1. Medium Access Control

2. Channel Allocation Static channel allocation in LANs and MANs
FDMA, TDMA, CDMA
Dynamic channel allocation in LANs and MANs
MAC protocols: with collisions, polling, token

3. Static Channel Allocation
Delay for one fast channel:
Delay for multiple FDM slower channels:

4. Poisson Process Probability of k arrivals in time t:
Probability that packet duration exceeds t:
Note that λ is the average packet arrival rate, and 1/μ is the average packet duration.

5. M/M/1 Queue
Queue equations, pj(t) is the probability that the number of packets at time t in a queue is j
The solution of the previous recursion for stationary probabilities to which pj(t) converge

6. Delay Little’s formula for average delay E[D]
E[Q] is the average number of packets in a queue
Delay is:

7. Static Channel Allocation
Delay for one fast channel:
where C is the channel bit-rate and E[L] is the average packet length.
Delay for multiple FDM N times slower channels:

8. Multiple Access Protocols
ALOHA
Carrier Sense Multiple Access (CSMA) protocols
CSMA/CD
CSMA/CA
Collision-Free protocols
Reservation based
Token based

9. Vulnerable period for the shaded frame.
Pure ALOHA
Vulnerable period for the shaded frame.

10. ALOHA Throughput
Throughput is S=GPs, where Ps is the probability of successful transmission.
The k frames per f frame slots is
Ps=e-fG
For pure ALOHA f=2, for slotted ALOHA f=1, so:

11. Throughput versus offered traffic for ALOHA systems.
Pure and Slotted ALOHA
Throughput versus offered traffic for ALOHA systems.

12. Carrier Sense Multiple Access
1-Persistant CSMA
Nonpersistant CSMA
P-Persistant CSMA

13. CSMA with Collision Detection
CSMA/CD can be in one of three states: contention, transmission, or idle.

14. Wireless LAN: CSMA-CA The MACA protocol. (a) A sending an RTS to B.
(b) B responding with a CTS to A.

15. DOCSIS (Data Over Cable Service Interface Specification)

16. Collision-Free Protocols:Reservations
The basic bit-map protocol.

17. Collision-Free Protocols: Bidding
The binary countdown protocol. A dash indicates silence.

18. Collision Free Protocols: Fiber Distributed Data Interface (FDDI)
Station transmits only when it has a token
Timers count the time while the token is away
Two timers determine how much data a station may transmit, so that the token delay is limited

19. Ethernet Ethernet, IEEE 802.3 10Base (10Mbps) Fast Ethernet (100Mbps)
Gigabit Ethernet

20. Ethernet MAC Sublayer Protocol
Frame formats. (a) DIX Ethernet, (b) IEEE
Preamble-synchronization, Type-upper layer protocol,
Pad-to make the minimum packet size 64B

21. CSMA with Collision Detection
CSMA/CD can be in one of three states: contention, transmission, or idle.

22. Back-Off Mechanism
After a collision, user accesses medium with probability 1/W where W is the window size.
With each collision W doubles.

23. Ethernet Performance

24. Throughput of CSMA/CD (Ethernet)
Assume that requests form a Poisson process with rate g, T is time slot duration, and Tp is a packet duration. The throughput equals S=Tp/(Tp+I), where I is the average time between packet transmissions.
The probability of a packet transmission is equal to the probability that there is only one request in some previous time slot which is Ps=gTe-gT.
The average time between transmissions is

25. Throughput of CSMA/CD The throughput is
It tends to 0 when g increases .
Protocol is unstable like ALOHA. g S l 1 2

26. Throughput of CSMA/CD The throughput is
If p is the packet generation probability and k is the number of active users
Ps=kp(1-p)k-1
The maximum throughput is achieved for p=1/k and it is tends to e when k tends to infinity

27. Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

28. The most common kinds of Ethernet cabling.
10Mbps Ethernet Cabling
The most common kinds of Ethernet cabling.

29. 10Mbps Ethernet Cabling Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.

30. Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
Ethernet Cabling
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

31. 10Mb Ethernet Coding
(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

32. 10 Mb Ethernet Collision Detection
10Base5 cabling,
Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

33. 10 Mb Ethernet Collision Detection
10Base2 and 10BaseT cabling,
Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

34. The original fast Ethernet cabling.

35. Fast Ethernet
Auto negotiation enables communication with 10Mb Ethernet
Manchester code → 4B/5B code
Full duplex mode is optional with using PAUSE command

36. A simple example of switched Ethernet.

37. Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation Ethernet.

