Notes - Broadband Wireless

The following are notes from Computer Networks written by Tanenbaum 5th edition.

• Broadband wireless, large antenna on a hill easier than digging many trenches for cables
• 802.16 WiMAX(Worldwide Interoperability for Microwave Access)
• fairly complicated like OSI so there is a WiMAX forum to define standards

Comparison of 802.16 with 802.11 and 3G

• why not just use 802.11 or 3G?
• more akin to 4 G combines both aspects
• connect devices to internet at megabit/sec speeds without cable or DSL
• designed to
• carry IP packets over air
• peer to peer VoIP stream media
• OFD based technology
• more like 3G in that
• tries to achieve high capacity
• uses more power and better antennas
• licensed spectrum around 2.5 GHz
• LTE(Long term evolution)
• collision course with 4G

The 802.16 Architecture and Protocol Stack

• base stations connect directly into backbone network, air interface to mobile and subscriber stations
• the following is the protocol stack 

The 802.16 Physical Layer

• utilizes 3.5 GHz or 2.5 GHZ
• transmissions over OFDM
• Symbols are sent with QPSK, QAM-16, QAM-64
• SNR ratio in order to reach distant stations use QPSK sends 2 bits per symbol coded for forward error correction
• common for noisy channels to tolerate bit erros
• 802.16 developers did not like certain parts of GSM and DAMPS
• chose flexible scheme OFDMA(Orthogonal Frequency Division Multiple Access)
• different sets of subcarriers so that more than one station can send/receive at once
• also chose to use TDD(Time Division Duplex) to alternate between sending and receiving
• could have done FDD(Frequency Division Duplex) but this is not as flexible and harder to implement

• one of new uplink bursts reserved for ranging
• process by which new stations adjust timing and request initial bandwidth during base station setup
• is a hope and transmit setup assumes no collision

The 802.16 MAC Sublayer Protocol

• data link layer divided into 3 sublayers
• encryption to keep data secret
• perform mutual authentication using RSA public-key cryptography using X.509 certificates
• Uplink Channel Services
• Constant bit rate service
• transmit voice
• Real-time variable bit rate service
• transmit multimedia
• Non Real-time variable bit rate service
• file transfer
• Best effort service
• everything else
• ethernet binary exponential backoff algorithm used
• all are connection oriented

The 802.16 Frame Structure

• All MAC frames begin with generic header, followed by CRC
• checksum surprisingly optional since no attempt is made to retransmit

• Generic Frame
• EC bit tells whether payload is encrypted
• type identifies frame type
• tells whether packing and fragmentation are present
• CI indicates presence or absence of checksum
• EK tells which encryption is being used
• length gives complete length of entire frame
• connection identifier tells which connection this frame belongs too
• Header CRC uses polynomial x⁸ + x² + x + 1
• Bandwidth request frame is different, starts with 1 bit instead of 0 and is just a command

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