1. Scheduling approaches to MAC
About random access:
Simple and easy to implement
In low-traffic, packet transfer has low-delay
However, limited throughput and in heavier traffic, packet delay has no bound. A station of bad luck may never have a chance to transfer its packet.
Scheduling approach:
provides orderly access to shared medium so that every station has chance to transfer

2. Scheduling approach—reservation protocol
The time line has two kinds of periods:
Reservation interval of fixed time length
Data transmission period of variable frames.
Suppose there are M stations, then the reservation interval has M minislots, and each station has one minislot.
Whenever a station wants to transfer a frame, it waits for reservation interval and broadcasts reservation bit in its minislot.
By listening to the reservation interval, every station knows which stations will transfer frames, and in which order.
The stations having reserved for their frame transfer their frames in that order
After data transmission period, next reservation interval begins.

3. Reservation protocol Frames-transmission 0 1 2 3 4 5 6 7
interval 1 1 1 1 3 5 1 1 3 7

4. Scheduling approach—polling protocol
Stations take turns accessing the medium:
At any time, only one station has access right to transfer into medium
After this station has done its transmission, the access right is handed over (by some mechanism) to the next station.
If the next station has frame to transfer, it transfers the frame, otherwise, the access right is handed over to the next next station.
After all stations are polled, next round polling from the station 1 begins.

5. Centralized polling vs. distributed polling
Centralized polling: a center host which polls the stations one by one
Distributed polling: station 1 will have the access right first, then station 1 passes the access right to the next station, which will passes the access right to the next next station, …

6. Interaction of polling messages and transmissions in polling systems… 1 2 3 4 5 M 1 2 t
packet transmissions
Figure 6.28

7. Token-passing rings – a distributed polling network
Station interfaces: are connected to form a
ring by point-to-point lines
Stations: are attached to the ring
by station interfaces.
token
Note: point-to-point lines, not a shared bus.
Token: a small frame, runs around the ring, whichever gets
the token, it has the right to transmit data frames.
The information flows in one direction.
Station interfaces have important functions.
Token-passing rings – a distributed polling network
Figure 6.30

8. Token-passing rings – a distributed polling system
Two modes of interface: 1. Listen mode, like a repeater but with some delay, because
every arriving bit will be copied into a 1-bit buffer and then copied out. While in the buffer, the
bit can be monitored, i.e., inspected and even modified. Therefore, 1-bit delay
2. Transmit mode: transmit frames from its attached station
When monitoring, if it finds the passing
bits are a data frame with its attached station
as destination, then it catches the frame,
i.e., copy the bits into its station (may
refrain from outputting them).
If it finds the passing bits are free token,
then if the station has information to send,
it will change the token to busy (set one
bit to 1) and change to transmit mode.
listen mode
transmit mode
input
from
ring
1 bit delay
output to
ring
delay
to station
from station
to station
from station
Token-passing rings – a distributed polling system
Figure 6.30

9. Removing a frame A transmitted frame needs to be removed (absorbed).
Who removes it?
Choice one: destination station
Choice two: transmitting station itself.
Choice two is preferred because the destination can insert ACK
into the frame and transmitter will get the ACK from its own
transmitted frame.

10. Approaches to token reinsertion: a). multitoken,
a). The free token is inserted immediately after the last bit of data frame is sent
b). Insert the free token after the last bit of busy token is received
c). Insert the free token after the last bit of data frame is received.
Generally: when a station seizes the token, it can only transmit limited
number of frames or limited time interval to avoid occupying too long. d a) b) c) d d d d d d
Busy token
Free token
Approaches to token reinsertion: a). multitoken,
b). single token, c)single packet
Figure 6.31

11. Comparison of scheduling & random access
Methodical orderly access: dynamic form of time division multiplexing, round-robin (only) when the stations have information to send.
Less variability in delay, supporting applications with stringent delay requirement. In high load, performance is good. E.g., token-ring may reach nearly 100 percent of performance when all stations have plenty of information to send.
Some channel bandwidth carries explicit scheduling information
Random access
Chaotic, unordered access
If rich bandwidth and light load, random access has low delay, otherwise, delay is undeterminably large.
Quite a lot bandwidth is used in collision to alert stations of the presence of other transmissions.