38. Gigabit Ethernet cabling.

39. Gigabit Ethernet Prioritization of fiber over copper
4B/5B coding → 8B/10B coding
Full duplex mode is preferred with PAUSE message
Carrier extension, and frame bursting introduced in half-duplex mode

40. IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.

41. A Sample HFC System HOME Secondary Hub RF Spectrum on coax:
Downstream: 500 MHz shared by ~50,000 (broadcast)
200 MHz by (narrowcast)
Upstream: ~37 MHz shared by 300
HOME
o o
o o
o o
o o
lup lb
ln (4ln/fiber)
Secondary Hub
Fiber
Node
o o
o o
o o
o o
o o
RF Spectrum on coax:
return
Capacity of the coax is “wasted.”
No architecture really looks exactly like this, but it is a well-defined starting point. I could have used
Lightwire as my starting point (in fact I originally did use Lightwire as my starting point) but systems like this are currently more common.
********OLD NOTES FROM HFC BASICS SLIDE***************
1. Drop is a coax bus, therefore all downstream traffic is broadcast, and terminal equipment in home selects the desired content.
2. Because it is coax, RF transmission is used.
3. Coax has a limited bandwidth, but is capable of maintaining a high SNR.
4. Fiber has virtually unlimited bandwidth, but it is difficult to maintain a high SNR (high SNR requires high power, which runs into non-linearities) This point is best illustrated by the fact that over longer distances the fiber carrying the analog AM-VSB signals doesn’t carry any other wavelengths.
5. The RF demarcation between analog broadcast signals and narrowcast signals is flexible.
The traditional method to increase capacity is to reduce the node size (analogous to cell size).
Upstream frequently split into 4 parts.
RF band is 5-42 MHz, this band can carry multiple RF channels. Channel width downstream 6MHz,
channel spacing upstream is 2MHz.
5-15 MHz is plagued with ingress noise.
All frequencies suffer from the funnel effect.
Bandwidth is shared. Coax drop downstream has 5Gbps capacity.
Services can be segregated by RF frequency.
For data the standard is DOCSIS (Data Over Cable Service Interface Specification).
Up to 10 Mbps transmission per RF channel is provided in the standard, but a peak rate of ~3 Mbps is more realistic.
A cable modem or set top box resides in the home, a CMTS, which coordinates traffic, resides in the headend.
80 broadcast channels QAM channels
(~150 video channels)
narrowcast
broadcast
5-42 MHz MHz MHz
Sheryl Woodward, AT&T Labs-Research

42. Justification for Using Shared Medium
Equivalent circuit rate (ECR) on a cable with many users is the rate of a dedicated link that would provide the same e.g. average delay (similar results is obtained for 90th percentile page delay). By Shankar, Jiang and Mishra:
where tON is the transmission tim, and tOFF is the think time, r is the channel rate, tON/(tON+tOFF)<<1, on periods have an exponential distribution.

43. Justification for Using Shared Medium
Let’s calculate how many users can be allocated one DOCSIS channel of 32Mbps to get the same experience as DSL user with dedicated rate of 2Mbps. According to traffic statistics page size is 68KB on average, and tOFF is 14.5s on average,
which is much more than 32/2=16 users. Price: high user speed.

44. DOCSIS MAC Protocol
Traffic that is transmitted downstream to the users is controlled by CMTS (cable modem termination system) in headend. It polices and shapes the traffic, and perform algorithms such are WFQ and RED.
Users requests are resolved at headend, and they are informed about the resolution through the downstream channel. If there is a collision of requests, users repeat their requests according to exponential back-off mechanism, otherwise they send data in specified time slot(s).

45. QoS in DOCSIS Service QoS parameters Access Mode Applications UGS
Unsolicited grant size, interval, jitter
Isonchronous
Videoconferencing, VoD, VoIP
UGS-AD
Unsolicited grant size, interval, jitter;polling interval, jitter
Isonchronous, periodic request polling
VoIP with silence supression
rtPS
Polling interval, jitter
Periodic request polling, piggybacking reservation
VoIP

46. QoS in DOCSIS Service QoS parameters Access Mode Applications nrtPS
Polling interval, min reserved rate, max sustained rate, priority
Periodic request polling, piggybacking reservation, immediate access
Demanding FTP BE
min reserved rate, max sustained rate, priority
Normal, piggybacking reservation, immediate access
Telnet, FTP, WWW
CIR
Unspecified

47. Performance for BE service in DOCSIS
Assume that requests form a Poisson process with rate g, T is time slot duration, and Tp is a packet duration. The throughput equals S=Tp/(Tp+I), where I is the average time between packet transmissions.
The probability of a packet transmission is equal to the probability that there is only one request in some previous time slot which is gTe-gT.
The average time between transmissions is

48. Performance for BE service in DOCSIS
The throughput is
It tends to 0 when g increases .
Protocol is unstable like ALOHA. g S l 1 2

49. Wireless LANs Distributed coordination function (DCF)
Point coordination function (PCF)

50. Part of the 802.11 protocol stack.

51. The 802.11 MAC Sublayer Protocol
(a) The hidden station problem.
(b) The exposed station problem.

52. Wireless LAN: CSMA-CA The MACA protocol. (a) A sending an RTS to B.
(b) B responding with a CTS to A.

53. The 802.11 MAC Sublayer Protocol
The use of virtual channel sensing using CSMA/CA.

54. The 802.11 MAC Sublayer Protocol
A fragment burst.

55. The 802.11 MAC Sublayer Protocol
Interframe spacing in

56. The 802.11 Frame Structure The 802.11 data frame.
Address 3 and 4-for source and dest base stations, Seq-fragment sequence number, Type-data or control, Subtype-RTS or CTS, MF-more fragments, More-more frames, W-WEP, O-frame sequence maintained

57. 802.11 Services Association Disassociation Reassociation Distribution
Authentication
Integration
Privacy