ECE544: Communication Networks-II, Spring 2006 D. Raychaudhuri & Max Ott Lecture II Includes...
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Transcript of ECE544: Communication Networks-II, Spring 2006 D. Raychaudhuri & Max Ott Lecture II Includes...
ECE544: Communication Networks-II, Spring 2006
D. Raychaudhuri & Max Ott
Lecture II
Includes teaching materials from L. Peterson
Today’s Lecture• Recap of network architecture &
top-down design– architecture paper discussion
• Shared media (MAC) protocols– Ethernet– Token ring– IEEE 802.11
Shared Media Networks
Cable (Ethernet)Token ring (FDDI)Wireless (802.11,
DTDMA)
Ethernet Overview• History
– developed by Xerox PARC in mid-1970s– roots in Aloha packet-radio network– standardized by Xerox, DEC, and Intel in 1978– similar to IEEE 802.3 standard
• CSMA/CD– carrier sense– multiple access– collision detection
• Frame Format
Destaddr
64 48 32
CRCPreamble Srcaddr
Type Body
1648
Ethernet (cont)• Addresses
– unique, 48-bit unicast address assigned to each adapter– example: 8:0:2b:e4:b1:2– broadcast: all 1s– multicast: first bit is 1
• Bandwidth: 10Mbps, 100Mbps, 1Gbps• Length: 2500m (500m segments with 4 repeaters)• Problem: Distributed algorithm that provides fair
access
Transmit Algorithm
• If line is idle…– send immediately– upper bound message size of 1500 bytes– must wait 9.6us between back-to-back
frames
• If line is busy…– wait until idle and transmit immediately– called 1-persistent (special case of p-
persistent)
Algorithm (cont)
• If collision…– jam for 32 bits, then stop transmitting frame– minimum frame is 64 bytes (header + 46 bytes
of data)– delay and try again
• 1st time: 0 or 51.2us• 2nd time: 0, 51.2, or 102.4us• 3rd time51.2, 102.4, or 153.6us• nth time: k x 51.2us, for randomly selected
k=0..2n - 1• give up after several tries (usually 16)• exponential backoff
CollisionsA B
A B
A B
A B
Ethernet Bridging• Bridge device for scaling network
span
Bridge
Ethernet A
Ethernet B
Pkt forwardingtable..can “learn”from packet source addr
Ethernet Performance• Max throughput <1 as a function of
span– instability can occur unless load is reduced
under congestion conditions– retransmission backoff policy for stability
Overloadregion
Normal operatingpoint
Capacity Limit~0.8
load lines
Offered Traffic
ThruTrafficmargin
stable policy(retx backoff)
stable policy(backoff too high)
unstable policy(no backoff)
Token Ring Overview
• Examples– 16Mbps IEEE 802.5 (based on earlier IBM
ring)– 100Mbps Fiber Distributed Data Interface
(FDDI)
Token Ring (cont)• Idea
– Frames flow in one direction: upstream to downstream
– special bit pattern (token) rotates around ring– must capture token before transmitting– release token after done transmitting
• immediate release• delayed release
– remove your frame when it comes back around– stations get round-robin service
• Frame Format
Control
8 8 8 24
CRCStart offrame
End offrame
Destaddr
Body
4848
Srcaddr
Status
32
Timed Token Algorithm• Token Holding Time (THT)
– upper limit on how long a station can hold the token
• Token Rotation Time (TRT)– how long it takes the token to traverse the ring.– TRT <= ActiveNodes x THT + RingLatency
• Target Token Rotation Time (TTRT)– agreed-upon upper bound on TRT
Algorithm (cont)
• Each node measures TRT between successive tokens– if measured-TRT > TTRT: token is late so don’t send– if measured-TRT < TTRT: token is early so OK to send
• Two classes of traffic– synchronous: can always send– asynchronous: can send only if token is early
• Worse case: 2xTTRT between seeing token• Back-to-back 2xTTRT rotations not possible
Token Maintenance• Lost Token
– no token when initializing ring– bit error corrupts token pattern– node holding token crashes
• Generating a Token (and agreeing on TTRT)– execute when join ring or suspect a failure– send a claim frame that includes the node’s TTRT bid– when receive claim frame, update the bid and forward– if your claim frame makes it all the way around the
ring:• your bid was the lowest• everyone knows TTRT• you insert new token
Maintenance (cont)
• Monitoring for a Valid Token– should periodically see valid transmission
(frame or token)– maximum gap = ring latency + max frame <
= 2.5ms– set timer at 2.5ms and send claim frame if it
fires
Wireless MAC
• Typically, three categories of radio MAC protocols :Simple packet-by-packet CSMA class protocols (802.11)Integrated packet/flow oriented protocols based on
dynamic TDMA (Hiperlan, 802.16, ...)CDMA-based (HDR, 3G, etc.)