12. Channelization Suppose M stations generate steady flow of information
Divide shared medium into M channels so that each station is allocated one channel
Four channelization mechanisms:
FDMA
TDMA
WDMA
CDMA

13. Time-Division Multiplexing (TDM)
(a)
Dedicated Lines A1 A2 B1 B2 C1 C2
(b)
Shared Line A1 C1 B1 A2 B2 C2
Figure 5.43

14. 1 1 2
MUX
MUX 2
. . .
. . . 22 23 24 b 1 2
. . . 24 b 24
frame 24
T-1 carrier system uses TDM to carry 24 digital signals in telephone system
Figure 4.4

15. Frequency-Division Multiplexing (FDM)
(a) Individual signals occupy W Hz C f B A W
(b) Combined signal fits into channel bandwidth A C B f
Figure 4.2

16. Wavelength-division multiplexing (WDM)1 2 n 1 2 n
1, 2, n
MUX
deMUX
Optical fiber
Figure 4.1

17. CDMA (Code-division multiple access)
Transmissions from different stations occupy entire frequency band at the same time
Different codes are used to separate different transmissions
A code is a unique binary pseudorandom sequence (such as c1,c2,…,cG) (each ci is a +1 or –1 signal)
A binary bit b (i.e., +1 signal) is spread by multiplying it with the code at the sender (which is a sequence of binary information)
At the receiver, the received binary string is multiplied by the same code to get back original bit.

18. CDMA (cont.) Since: However, if a different code d1,d2,…,dG is used:
b(c1+c2+…+cG) at the sender
The received information is multiplied by c1,c2,…,cG to get b(c12+c22+…+cG2) = b1=b.
Due to each ci2=1 no matter ci=1 or –1
So the receiver gets back b.
However, if a different code d1,d2,…,dG is used:
The multiplication will be c1  d1+c2  d2 +…+cG  dG
Since ci and di are independent, there will be equal number of +1 and –1, so the result will be 0,
Thus the b can not be obtained.

19. CDMA (cont.)
In order for different codes are randomized, the sequence length G must be long enough.
G is called spreading factor.
Using LSFR (Left-Shift-Feedback-Register).

20. Channelization: CDMA Code Division Multiple Access
Channels determined by a code used in modulation and demodulation
Stations transmit over entire frequency band all of the time! 1
Frequency 2 W 3
Time

21. CDMA Spread Spectrum Signal
Binary
information
R1 bps
W1 Hz
Unique user binary random sequence
Digital
modulation
Radio
antenna
Transmitter from one user
R >> R1bps
W >> W1 Hz
User information mapped into: +1 or -1 for T sec.
Multiply user information by pseudo- random binary pattern of G “chips” of +1’s and -1’s
Resulting spread spectrum signal occupies G times more bandwidth: W = GW1
Modulate the spread signal by sinusoid at appropriate fc

22. CDMA Demodulation  Recover spread spectrum signal
Signal and residual
interference
Correlate to
user binary
random sequence
Signals
from all
transmitters
Digital
demodulation
Binary
information 
Recover spread spectrum signal
Synchronize to and multiply spread signal by same pseudo-random binary pattern used at the transmitter
In absence of other transmitters & noise, we should recover the original +1 or -1 of user information
Other transmitters using different codes appear as residual noise

23. Pseudorandom pattern generator
Feedback shift register with appropriate feedback taps can be used to generate pseudorandom sequence R0 R1 R2
g(x) = x3 + x2 + 1 g0 g2 g3
The coefficients of a
primitive generator polynomial
determine the feedback taps
Time R0 R1 R2
Sequence repeats
from here onwards
output

