NETW 501 Communication Networks Tutorial 7 By Mariham
NETW 501 Communication Networks Tutorial 7 By: Mariham Wasfy
Outline • Scheduling Dynamic MAC • Channelization Static MAC
Outline • Scheduling Dynamic MAC o Reservation o Polling o Token Passing • Channelization Static MAC o FDMA o TDMA o CDMA
�� Recap MAC Keep the same order every time There is NO controller between hosts Static Channelization - FDMA - TDMA - CDMA Dynamic Random Access ALOHA Slotted ALOHA CSMA/CD Different order every time Scheduling ng - Reservation - Polling - Token Passing There is controller between hosts
Scheduling o Reservation o Polling o Token Passing
Scheduling o Reservation o Polling o Token Passing
Reservation • Star topology A I want to send 2 frames B I want to send 3 frames E I want to send 1 frame D I want to send 1 frame I want to C send 2 frames
Reservation 1. Each host � will send request to send x frames to controller A E D B C 8 9 2 7 frames in total 5
Reservation 2. Controller will process requests by assigning slots to each host RANDOMLY E A D B C 1 2 3 4 5 6 7 8 9 A C D B A B B C E ∴ Number of frames = Number of slots
Reservation 3. Each host will send its �� frame/s in assigned slot/s E A D B C 1 2 3 4 5 6 7 8 9 A C D B A B B C E
Reservation 1. • Time Efficiency (ղ) Hosts send the requests to the controller 2. The controller Assigning time slots among hosts E A 1 2 3 4 5 6 7 8 9 D B C A C D B A B B C E 3. Hosts sending their frames in 3. in the assigned time slots
Reservation • Time Efficiency (ղ) Hosts send The controller requests to the controller Assigning time slots among hosts Hosts sending their frames in assigned time slots
Reservation • Time Efficiency (ղ) Total number of frames/host (if it’s same for all hosts) Total number of hosts Reservation time (step 1 + 2)
Scheduling o Reservation o Polling o Token Passing
Polling • Star topology • Polling: o o A E Round Robin Fashion (fair) Pre-determined order Controller Host Poll Question: Do you have something to send? Controller Poll Answer: Yes/No Host D B C
Polling A Controller Host Poll Question: Do you have something to send? Controller Poll Answer: Yes/No E Host D B C
Polling • Transmission limited by each host: 1. 2. 3. 4. Exhaustive Gated Frame limited Time limited A E D B C
Polling • Transmission limited by each host: 1. 2. 3. 4. Exhaustive Gated (buffer limited) Frame limited Time limited A E D B C
Polling • Transmission limited by each host: A E 1. Exhaustive 2. Gated 3. Frame limited (certain number of frames per round) For instance 1 frame per round 4. Time limited D B C
Polling • Transmission limited by each host: 1. 2. 3. 4. Exhaustive Gated Frame limited Time limited A E D B C
Polling • Time Efficiency (ղ)
Polling • Time Efficiency (ղ)
Polling • Time Efficiency (ղ) – DIFFERENT number of frames to be send per host Total number of frames to be transmitted by ALL hosts Poll Question and Poll Answer Total number of host/stations
Polling • Time Efficiency (ղ) - SAME number of frames to be send per host Total number of host/stations Poll Question and Poll Answer Total number of frames to be transmitted by ALL hosts
Polling • Time Efficiency (ղ) Total number of host/stations Poll Question and Poll Answer Total number of frames to be transmitted by EACH hosts
Scheduling o Reservation o Polling o Token Passing
Token Passing A • Ring topology Token Flag: Data: Source: Destination: • • current 1 Inserted if the token contains data (busy) bysame source Insert Data • • If host as destination -> COPY • • current 0 Remove if thehost token is empty (free) by isn’t source too Remove Data when get back it • • If thetoken destination -> to discard and • Copied by destination FORWARD B E C D
Token Passing A send to C 1 C A A 11 C A B 10 C A 1 1 C A C D C A Is == C? Is EA B C == == C? C? D Is No No Yes No But it’s the source and of course Then discard COPY ONLY and forward Then discard and the token passed by C and C copied the data. So A now can remove the data, source and destination and turn flag to 0. E
Token Passing • Time Efficiency (ղ) Should return back to source Total number of host/stations Where Tcopy is very small value can be ignored
Token Passing • Time Efficiency (ղ) Total number of host/stations Should return back to source
Token Passing • Star topology A E D B C
Token Passing • Time Efficiency (ղ) To and from controller Should return back to source Total number of host/stations
Channelization o FDMA o TDMA o CDMA
Channelization o FDMA o TDMA o CDMA
FDMA – Frequency Division Multiplexing Access Telephone Line Splitter Calls Download Upload 0 4 5 25 26 40 KHz Guard Bands to avoid interference
FDMA – Frequency Division Multiplexing Access Frequency ∴ 3 users �� Time
Channelization o FDMA o TDMA o CDMA
TDMA – Time Division Multiplexing Access
TDMA – Time Division Multiplexing Access Frequency ∴ 3 users �� Time
Channelization o FDMA o TDMA o CDMA
CDMA – Code Division Multiplexing Access
CDMA – Code Division Multiplexing Access Frequency ∴ 6 users ���� Time
Tutorial
1. A wireless LAN using Polling to provide communications between 10 work stations and a central base station. The system uses a channel operating at 25 Mbps. Assume that half the stations (station of type A) are 100 meters away from the base station and are given opportunity to send 2 frames per poll. The rest (stations of type B) are 50 meters away from the base station and are allowed to send 4 frames per poll. The polling messages are 64 bytes long. Also assume that frames are of constant length 1250 bytes and those stations indicate that they have no frames to transmit with a 64 bytes message. The channels are error-free. a. Calculate the maximum time that could elapse between 2 consecutive polls at any station. b. Given the value you have computed in (a). What is the maximum arrival rate of frames that station of type A may support?
