Transcript ECE544 - WINLAB
ECE544: Communication Networks-II Spring 2013 D. Raychaudhuri Lecture II
Includes teaching materials from L. Peterson, J. Kurose
Today’s Lecture
• Recap of network architecture & top down design
– architecture paper discussion
• Shared media (MAC) protocols
– Ethernet – Token ring – IEEE 802.11
Link Layer: Introduction
• Some terminology: – hub/repeater (layer 1), bridge/LAN switch (layer 2), router (layer 3), host (layers 1-3 + app) – Links are communication channels that connect adjacent nodes along communication path (point-to-point, shared, wired, wireless) – Layer-2 frame: encapsulates payload/datagram/IP packet/service unit LAN 1 LAN 2 Switch Host Link Lin k Switch Router Link Host
Link Layer Services
• Data-link layer: transfer datagram from one node to adjacent node over a link – Framing: encapsulate datagram into frame, adding header, trailer. • Identify what set of bits constitute a frame, that is, determining the beginning and the end of a frame – channel access if shared medium • MAC addresses used in frame headers to identify source, destination • different from IP address!
– Reliable delivery between adjacent nodes • Error detection • Error recovery: forward error correction code, retransmission (ARQ)
Link Layer Communication • Link layer implemented in adaptor (NIC) and driver (Ethernet card, WLAN card) • Sending side: encapsulates higher layer payload in a frame, adds error checking bits, flow control, etc.
• Receiving side: error detection, flow control, extracts payload, passes to the receiving node Link layer protocol Sending node Datagram Frame Adaptor CPU Host Cache Frame Adaptor Memory I/O bus Control status register Bus interface NIC Link interface Network Datagram Recv node
Layer 2 vs. Layer 3 • Layer 2 switching – Based on MAC address – Self configuring and plug & play – Transparent to protocols above the MAC layer – Fast and inexpensive – Does not limit the scope of broadcasts – Does not scale to extremely large networks • Layer 3 routing – Based on IP address – Must get IP address (DHCP or manual assign) – Easily connect LANs that uses different link protocols – Scalable to large network by subnet routing – Broadcast limited only in a subnet
• Link Layer Techniques – Encoding (more Physical Layer stuff) – Framing & PPP Protocol – Error Detection & Correction – ARQ • Self study topics (see Ch2 & slides)
Binary Encoding
• Binary Encoding: signal turn the binary data (bits) into signals to transmit on cable or optical fiber link (physical layer stuff, but better to know) • Baseband, not modulate to high frequency • Nonreturn To Zero (NRZ): 1=high signal, 0=low – May stay on high or low signal too long for a long strings of consecutive 1s or 0s => baseline wander, clock recovery problems.
• Nonreturn to Zero Inverted (NRZI): 1 = signal transition (low to high, or high to low), 0=no change.
– Solve the problem of consecutive 1s, but not consecutive 0s
• •
Manchester Encoding
Manchester Encoding: NRZ_encode data XOR clock – Clock cycle (a low/high pair) = 2 x signal interval – Baud rate (the signal change rate) = 2 x bitrate – 0 =high-to-low transition, 1 = low-to-high transition – Clock recovery Variation: Differential Manchester – 1 = the first half of the signal equal to the last half of the previous bit’s signal – 0 = the first half of the signal opposite to the last half of the previous bit’s signal 0 1 0 0 Bits NRZ Clock Manchester 0 0 1 1 1 1 0 1 NRZI
Point-to-Point Data Link Protocol
• Two types of links – point-to-point link (easier than broadcast link) • one sender, one receiver on the link, NO Media Access Control • no need for explicit MAC addressing • e.g., dialup link, ISDN line – Broadcast (shared wire or medium) • popular point-to-point DLC protocols: – PPP (point-to-point protocol): byte-oriented • PPP for dial-up access • PPP over Ethernet (DSL) – HDLC (High level data link control): bit-oriented Modem PPP
PPP Functions
• Framing: encapsulation of network-layer datagram in data link frame – Identify what set of bits constitute a frame, that is, time determining the beginning and the end of a frame • carry data of any network layer protocol (not just IP) at same – ability to demultiplex upwards • bit transparency: must carry any bit pattern in the data field • error detection (no correction) • connection liveness: detect, signal link failure to network layer • network layer address negotiation: endpoint can learn/configure each other’s network address • PPP – no error correction/recovery – no flow control – out of order delivery OK – no need to support multipoint links (e.g., polling)
PPP Data Frame
• Flag: delimiter (framing) • Address: • Control: • Protocol: upper layer protocol to which frame carried (e.g. IP) • Info: upper layer data • Check: CRC Octet: 1 1 1 1 or 2 01111110 flag 11111111 address 00000011 protocol control variable info 2 or 4 CRC 1 01111110
Byte Stuff
• “data transparency”requirement: data field must be allowed to include flag pattern <01111110> – Q: is received <01111110> data or flag?
• Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte • Receiver: – two 01111110 bytes in a row: discard first byte, continue data reception – single 01111110: flag byte
PPP Link Control Protocol (LCP) • Before exchanging network-layer data, data link peers must
– configure PPP link (max. frame length, authentication)
• learn/configure network
– layer information – for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address
High-Level Data Link Control (HDLC)
• Bit oriented protocol: view the frame as a collection of bits, does not care byte boundaries.
• Sentinel characters 01111110 transmitted as the link is idle for synchronization • Bit stuffing: to distinguish the data pattern 01111110 in the body from the special “beginning/end” sequence – after transmitting any 5 consecutive 1s in body, insert a 0 – 011111xxxx => 0111110xxx
Bits:
8 16 01111110 Beginning sequence Header variable body 16 CRC 8 01111110 ending sequence
Error Detection
• EDC= Error Detection and Correction bits (redundancy) • D = Data protected by error checking, may include header fields • Error detection not 100% reliable!
– protocol may miss some errors, but rarely – larger EDC field yields better detection and correction
Parity Checking
• Single Bit Parity: – Detect single bit errors • Two Dimensional Bit Parity: – Detect and correct single bit error
Internet Checksum
• Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer) Sender: • treat segment contents as sequence of 16-bit integers • checksum: addition (1’s complement sum) of segment contents, and take the ones complement of the result • sender puts checksum value into UDP checksum field Receiver: • compute checksum of received segment • check if computed checksum equals checksum field value: – NO -error detected – YES -no error detected. But maybe errors (internet checksum not very strong for error detection, but simple)
Cyclic Redundancy Check (CRC) • A (n+1)-bit message M can be represented as a polynomial of degree n. For example, – X = 10011010; – M(X) = X 7 + X 4 + X 3 + X • Choose k+1 bit pattern (divisor), C(X), a polyn of degree k • goal: get k CRC bits, Y , such that – P=
– can detect all burst errors less than k+1 bits n bits k bits Y: CRC M: data bits to be sent M x 2 k XOR R
CRC Example
• Goal: design P(X) such that it is exactly divisible by C(X) • T(X) = M(X) X k (add k zero’s to the end of the message)
R
remainder T
[
C
( (
X X
) ) ] • Subtract the remainder from T(X) to get P(X).
– P(X) is now exactly divisible by C(X).
