Cellular Internet Access - Computer Networks and Protocols - Lecture Notes, Study notes of Computer Networks

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Cellular internet access Tuesday, November 08, 2005 8:21 AM Docsity.com

Thursday, March 08, 2012 6:06 PM Docsity.com

Special signal runs at higher than data rate (chipping rate) 1: leave normal signal alone.

• 1: invert. Sends 1's and - 1's (mapped to 1's and 0's) Code-division multiple access (CDMA) 1's: higher frequency. 0's: lower frequency. CDMA match: all senders at same frequency mismatch: senders differ in frequency wonderful: we can distinguish between match and mismatch! 0: match 1: match mismatch State of transmission is either Note: utilizes frequency modulation. every sender has unique codeset. sender signals are additive in CDMA code: matched signals reinforce, mixed signals cancel (interfere). can still match in presence of interference. Advantage of CDMA: resistant to interference CDMA Tuesday, November 01, 2005 9:35 AM Docsity.com

CDMA coding Tuesday, November 01, 2005 9:35 AM Docsity.com

 multiple overlapping codes subtle trick: if there is a part of a code sequence free of interference, then one can read the bit even though the rest of the code is interfered with!

subtle trick: insure that for multiple transmissions, there are some bits in the chipper that get through in the clear regardless of what's sent, enough to tell whether there's a 0 or a 1 in the transmission.

design theory: create codes whose overlap and interference does not prohibit communications

Idea of CDMA: Idea of CDMA Tuesday, November 01, 2005 9:35 AM Docsity.com

Building a CDMA codeset:

Building a CDMA codeset Tuesday, November 01, 2005 9:35 AM Docsity.com

 high availability required  no disruption allowed. parameters based upon maximum vehicle speed

Mobility management in cellular networks  Maintaining a phone call.  Maintaining TCP connectivity. Two parts: Mobility in Cellular Networks Tuesday, November 08, 2005 9:03 AM Docsity.com

BS: Base Station: a radio tower home MSC: home switching center for a registered phone visiting MSC: where the phone happens to be now. MSC: Mobile switching center HLR : Home location register: where are phones now? VLR : Visited location register: what phones are visiting a tower zone now? Location register: a database of where phones are. Handoff: the process of switching which tower handles a call or connection. Terms Terms Tuesday, November 08, 2005 9:03 AM Docsity.com

 home MSC switches call to visiting MSC  visiting MSC looks up phone in VLR visiting MSC contacts phone via appropriate route.

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old base station (BS) informs MSC of intent to hand off and the BSs to which it is to be handed off.

 MSC allocates path to new BS  new BS activates radio channel  new BS signals ready to MSC  MSC instructs mobile to do handoff mobile and new BS exchange a few messages to validate handoff

mobile sends "handoff complete" message to MSC.

 MSC releases old BS path.  old BS deallocates channel. Handoffs: how a cellular station switches towers Handoffs Tuesday, November 08, 2005 9:19 AM Docsity.com

 MSC where call originated Handles handoffs to out-of-MSC towers

 Anchor MSC: manages switching What happens when a cellphone switches MSC's during a call Anchor MSC Tuesday, November 15, 2005 8:47 AM Docsity.com

The cellphone vendor maintains the "home address space and home agent" Packets travel the same path as calls (with the same switching). Remote agent is in the foreign MSC. So it's completely transparent to the user. Basically it's a "mobile-IP" scenerio, except that How does IP addressing work to cellphones? How does IP addressing work to cellphones Thursday, March 08, 2012 4:57 PM Docsity.com

Common ad-hoc protocol: flooding Broadcast a packet. All receivers rebroadcast. Use a sequence number to avoid rebroadcasting something twice. Flooding Flooding Thursday, March 08, 2012 9:46 AM Docsity.com

Network is silent until there is a need to communicate. "Needy node" broadcasts intent to communicate (a MAC layer concept). This creates a distance vector neighbor exchange in which each neighbor hop adds distance. Distance vector packets are sequenced; if you get one with a later sequence number than you already sent, ignore it. Ad Hoc On-Demand Distance Vector Routing (AODV) AODV Thursday, March 08, 2012 9:42 AM Docsity.com