Advanced Computer Networks: Lecture 12 - Dr. Amir Qayyum
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CS716
Advanced Computer Networks
By Dr. Amir Qayyum
1
Lecture No. 12
Fiber Distributed Data Interface
• Similar to 802.5/IBM token rings but runs on fiber
• Consists of a dual ring: two independent rings that
transmit data in opposite directions at 100Mbps
• Tolerates a single link break or node failure (self
healing ring)
(a)
(b)
3
FDDI Concentrator
• Allows nodes to attach using a single cable SAS
• Dual connected nodes still exist DAS
• Concentrator attaches several SASs to dual ring
– Uses optical bypass to isolate failed SAS
Upstream
neighbor (DAS)
Downstream
neighbor (DAS)
Concentrator (DAS)
SAS
SAS
SAS
SAS
4
FDDI – Physical Properties
• Variable size buffer (9 80 bits)
between input and output
interfaces (10 ns bit time)
– Not required to fill buffer before
starting transmission
• Maximum 500 stations, maximum
2 km distance between any pair of
stations
5
FDDI – Physical Properties
• Total 200 km fiber: dual nature
implies 100 km cable
connecting all stations
• Physical media can be coax or
twisted pair cable
• Uses 4B/5B encoding
6
Timed Token Algorithm
• Token Holding Time (THT)
– Upper limit on how long a station can
hold the token
– configured to some suitable value
• Token Rotation Time (TRT)
– How long it takes the token to traverse
the ring (time since a host released the
token)
– TRT <= ActiveNodes x THT +
RingLatency
7
Timed Token Algorithm
• Target Token Rotation Time
(TTRT)
– “agreedupon” or negotiated
upper bound on TRT
8
MAC Algorithm
• Each node measures TRT
between successive token
arrivals
• If measuredTRT > TTRT
– Token is late
– Can not send data
9
MAC Algorithm
• If measuredTRT < TTRT
– Token is early so OK to send
– Send data for remaining time until
either
• No more data to send
• THT >= (TTRT – measured TRT)
10
FDDI MAC Issue
• If a node has lots of data, it
holds the token for the
maximum allowed time
• When a downstream neighbor
gets the token, its measured
TRT >= TTRT
– It cannot transmit its frame!
11
FDDI MAC Issue
• What if the
downstream neighbor
has some urgent data
to send ?
12
FDDI Traffic Classes
• Synchronous traffic
–Latency sensitive
–Gets higher priority
–Can always send data
13
FDDI Traffic Classes
• Asynchronous traffic
– Sensitive to throughput rather
than delay
– Lower priority
– Can send only if token is early
• May cause the time to exceed by
one FDDI frame
14
Bounded Priority Traffic
• If a node has large amount of
synchronous data
– It will send regardless of measured TRT
– TTRT will become meaningless !!!
• Therefore, total synchronous data
during one token rotation is bounded
by TTRT
15
Bounded Priority Traffic
• Worse case: 2xTTRT between
seeing token
– One TTRT is consumed first by
asynchronous data
– Another TTRT is then consumed by
synchronous data
• Backtoback 2xTTRT rotations
not possible
16
Token Maintenance
• Monitoring for the lost token
– No token when initializing ring
– Bit errors corrupt token pattern
– Node holding the token crashes
17
Token Maintenance
• Monitoring for a valid token
– Should periodically see valid
transmission (frame or token)
– Max. gap = ring latency + max
frame <= 2.5ms
• Set 2.5ms timer; start
negotiations if it fires
18
Token Maintenance
• The procedure when a node
– Joins the ring (startup)
– Suspects a failure
• Claim frame is used in order to
– Generate a new Token
– Agree on TTRT (so that an
application can meet its timing
constraints)
• A node can send a claim frame
without holding the token
19
Token Maintenance Procedure
• A node sends a claim frame including
its TTRT bid
• When a node receives a claim frame, it
compares the bid with its own bid
– If its bid is higher, it updates TTRT &
forward the frame
– If its bid is lower, it replaces with its own
claim frame
20
– If bids are equal, higher address node wins
Token Maintenance Procedure
• If a node’s claim frame
returns back to it, it knows:
–Its bid was the lowest
–Everyone knows TTRT
–It can now insert new token
21
Frame Format
• 4B/5B control symbols for start and end of frame
• Control Field
– 1st bit: asynchronous (0) versus synchronous (1) data
– 2nd bit: 16bit (0) versus 48bit (1) addresses
– Last 6 bits: demux key (includes reserved patterns for
token and claim frame)
• Status Field
– From receiver back to sender; error in frame
– Recognized address; accepted frame (flow control)
8
8
48
48
Start of
frame
Control
Dest
addr
Src
addr
Variable
Body
32
CRC
8
End of
frame
24
Status
22
Feedback
• Error detection
– Host attaches “error” marker to frame
– Sender detects error marker, resends later
• Flow control
– Host attaches “my address but did not
copy”
– Sender detects problem, resends later
(backs off)
23
Wireless LANs
24
Wireless LANs
• IEEE 802.11 standard
– Designed for use in a small area (offices,
campuses)
• Bandwidth: 1, 2 or 11 Mbps
– Up to 56Mbps in newer 802.11a standard
• Targets three physical media
– Two spread spectrum radio (2.4GHz freq)
– One diffused infrared (10m range, 850 25
nm band)