24. Channelization in Code Space
Each channel uses a different pseudorandom code
Codes should have low cross-correlation
If they differ in approximately half the bits the correlation between codes is close to zero and the effect at the output of each other’s receiver is small
As number of users increases, effect of other users on a given receiver increases as additive noise
CDMA has gradual increase in BER due to noise as number of users is increased
Interference between channels can be eliminated if codes are selected so they are orthogonal and if receivers and transmitters are synchronized
Shown in next example

25. Example: CDMA with 3 users
Assume three users share same medium
Users are synchronized & use different 4-bit orthogonal codes: {-1,-1,-1,-1}, {-1, +1,-1,+1}, {-1,-1,+1,+1}, {-1,+1,+1,-1},
User 1 x -1 +1
Receiver +1 -1 +1 +
User 2 x
User 3 x +1 -1
Shared
Medium

26. Sum signal is input to receiver
Channel 1: 110 -> > (-1,-1,-1,-1),(-1,-1,-1,-1),(+1,+1,+1,+1)
Channel 2: 010 -> > (+1,-1,+1,-1),(-1,+1,-1,+1),(+1,-1,+1,-1)
Channel 3: 001 -> > (+1,+1,-1,-1),(+1,+1,-1,-1),(-1,-1,+1,+1)
Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)
Channel 1
Channel 2
Channel 3
Sum Signal

27. Example: Receiver for Station 2
Each receiver takes sum signal and integrates by code sequence of desired transmitter
Integrate over T seconds to smooth out noise
Decoding signal from station 2
Integrate
over T sec + x
Shared
Medium

28. Decoding at Receiver 2 = X
Sum Signal: (+1,-1,-1,-3),(-1,+1,-3,-1),(+1,-1,+3,+1)
Channel 2 Sequence: (-1,+1,-1,+1),(-1,+1,-1,+1),(-1,+1,-1,+1)
Correlator Output: (-1,-1,+1,-3),(+1,+1,+3,-1),(-1,-1,-3,+1)
Integrated Output: , ,
Binary Output: , ,
Sum Signal
Channel 2
Sequence
Correlator
Output
Integrator -4 +4 X =

29. Walsh Functions
Walsh functions are provide orthogonal code sequences by mapping 0 to -1 and 1 to +1: each row is a code, rows are orthogonal.
Walsh matrices constructed recursively as follows:
W2n=
Wn Wn
Wn Wnc
W8=
0 0
0 1
1 1
1 0
W2=
0 0
0 1
W1=
W4=
0 0
0 1
1 1
1 0

30. LAN bridges
Repeater: used to connect two or more networks at physical layer
Bridge: used to connected two or more networks at data link or MAC layer
Router: used to connect two or more networks at network layer
Gateway: connect two or more networks at higher layers.

31. LAN bridges’ functions
Bridges are generally used to connect LANs by
Extending a LAN which reach saturation, called bridged/extended LAN
Connect different departments’ LANs, these LANs
Use different network layer protocols, bridges are fine because bridges operate at data link layer which supports different network layer protocols
Located in different building, bridges are fine because bridges can be connected by point-to-point link
Are different LANs: bridges need to have ability to convert between different frame format
Security is big problem in LAN, why?
So bridges need to have some ability of dealing with security issue such as filtering frames, controlling flow of frames in and out.
Bridge is better than repeater when connect two exact same LANs because bridge has the ability to reduce traffic by confining local traffic.
Bridge will monitor MAC address of frames so it can not be in physical layer
Bridge have no routing ability so it is not in network layer.

32. Interconnection by a bridge
Bridges generally connect the same types of LANs, so they generally
operate at MAC layer.
Network
Network
Bridge
LLC
LLC
MAC
MAC
MAC
MAC
Physical
Physical
Physical
Physical
Interconnection by a bridge
Figure 6.80

33. A bridged LAN
Port #1
Port #2
Figure 6.79

34. Three type bridges
Transparent bridges: means that stations are completely unaware of the presence of bridges. Used in Ethernet LANs. No burden in stations, bridges take care of all connection related functions.
Source routing bridge: used in token-ring and FDDI LANs. Burden on stations. Source station needs to give route to the destination. Bridges just forward frames based on the route in the frame.
Mixed-media bridges: used to interconnect LANs of
different types. These bridges have abilities of converting
between LANs.

