1 Wireless and Mobile Networks EECS 489 Computer Networks Z. Morley Mao Monday March 19, 2007 Acknowledgement: Some slides taken from Kurose&Ross

2 Mobile IP r RFC 3220 r has many features we’ve seen: m home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) r three components to standard: m indirect routing of datagrams m agent discovery m registration with home agent

3 Mobile IP: indirect routing Permanent address: Care-of address: dest: packet sent by correspondent dest: dest: packet sent by home agent to foreign agent: a packet within a packet dest: foreign-agent-to-mobile packet

4 Mobile IP: agent discovery r agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) R bit: registration required H,F bits: home and/or foreign agent

5 Mobile IP: registration example

6 Components of cellular network architecture correspondent MSC wired public telephone network different cellular networks, operated by different providers recall:

7 Handling mobility in cellular networks r home network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon) m home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network) r visited network: network in which mobile currently resides m visitor location register (VLR): database with entry for each user currently in network m could be home network

8 Public switched telephone network mobile user home Mobile Switching Center HLR home network visited network correspondent Mobile Switching Center VLR GSM: indirect routing to mobile 1 call routed to home network 2 home MSC consults HLR, gets roaming number of mobile in visited network 3 home MSC sets up 2 nd leg of call to MSC in visited network 4 MSC in visited network completes call through base station to mobile

9 Mobile Switching Center VLR old BS new BS old routing new routing GSM: handoff with common MSC r Handoff goal: route call via new base station (without interruption) r reasons for handoff: m stronger signal to/from new BS (continuing connectivity, less battery drain) m load balance: free up channel in current BS m GSM doesn’t mandate why to perform handoff (policy), only how (mechanism) r handoff initiated by old BS

10 Mobile Switching Center VLR old BS GSM: handoff with common MSC new BS 1. old BS informs MSC of impending handoff, provides list of 1 + new BSs 2. MSC sets up path (allocates resources) to new BS 3. new BS allocates radio channel for use by mobile 4. new BS signals MSC, old BS: ready 5. old BS tells mobile: perform handoff to new BS 6. mobile, new BS signal to activate new channel 7. mobile signals via new BS to MSC: handoff complete. MSC reroutes call 8 MSC-old-BS resources released

11 home network Home MSC PSTN correspondent MSC anchor MSC MSC (a) before handoff GSM: handoff between MSCs r anchor MSC: first MSC visited during call m call remains routed through anchor MSC r new MSCs add on to end of MSC chain as mobile moves to new MSC r IS-41 allows optional path minimization step to shorten multi-MSC chain

12 home network Home MSC PSTN correspondent MSC anchor MSC MSC (b) after handoff GSM: handoff between MSCs r anchor MSC: first MSC visited during cal m call remains routed through anchor MSC r new MSCs add on to end of MSC chain as mobile moves to new MSC r IS-41 allows optional path minimization step to shorten multi-MSC chain

13 Mobility: GSM versus Mobile IP GSM elementComment on GSM elementMobile IP element Home systemNetwork to which the mobile user’s permanent phone number belongs Home network Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR) Home MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information Home agent Visited SystemNetwork other than home system where mobile user is currently residing Visited network Visited Mobile services Switching Center. Visitor Location Record (VLR) Visited MSC: responsible for setting up calls to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user Foreign agent Mobile Station Roaming Number (MSRN), or “roaming number” Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent. Care-of- address

14 Wireless, mobility: impact on higher layer protocols r logically, impact should be minimal … m best effort service model remains unchanged m TCP and UDP can (and do) run over wireless, mobile r … but performance-wise: m packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff m TCP interprets loss as congestion, will decrease congestion window un-necessarily m delay impairments for real-time traffic m limited bandwidth of wireless links

15 Chapter 6 Summary Wireless r wireless links: m capacity, distance m channel impairments m CDMA r IEEE (“wi-fi”) m CSMA/CA reflects wireless channel characteristics r cellular access m architecture m standards (e.g., GSM, CDMA-2000, UMTS) Mobility r principles: addressing, routing to mobile users m home, visited networks m direct, indirect routing m care-of-addresses r case studies m mobile IP m mobility in GSM r impact on higher-layer protocols

16 MPLS is used today! r % traceroute -q1 -w2 r traceroute to ( ), 30 hops max, 46 byte packets r 1 eecscomp2-4 ( ) ms r ( ) ms r 3 ge-caen-bin-seb.r-bin-seb.umnet.umich.edu ( ) ms r 4 pc-bin-arb-seb.r-bin-arb.umnet.umich.edu ( ) ms r 5 ge-1-1-0x984.aa1.mich.net ( ) ms r ( ) ms r 7 brvwil1wcx2-pos14-1.wcg.net ( ) ms r ( ) ms r 9 te-4-4.car2.Chicago1.Level3.net ( ) ms r 10 ae car4.Chicago1.Level3.net ( ) ms r 11 ggr2-p3110.cgcil.ip.att.net ( ) ms r 12 tbr2-p cgcil.ip.att.net ( ) ms r MPLS Label=32719 CoS=0 TTL=0 S=1 r 13 tbr2-cl3641.phlpa.ip.att.net ( ) ms r MPLS Label=31344 CoS=0 TTL=0 S=1 r 14 tbr1-cl9.wswdc.ip.att.net ( ) ms r MPLS Label=31295 CoS=0 TTL=0 S=1 r 15 gbr5-p10.wswdc.ip.att.net ( ) ms r MPLS Label=317 CoS=0 TTL=0 S=1 r 16 gbr1-s83.mdtva.ip.att.net ( ) ms r ( ) ms r 18 * CoS: Class of Service S: Stacking bit

17 Boeing Connexion Mobility Service

18 Example problem 1 r If all the links in the Internet were to provide the reliable delivery service, would the TCP reliable delivery service be redundant? Why or why not?