Wireless LANs
• IEEE 802.11a & b, Hiperlan2• Bandwidth: ~1-20 Mbps• Physical Media
– spread spectrum radio (2.4, 5 GHz)– diffused infrared (10m)
Spread Spectrum• Idea
– spread signal over wider frequency band than required
– originally deigned to thwart jamming
• Frequency Hopping– transmit over random sequence of frequencies– sender and receiver share…
• pseudorandom number generator• seed
– 802.11 uses 79 x 1MHz-wide frequency bands
Spread Spectrum (cont)• Direct Sequence
– for each bit, send XOR of that bit and n random bits
– random sequence known to both sender and receiver
– called n-bit chipping code – 802.11 defines an 11-bit chipping code
Random sequence: 0100101101011001
Data stream: 1010
XOR of the two: 1011101110101001
0
0
0
1
1
1
Collision Avoidance
• Similar to Ethernet• Problem: hidden and exposed
nodes
A B C D
RTS/CTS Procedure• Sender transmits RequestToSend (RTS) frame• Receiver replies with ClearToSend (CTS)
frame• Neighbors…
– see CTS: keep quiet– see RTS but not CTS: ok to transmit
• Receive sends ACK when has frame– neighbors silent until see ACK
• Collisions– no collisions detection– known when don’t receive CTS– exponential backoff
RTS/CTS Procedure
RTS CTS
length (L)
Data Packet ACK
L
RTS collisionexponentialbackoff
Modempreamble
TDM Downlink
Frame Header
D-TDMA UplinkS-ALOHA
control
User B User C
TDMA Frame (~1-2 ms)
Cell Sequence No.
GFC VPIVPI VCI
VCIVCI PTI CLP
HEC
Standard ATMPayload
(48 bytes)
CRC-16
WATM Cell
BW Req/Alloc Pkt
AP IdentifierTDMA Frame No.
# Up contention slots
Superframe size
# Dn Control Slots
# Dn data cells
Reserved (8 Bytes)
CRC-16
# Slots Req/Start..
Resvd/# Slots Alloc
Type RSN
Mobile ID
RT....Slot
CRC-16
# Data Cells# Control Pkts
Reserved
CRC-16
Mobile ID
Uplink Subframe Header
Burst from User ATo Base Station
Burst from Base Station -> Mobiles
TDMA/TDD MAC• MAC protocol used in some broadband
wireless scenarios (Hiperlan, 802.16, WATM) supports flow QoS, etc.
Supporting Mobility• Case 1: ad hoc net working• Case 2: access points (AP)
– tethered– each mobile node associates with an AP
B
H
A
F
G
D
AP-2
AP-3AP-1
EC
Distribution system
Mobility (cont)• Scanning (selecting an AP)
– node sends Probe frame– all AP’s w/in reach reply with ProbeResponse frame– node selects one AP; sends it AssociateRequest
frame– AP replies with AssociationResponse frame– new AP informs old AP via tethered network
• When– active: when join or move– passive: AP periodically sends Beacon frame
27
Today’s Homework• Peterson & Davie, Chap 2, 3rd ed
2.62.182.232.332.442.42
Download and review Ethernet and 802.11b specs
Due next Fri (2/4)
28
Class Presentations• Performance evaluation of CSMA/CD• Error control protocols: HDLC summary
and sliding window implementation• Host interface: 802.11 driver code example• ATM LAN emulation protocol (LANE)• Switch fabric tutorial: crossbar, knockout,
banyan; shared media; self-routing• VLAN and VPN• …more TBA