35. Transparent bridges Three basic functions:
Forwards frames from one LAN to another
Learns where stations are attached to the LAN
Prevents loops in the topology
A transparent bridge is configured in “promiscuous” mode

36. Forwarding and forwarding table of bridges
MAC address
Port
Port is a physical interface, a bridge have two or more ports
For every station in bridged LAN, the port number indicates
which part (direction) of the bridge this station is attached to.
When a frame comes to a bridge, the bridge will forward to the
port corresponding to the MAC address in the table, which is the
destination physical address in the frame.
4. Question: how to establish forwarding table?
Manually set up by administrator. Good?
No, automatically set up by self learning

37. Bridge learning
When a bridge receives a frame, it searches through the forwarding table:
1. For source address, if not found, adds source address along coming port # into table
2. For destination address, if found, forwards the frame to the corresponding port,
except the corresponding port # being same as the coming port #
3. otherwise (not found), floods the frame to all ports except the coming port

38. Any problem with it? 1.S1->S5 4.S2->S1 2.S3->S2 3.S4->S3
LAN1
LAN2
LAN3
Bridge1
Bridge 2
port 1
port 2
port 1
port 2
Address Port
Address Port S1 1 S1 1 S3 2 S3 1 S4 2 S4 2 S2 1
Any problem with it?
Figure 6.85

39. LAN topology is dynamic
Life is never static in real world. The bridged LAN change constantly.
Add station:
Easy, learn again.
Move station:
When find same MAC address, but coming from a
different port, update the port number.
Remove station:
Timer, associate every entry (a station) in forwarding
table a timer, when timer times out, remove the entry
from the table.
Moreover, when receiving a frame with a source
MAC address, if its entry is already in the table, then
refresh the timer.
Any more problem?

40. Loop in the topology will cause flooding forever
S1->S5
LAN1
LAN2
LAN3
Bridge1
Bridge 2
port 1
port 2
port 1
port 2
Bridge3
Figure 6.85

41. Spanning tree to break loop
Main idea: maintain a spanning tree to include all stations but disable some ports automatically. Thus, remove loop.
Assumptions: unique LAN IDs, unique bridge IDs, and unique port IDs. The
lowest ID is used to break a tie.

42. Steps of spanning tree
Select a root bridge which is the bridge with the lowest bridge ID.
For each bridge except the root bridge, determine the root port which is the port with least-cost path (& lowest ID) to root bridge.
For each LAN, select a designated bridge, which is the bridge offering the least-cost path (& lowest ID) from the LAN to root bridge. The port connecting the LAN and the designated bridge is called a designate port.
All root ports and designated ports are put into forwarding table. These are only ports that are allowed to forward frames. The other ports are placed into a “blocking” state.

43. LAN1 (1) B1 B2 (1) (2) (2) (3) B3 LAN2 (2) (1) B4 (2) LAN3 (1) B5 (2)
Figure 6.86

44. Figure 6.87

45. Source routing bridges
Putting burden on end stations, bridges are mainly responsible for forwarding.
Each station determines the route to the destination and put the routing information in the header of the frame.
Source routing information is inserted only when source and destination are in different LANs and is indicated by I/G bit in source address.