19 Example problem 1 r If all the links in the Internet were to provide the reliable delivery service, would the TCP reliable delivery service be redundant? Why or why not? r Ans: TCP is still needed to ensure end-to- end reliability. Packets may be lost due to routing loops or equipment failures. TCP also provide in-order delivery.

20 Example problem 2 r Suppose two nodes start to transmit at the same time a packet of length L over a broadcast channel of rate R. Denote the propagation delay between the two nodes as t prop. Will there be collision if t prop < L/R? Why or why not?

21 Example problem 2 r Suppose two nodes start to transmit at the same time a packet of length L over a broadcast channel of rate R. Denote the propagation delay between the two nodes as t prop. Will there be collision if t prop < L/R? Why or why not? r Ans: Yes, there will be collision, because while a node is transmitting it will start to receive a packet from the other node.

22 Review: Ethernet CSMA/CD algorithm 1. Adaptor receives datagram from net layer & creates frame 2. If adapter senses channel idle (for 96 bit times), it starts to transmit frame. If it senses channel busy, waits until channel idle (for 96 bit times), and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2 m -1}. Adapter waits K·512 bit times and returns to Step 2

23 Review: Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time:.1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Exponential Backoff: r Goal: adapt retransmission attempts to estimated current load m heavy load: random wait will be longer r first collision: choose K from {0,1}; delay is K· 512 bit transmission times r after second collision: choose K from {0,1,2,3}… r after ten collisions, choose K from {0,1,2,3,4,…,1023}

24 Example problem 3 r In CSMA/CD, after the fifth collision, what is the probability that a node chooses K=4? The result K=4 corresponds to a delay of how many seconds on a 10 Mbps Ethernet?

25 Example problem 3 r In CSMA/CD, after the fifth collision, what is the probability that a node chooses K=4? The result K=4 corresponds to a delay of how many seconds on a 10 Mbps Ethernet? r Ans: After the fifth collision, the adapter chooses from {0,1,2,…., 2^5-1}. The probability that it chooses 4 is 1/32. r It waits K· 512 bit transmission times m 4x512bit/(10Mbps) = microseconds.

26 Example problem 4 r Suppose nodes A and B are on the same 10Mbps Ethernet segment and the propagation delay between the two nodes is 225 bit times. Suppose A and B send frames at the same time, the frames collide, and then A and B choose different values of K in the CSMA/CD algorithm. Assuming no other nodes are active, can the retransmissions from A and B collide?

27 r Suppose nodes A and B are on the same 10Mbps Ethernet segment and the propagation delay between the two nodes is 225 bit times. Suppose A and B send frames at the same time, the frames collide, and then A and B choose different values of K in the CSMA/CD algorithm. Assuming no other nodes are active, can the retransmissions from A and B collide? r Ans: Assuming A chooses K=0, B chooses K=1, basically we need to find out whether A’s retransmission will reach B before B’s scheduled retransmission time. (It’s unlikely that A can finish retransmission before B retransmits.)

28 r Suppose nodes A and B are on the same 10Mbps Ethernet segment and the propagation delay btw. the two nodes is 225 bit times. Suppose A and B send frames at the same time, the frames collide, and then A and B choose different values of K in the CSMA/CD algorithm. Assuming no other nodes are active, can the retransmissions from A and B collide? r Draw a time line: m t=0A and B begin transmission m t=225A and B detect collision m t=225+48=273A and B finish sending Jam signal m =498B’s last bit arrives at A; A detects an idle channel m =594A starts transmitting m =785B returns to step2, B must sense idle channel for 96 bit times: =881 m =819A’s transmission reaches B m 819<881: A’s retransmission reaches B before B can send.

29 Example problem 5 r Why are link layer acknowledgements used in but not in wired Ethernet?

30 Example problem 5 r Why are link layer acknowledgements used in but not in wired Ethernet? r Ans: Two reasons why uses ACKs: (1) wireless channel’s bit error rate is much higher. (2) in wired Ethernet, a transmitting station can detect when there has and hasn’t been a collision, but in a station cannot detect a collision due to hidden terminal problem.

31 Example problem 6 r Suppose the IEEE RTS and CTS frames were as long as the standard DATA frames. Would there be any advantage to using the CTS and RTS frames? Why and why not?

32 Example problem 6 r Suppose the IEEE RTS and CTS frames were as long as the standard DATA frames. Would there be any advantage to using the CTS and RTS frames? Why and why not? r Ans: No, there wouldn’t be any advantage. Suppose there are two stations that want to transmit at the same time, and they both use RTS/CTS. If the RTS frame is as long as a DATA frame, the channel would be wasted for as long as it would have been wasted for colliding DATA frames. Thus, the RTS/CTS exchange is only useful when they are significantly smaller than DATA frames.

33 Example problem 7 r Consider three LANs interconnected by two routers r Assign IPs to all the interfaces. Subnet 1 uses addresses of the form xxx, subnet 2 uses addresses of the form xxx, and subnet 3 uses addresses of the form xxx r Assign MAC addresses (arbitrarily). r Consider sending an IP datagram from Host A to Host F, suppose all the ARP tables are up-to-date. Enumerate all the steps. Subnet 1 A B Subnet 3 E F C D Subnet 2

34 Example problem 7 Subnet 1 A B Subnet 3 E F C D Subnet

Subnet 1 A B Subnet 3 E F C D Subnet Forwarding table in A determines that the datagram should be routed to interface Host A uses ARP to determine the LAN address for , namely The adapter in A creates an Ethernet packet with destination address of The first router receives the packet and extracts the datagram. The forwarding table in this router indicates that the datagram is to be routed to The first router then uses ARP to obtain the associated Ethernet address: The process continues until the packet has reached host F.