1 6 2 7 3 100 m Central Station 50 m 8 4 9 5 10 Type A Type B
2. Assume Round Robin Polling applied for 4 users A, B, C, D. Each user is allowed to transmit only 2 frames per poll. Assume at t = 0 the polling order starts with station A followed by B, C then D. If stations A, B, C & D need to transmit 9, 5, 7, 6 frames respectively. Assume frame size is 1500 bytes and the transmission rate is 1 Mbps. Assume the time used to send the polling messages are negligible as well as the propagation time. At what time will station B be able to complete transmission of all its frames?
9 A 5 B 7 C 6 D Central Station
Cycle A B C D #1 2 sent out of 9 (2/9) 7 remaining 2 sent out of 5 (2/5) 3 remaining 2 sent out of 7 (2/7) 5 remaining 2 sent out of 6 (2/6) 4 remaining #2 Other 2 sent out of 9 (4/9) 5 remaining Other 2 sent out of 5 (4/5) 1 remaining Other 2 sent out of 7 (4/7) 3 remaining Other 2 sent out of 6 (4/6) 2 remaining #3 Other 2 sent out of 9 (6/9) 3 remaining Other 1 sent out of 5 (5/5) 0 remaining ∴ Stop here as B finishes
3. If Reservation is used as multiple access protocol and No. of stations = No. of mini -slots (M) = 6. Each mini-slot can reserve up to 4 frames (k = 4) and frame transmission time (X) is 1 msec. and reservation delay (v) = 50 µsec. Calculate the system maximum efficiency.
1 2 3 4 Central Station 5 6
4. A wireless LAN uses polling to provide communications between M workstations and a central base station. The system uses a channel operating at 25 Mbps. Assume that all stations 100 meters from the base station and that polling messages are 64 bytes long. Assume that frames are of constant length of 1250 bytes, the frame arrival rate is 40 frames/sec and that stations indicate that they have no frames to transmit with a 64 byte message. What is the maximum possible number of stations that can be supported if stations are allowed to transmit 4 frames per poll?
Given: • M Stations • R = 25 Mbps • D = 100 m • Lpoll. Question = Lpoll. Answer = 64 B • Lframe = 1250 B • Frame limited poll: 4 frames/poll
Stations
Given: • Available bandwidth is 890 → 960 MHz. • 890 → 915 for the uplink • 935 → 960 for the downlink • Two operators share the same bandwidth �� Operator 1 890 + 12. 5 = 902. 5 Upload 890 – 902. 5 Upload 902. 5 - 915 Download 947. 5 - 960 Download 935 – 947. 5 Upload Uplink 915 – 890 = 25 902. 5 915 890 �� Operator 2 Download 960 – 935 = 25 935 947. 5 960 935 + 12. 5 = 947. 5 MHz
Given: • FDMA: Each user has 200 KHz • TDMA: 8 users in 8 time slots for upload and same for download Technique Upload Channels Download Channels FDMA 62 62
Given: • FDMA: Each user has 200 KHz • TDMA: 8 users in 8 time slots for upload and same for download Technique Upload Channels Download Channels TDMA 8 8
Given: • FDMA: Each user has 200 KHz • TDMA: 8 users in 8 time slots for upload and same for download Technique Upload Channels Download Channels FDMA 62 62 TDMA 8 8
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