46. Frame format for source routing
Control field contains type of frame, routing information and length of it.
2. Designator contains a 12-bit LAN number and a 4-bit bridge number
3. The highest bit in source address indicates whether it is source routing.
Routing
Route
Route
Route
Control
Designator-1
Designator-2
Designator-m
2 bytes
2 bytes
2 bytes
2 bytes
Destination
Source
Routing
Data
FCS
Address
Address
Information
Question: how to find the route in the first place?
Figure 6.88

47. Route discovery in source routing
Source broadcasts a single-route broadcast frame with no route designator, which will arrive the destination eventually
-Spanning tree algorithm may be used to confine flooding.
Whenever a bridge gets the frame, it inserts its number and out-going LANs number into the frame and forwards to the out-going LAN. (The first bridge also inserts its in-coming LAN number into it).
When the destination gets the frame, it replies with a broadcast of “all-route broadcast” frame with no route designator..
The bridges inserts its ID and out-going LAN number and forwards to the out-going LAN (only for this LAN whose number was not recorded in the frame to prevent loop yet).
The source will get returned frames with all possible routes,
so source can select best one (maybe cache it)

48. Interconnection with source routing bridges
Assume that S1 wants to send a frame to S3 * * * *
Suppose B1, B3, B4,B6 are part of spanning tree
Figure 6.89

49. Routes followed by single-route broadcast frames
LAN3 B6
LAN5
LAN1 B1
LAN2 B4
LAN4
Figure 6.90

50. Routes followed by all-routes broadcast frames
LAN1 B1
LAN2 B5 B4
LAN4 B7 B1
LAN1 B2 B6
LAN3 B3
LAN2 B5 B4
LAN4 B7 B1
LAN1 B2 B4
LAN2 B5
LAN4 B3
LAN5 B7 B3 B1
LAN1 B2
LAN3 B5 B6 B4
LAN2 B2
LAN1 B1 B3
LAN3 B7
LAN4 B5 B6 B3 B2
LAN1 B1
LAN2 B4 B5
LAN3 B3
LAN2 B1
LAN1 B2 B4 B6
Figure 6.91

51. Mixed-media bridges Such as connect Ethernet and token-ring networks
Convert between two address representations
Ethernet has maximum size of 1500 bytes but token-ring has no explicit limit. Drop frame if it is too long because bridges do not do segmentation
Token-ring has three status bits A, C and E, but
no these bits in Ethernet, so no conversion for them
Different transmission rate, so bridges need buffer to store extra frame temporarily.

52. IEEE LAN standards 802.2 LLC 802.3 CSMA-CD (I.e., Ethernet)
Frame length 64 –1518 (or client data: 46—1500)
802.4 token-bus
802.5 token-ring
FDDI (Fiber Distributed Data Interface)
wireless networks

53. Ethernet --history
1970s, Robert Metcalfe etc. Xerox, connecting workstations
1980s, DEC, Intel, Xerox, “DIX” Ethernet standard 10Mpbs
The basis for IEEE standard, “thick” coaxial cable in 1985.
802.3 and “DIX” Ethernet differ in one header field definition.
Expended to “thin” coaxial, twisted-pair, optical fiber.
1995: 100Mbps, 1998: 1Gbps, 2002: 10Gbps.

54. Ethernet -- basis Bus based coaxial cable CDMA-CD with 1-persistent
Minislot time: two propagation delays
10Mbps, 2500 meters, 4 repeaters
(Truncated) binary exponential backoff algorithm
Three periods and performance

55. Ethernet --backoff (Truncated) binary exponential backoff algorithm
Whenever collision, wait for B minislots
B is determined as followed:
After 1th collision, B is selected from 0 to 1
After 2th collision, B is selected from 0 to 3
After 3th collision, B is selected from 0 to 7 …
After nth collision, B is selected from 0 to
2n-1 if n<10 and if n<=16, otherwise, give up.

56. Ethernet--performance
Three periods: idle, contention, transmission
During saturation, no idle period
A L/R transmission period
A tprop period for finding the end of transmission
A contention period of B minislots, in average B=e=2.71.
So performance is:
(L/R)/(L/R+ tprop+2e tprop)=1/(1+(1+2e) tpropR/L)=
1/(1+(1+2e)a)=1/(1+6.44a), where a= tprop R/L.

