for initial ranging opportunities by scanning the UL-MAP information present in every frame. After a random backoff time, the SS sends the ranging request message (RNG-REQ) to the BS. The BS calculates the timing advance value that the SS must use in UL from now on, and sends this information to the SS in the RNG-RSP message. 49

Connection establishment (2)
Initial ranging and negotiation of SS capabilities (2): The BS also sends power control information, as well as the CID for the basic management connection and the primary management connection to the SS. Using the primary management connection, the SS reports its PHY capabilities, including the modulation and coding schemes it supports and whether, in an FDD system, it is half-duplex or full-duplex. The BS, in its response, can deny the use of any capability reported by the SS. SS authentication (using privacy sublayer): Each SS contains both a manufacturer-issued factory-installed X.509 digital certificate and the certificate of the manufacturer. After sending these certificates (and the public key of the SS) to the BS, the BS can authenticate the SS. If authentication is successful, the BS sends the Authorization Key (AK), encrypted with the public key of the SS, to the SS. The AK is used both by SS and BS for securing further information flow. 50

SS registration: Upon successful authentication, the SS registers with the network. The response from the BS contains the CID for a secondary management connection. Contrary to the basic and primary management connection, this secondary management connection is secured. At this point, the capabilities related to connection setup and MAC operation, as well as the IP version used, are determined. (Remember that the secondary management connection is IP-based.) 51

IP connectivity: After registration, using the secondary management connection, the SS is allocated an IP address via DHCP and establishes the time of the day via the Internet Time Protocol. The DHCP server also provides the address of the TFTP server from which the SS can request a configuration file. This file provides a standard interface for providing vendor-specific configuration information. Connection setup: Finally, the secondary management connection is also used for setting up one or more transport connections. These transport connections carry the actual user data (IP traffic, VoIP traffic, etc.) between BS and SS. IEEE uses the concept of service flows to define unidirectional transport of packets on either downlink or uplink. Service flows are characterized by a certain set of QoS parameters, and are established using a three-way handshaking establishment procedure. 52

Summary: Dynamic QoS management
In summary, IEEE offers the following mechanisms for dynamically managing QoS and bandwidth: In the PHY layer by adjusting the DL and UL burst profiles (modulation and coding combination) on a per-frame basis. In the MAC layer through fragmentation and packing (both can be done at the same time). At higher protocol layers by using scheduling algorithms in the base station. These algorithms are not specified in the IEEE standard. 53

WirelessMAN-OFDM PHY WirelessMAN-OFDM is based on 256 subcarriers, of which 200 subcarriers are used: 192 data subcarriers + 8 pilot subcarriers. There are 56 ”nulls” (center carrier, 28 lower frequency and 27 higher frequency guard carriers). 54

Modulation and coding affect user data rate
The 192 data subcarriers carry 192 data symbols in parallel (= transmitted at the same time). Each symbol carries 1 bit (BPSK), 2 bits (QPSK), 4 bits (16-QAM), or 6 bits (64-QAM) of channel information (corresponding to the channel bit rate after channel coding, not to be confused with the user bit rate before channel coding). The inner convolutional coding reduces the usable number of bits to 1/2, 2/3, or 3/4 of the channel information. The outer Reed-Solomon block coding furthermore reduces the usable number of bits about 10 %. 55

Subcarrier signal in time domain
Guard time for preventing intersymbol interference In the receiver, FFT is calculated only during this time Tg Tb Next symbol Time Ts IEEE offers four values for G = Tg/Tb: G = 1/4, 1/8, 1/16 or 1/32. (802.11a/g offers only one value: G = 1/4) 56

Subcarrier signal in time domain (2)
Tg Tb Next symbol Time IEEE offers various bandwidth choices. The bandwidth is typically an integer multiple of 1.25, 1.5 or 1.75 MHz. (802.11a/g offers only a fixed channel bandwidth: MHz) Since the number of subcarriers is fixed, a certain bandwidth is translated into a certain subcarrier spacing Df = 1/Tb. 57

802.16e (Mobile WiMAX) PHY MAC Extensive Security Enhancements
Kept the fixed PHYs but added “Scalable OFDMA” (128, 512, and 1024 FFT) Expanded MIMO Support MAC Hybrid ARQ Handover Support (Make-Before-Break & Macro-Diversity Handover) Sleep mode Idle mode (paging) Scanning for neighbors Advertisement of neighbors Efficient network re-entry phase Extensive Security Enhancements 58

Resources 802.16(e) specification
HUT Communications Laboratory, “Wireless Personal, Local, Metropolitan and Wide Area Networks” Rohde & Schwarz, 59

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