57. Ethernet--performance
Why B=e in average?
Suppose n stations and each station transmits during a contention period with probability p,
The probability that one station transmits successfully is Psuccess=np(1-p)n-1.
For maximum throughput, p should be selected to maximize Psuccess, i.e., p=1/n.
So Psuccessmax=n(1/n)(1-1/n)n-1=(1-1/n)n-11/e.
Since probability of success in one minislot is Psuccessmax , the average number of minislots that elapse until a station captures the channel is
1/ Psuccessmax =e.

58. IEEE 802.3 MAC Frame Every frame transmission begins “from scratch”7 1 6 6 2 4
Destination
address
Source
address
Preamble SD
Length
Information
Pad
FCS
Synch
Start
frame
bytes
Every frame transmission begins “from scratch”
Preamble helps receivers synchronize their clocks to transmitter clock
7 bytes of generate a square wave
Start frame byte changes to
Receivers look for change in 10 pattern

59. IEEE 802.3 MAC Frame 802.3 MAC Frame Preamble SD Destination address
Source
Length
Information
Pad
FCS 7 1 6 2 4
bytes
Synch
Start
frame
Single address
Group address
Destination address
single address
group address
broadcast =
Addresses
local or global
Global addresses
first 24 bits assigned to manufacturer;
next 24 bits assigned by manufacturer
Cisco C
3COM C
Local address
Global address
802.3 MAC Frame

60. IEEE 802.3 MAC Frame Length: # bytes in information field
Preamble SD
Destination
address
Source
Length
Information
Pad
FCS 7 1 6 2 4
bytes
Synch
Start
frame
802.3 MAC Frame
Length: # bytes in information field
Max frame 1518 bytes, excluding preamble & SD
Max information 1500 bytes: 05DC
Pad: ensures min frame of 64 bytes
FCS: CCITT-32 CRC, covers addresses, length, information, pad fields
NIC discards frames with improper lengths or failed CRC

61. DIX Ethernet II Frame Structure
Preamble SD
Destination
address
Source
Type
Information
FCS 7 1 6 2 4
bytes
Synch
Start
frame
Ethernet frame
DIX: Digital, Intel, Xerox joint Ethernet specification
Type Field: to identify protocol of PDU in information field, e.g. IP, ARP
Framing: How does receiver know frame length?
physical layer signal, byte count, FCS

62. IEEE 802.3 Physical Layer (a) (b) Hubs & Switches!
Table 6.2 IEEE Mbps medium alternatives
10base5
10base2
10baseT
10baseFX
Medium
Thick coax
Thin coax
Twisted pair
Optical fiber
Max. Segment Length
500 m
200 m
100 m
2 km
Topology
Bus
Star
Point-to-point link
Hubs & Switches!
(a)
transceivers
(b)
Thick Coax: Stiff, hard to work with
T connectors flaky

63. Ethernet Hubs & Switches
(a)
     
Single collision domain
(b)
   
High-Speed backplane or interconnection fabric
Twisted Pair Cheap
Easy to work with
Reliable
Star-topology CSMA-CD
Twisted Pair Cheap
Bridging increases scalability
Separate collision domains
Full duplex operation

64. Ethernet Scalability
a = .01
a = .1
a = .2
CSMA-CD maximum throughput depends on normalized delay-bandwidth product a=tprop/X=tpropR/L
x10 increase in bit rate = x10 decrease in X
To keep a constant need to either: decrease tprop (distance) by x10; or increase frame length x10

65. Fast Ethernet IEEE 802.3u 100Mbps
CSMA-CD is sensitive to a, since rate increases 10 times, to keep same performance with Ethernet, ???
Either increase the frame size 10 times (to 640 bytes) or reduce maximum
Distance 10 times (250 meters).
The final decision is to keep the frame size and procedure unchanged but
To define a set of physical layers based on hub (start) topology involving
Twisted-pair and optical fiber.

66. Fast Ethernet Two modes:
All incoming lines are logically connected to a single collision domain and CSMA-CD is used
Incoming frames are buffered and then switched internally within the hub, CSMA-CD is not used but use multiplexing and switching.
Fast Ethernet is deployed in departmental networks
Aggregate traffic from shared 10Mbps LANs
Provide greater bandwidth to a server
Provide greater bandwidth to certain users.

67. Gigabit Ethernet 802.3z, 1998, 1Gbps.
Frame size extended to 512 bytes.
Moreover frame bursting: a burst of small frames.
Preserve the frame structure but operate primarily in a switched mode.

68. Optical fiber multimode
Fast Ethernet
Table 6.4 IEEE Mbps Ethernet medium alternatives
100baseT4
100baseT
100baseFX
Medium
Twisted pair category 3
UTP 4 pairs
Twisted pair category 5
UTP two pairs
Optical fiber multimode
Two strands
Max. Segment Length
100 m
2 km
Topology
Star
To preserve compatibility with 10 Mbps Ethernet:
Same frame format, same interfaces, same protocols
Hub topology only with twisted pair & fiber
Bus topology & coaxial cable abandoned
Category 3 twisted pair (ordinary telephone grade) requires 4 pairs
Category 5 twisted pair requires 2 pairs (most popular)
Most prevalent LAN today

69. Gigabit Ethernet Slot time increased to 512 bytes
Table 6.3 IEEE Gbps Fast Ethernet medium alternatives
1000baseSX
1000baseLX
1000baseCX
1000baseT
Medium
Optical fiber
multimode
Two strands
single mode
Shielded copper cable
Twisted pair category 5
UTP
Max. Segment Length
550 m
5 km
25 m
100 m
Topology
Star
Slot time increased to 512 bytes
Small frames need to be extended to 512 B
Frame bursting to allow stations to transmit burst of short frames
Frame structure preserved but CSMA-CD essentially abandoned
Extensive deployment in backbone of enterprise data networks and in server farms

70. 10 Gigabit Ethernet Frame structure preserved
Table 6.5 IEEE Gbps Ethernet medium alternatives
10GbaseSR
10GBaseLR
10GbaseEW
10GbaseLX4
Medium
Two optical fibers
Multimode at 850 nm
64B66B code
Single-mode at 1310 nm
64B66B
Single-mode at 1550 nm
SONET compatibility
Two optical fibers multimode/single-mode with four wavelengths at 1310 nm band
8B10B code
Max. Segment Length
300 m
10 km
40 km
300 m – 10 km
Frame structure preserved
CSMA-CD protocol officially abandoned
LAN PHY for local network applications
WAN PHY for wide area interconnection using SONET OC-192c
Extensive deployment in metro networks anticipated

71. Typical Ethernet Deployment
Server
100 Mbps links
10 Mbps links
Gigabit Ethernet links
Server farm
Department A
Department B
Department C
Hub
Ethernet switch
Switch/router

72. IEEE 802.5 Ring LAN Unidirectional ring network
4 Mbps and 16 Mbps on twisted pair
Differential Manchester line coding
Token passing protocol provides access
Fairness
Access priorities
Breaks in ring bring entire network down
Reliability by using star topology

73. Star Topology Ring LAN
Stations connected in star fashion to wiring closet
Use existing telephone wiring
Ring implemented inside equipment box
Relays can bypass failed links or stations
Wiring Center A B C D E

74. Token Frame Format Data frame format Token frame format SD FC AC
Destination
address
Source
Information
FCS 1 4 ED 6 FS
Data frame format SD AC ED
Token frame format
J, K nondata symbols (line code)
J begins as “0” but no transition
K begins as “1” but no transition
Starting
delimiter
J K J K 0
Access
control
PPP=priority; T=token bit
M=monitor bit; RRR=reservation
T=0 token; T=1 data
P P P T M
R R R
Ending
delimiter
I = intermediate-frame bit
E = error-detection bit I E
J K J K 1

75. Data Frame Format Data frame format SD FC AC Destination address
Source
Information
FCS 1 4 ED 6 FS
Data frame format
Frame
control
FF = frame type; FF=01 data frame
FF=00 MAC control frame
ZZZZZZ type of MAC control
F F
Z Z Z Z Z Z
Addressing
48 bit format as in 802.3
Information
Length limited by allowable token holding time
FCS
CCITT-32 CRC
A = address-recognized bit
xx = undefined
C = frame-copied bit
Frame
status A C
x x A C
x x

76. Other Ring Functions Priority Operation
PPP provides 8 levels of priority
Stations wait for token of equal or lower priority
Use RRR bits to “bid up” priority of next token
Ring Maintenance
Sending station must remove its frames
Error conditions
Orphan frames, disappeared token, frame corruption
Active monitor: a station responsible for error corrections.
MAC control frames and protocol are needed.
Beacon frames for detecting neighbors and broken links.
Election protocol.

77. Ring Latency & Ring Reinsertion
M stations
b bit delay at each station
b=2.5 bits (using Manchester coding)
Ring Latency:
t’ = d/n + Mb/R seconds
t’R = dR/n + Mb bits
Example
Case 1: R=4 Mbps, M=20, 100 meter separation
Latency = 20x100x4x106/(2x108)+20x2.5=90 bits
Case 2: R=16 Mbps, M=80
Latency = 840 bits

78. (a) Low Latency (90 bit) Ring Efficiency=400/490=82%
t = 400, transmission of last bit
t = 490, last bit received, reinsert token
t = 90, return
of first bit
t = 0, A begins frame
Reinsert token after frame return (400: frame length)
results in low efficiency.
(b) High Latency (840 bit) Ring
Efficiency=400/1240=32% A A A A
t = 1240, last bit
received, reinsert
token
t = 400, transmit
last bit
t = 840, arrival
first frame bit
t = 0, A begins frame

79. (a) Low Latency (90 bit) Ring (suppose header=120 bits)
Efficiency=400/400=100% A A A A
t = 90, return
of first bit
t = 210, return of header
t = 400, last bit enters ring, reinsert token
t = 0, A begins frame
Reinsert token immediately after completion of frame transmission: no idle period, 100%.
(b) High Latency (840 bit) Ring
Efficiency=400/960=42% A A A A
t = 840, arrival
first frame bit
t = 400, transmit
last bit
t = 960, last bit of header
received, reinsert token
t = 0, A begins frame

80. Fiber Distributed Data Interface (FDDI)
Token ring protocol for LAN/MAN
Counter-rotating dual ring topology
100 Mbps on optical fiber
Up to 200 km diameter, up to 500 stations
Station has 10-bit “elastic” buffer to absorb timing differences between input & output
Max frame 40,000 bits
km gives ring latency of 105,000 bits
FDDI has option to operate in multitoken mode

81. Dual ring becomes a single ringX
Dual ring becomes a single ring

82. FDDI Frame Format Data Frame Format Token Frame Format SD Destination
Address
Source
Information
FCS 8 4 ED FC 6 1 FS
PRE
Preamble
Frame
control
CLFFZZZZ C = synch/asynch
L = address length (16 or 48 bits)
FF = LLC/MAC control/reserved frame type
CLFFZZZZ = or denotes token frame SD FC ED
Token Frame Format
PRE

83. Timed Token Operation Station Operation Two traffic types
Maintain Token Rotation Timer (TRT): time since station last received token
When token arrives, find Token Holding Time
THT = TTRT – TRT
THT > 0, station can send all synchronous traffic up to Si + THT-Si asynchronous traffic.
THT < 0, station can only send synchronous traffic up to Si
As ring activity increases, TRT increases and asynch traffic throttled down
Two traffic types
Synchronous
Asynchronous
All stations in FDDI ring agree on target token rotation time (TTRT)
Station i has Si max time to send synch traffic
Token rotation time is less than 2*TTRT if
S1 + S2 + … + SM-1 + SM < TTRT
FDDI guarantees access delay to synch traffic