• Corresponding to the complete transmitted message (Remember – all addition/subtract use modulo-2 arithmetic)
Automatic Repeat reQuest(ARQ)
• Stop-and-wait ARQ – Transmit a frame and wait for acknowledge – If positive acknowledge (ACK) from receiver, send next frame – If ACK does not arrive after a certain period of time (Timeout), retransmits the frame – Simple, low efficiency • Go-back-N ARQ – Transmit frames continuously, no waiting – The receiver only acks the highest-numbered frames received in sequence – ACK comes back after a round-trip delay – If timeout, the sender retransmits the frames that are not acked and N-1 succeeding frames that were transmitted during the round-trip delay (N frames transmitted during a round-trip delay) – Need buffer at transmitter, does not have to buffer the frames at the receiver, – moderate efficiency and complexity. Less efficient when the round-trip delay is large and data transmission rate is high • Selective-repeat – Transmit continuously, no waiting – The receiver acks all successfully received frames – The sender only retransmits (repeats) the unacked frames when their timers expire – Most efficient, but most complex, buffer needed at both transmitter and receiver, need per frame timer
Sliding Window
• Reliable delivery: retransmission • Ordered delivery: preserve the order in which the frames are transmitted – Receiver does not pass along (buffer) out-of-order frames • Flow control: feedback mechanism by which the receiver is able to throttle the sender – Inform the sender of how much frames the receiver has room to receive
Sliding Window (Cont)
• Send window size ( transmit, • LAR SWS ): the upper bound on the number of unacked frames that the sender can – set according to the round-trip delay to keep the pipe full (recall: bandwidth x delay product represents the amount of data that could be in transit) : the sequence # of the last ack received • LFS • LAF : the sequence # of the last frame sent • Receiver window size ( willing to accept RWS ): the upper bound on the number of out-of-order frames that the receiver is : the sequence # of the largest acceptable frame • LFR : the sequence # of the last frame received • SeqNumToAck : the largest sequence # not yet acked, such that
all frames
with seq # <= SeqnumToAck have been received
LAR SWS
Sliding Window
LFS Sender Receiver 1 2 3 4 5 6 7 8 9 3 4 5 6 7 8 9 10 11 12 Ack 1 Ack 2 Ack 2 Ack 2 Ack 2 Ack 3 Ack 4 Ack 5 Ack 6 1 2 3 4 5 6 7 8 9 3 4 5 6 7 8 9 10 11 12 LFR Error RWS LAF SeqNumToAck • LFS-LAR<=SWS, LAF-LFR<=RWS • Finite Seq. # wraps around: – SWS < (MaxSeqNum+1)/2 when RWS=SWS to distinguish between different incarnations of the same seq. #
Shared Media Networks
• MAC (medium access control) – ALOHA, Slotted ALOHA – CSMA/CD, CSMA/CA – Token Ring – TDMA, Dynamic TDMA – FDMA, CDMA • LAN Technologies – IEEE 802.3 Ethernet – IEEE 802.5 Token Ring – IEEE 802.11 Wireless LAN
End host Application Presentation Session Transport Network Data link
Medium Access Sublayer
LLC MAC
Applications TCP/UDP IP Subnet
• Medium access control (MAC) sublayer is not relevant on point-to-point links • The MAC sublayer is only used in broadcast or shared medium/channel networks • All communication entities “share” a common channel – Wired networks: Ethernet LAN – Wireless & Mobile Networks: Satellite, Cellular, Wireless LAN,
Physical Physical
Media Access Protocol
• Shared broadcast channel – two or more simultaneous transmissions by nodes: interference • Collision if node receives two or more signals at the same time • MAC protocol – Determines how nodes share channel, i.e., determine when node can transmit • Ideally, if broadcast channel of rate R bps – When one node wants to transmit, it can send at rate R.
– When M nodes want to transmit, each can send at average rate R/M (fairness)
MAC Classification
• Channel Partitioning – divide channel into smaller “pieces” (time slots, frequency, code) – allocate piece to node for exclusive use • TDMA, CDMA, FDMA • Random Access – channel not divided • When node has frame to send, transmit with the total channel bandwidth – No coordination between nodes, control is completely distributed – two or more nodes transmit simultaneously ➜ “collision” – random access MAC protocol should specify: • how to detect collisions • how to recover from collisions (e.g., via delayed retransmissions) • Examples: ALOHA, Slotted ALOHA, CSMA/CD, CSMA/CA • “Taking turns” – Nodes take turns • Token ring • Hybrid – Combine two or more techniques together
Pure (Unslotted) ALOHA
• Early packet radio network created at the U. of Hawaii in 1970 • Uplink channel (clients->hub) and downlink channel (hub->clients) uses different frequencies • Client nodes send data frames to the central hub using the shared uplink channel.
• The hub immediately re-send the received frames, allowing clients to determine whether or not their data • had been received properly.
Simplest form of random access, provides basis for more advance contention MAC Hub Client
Aloha Algorithm
• Aloha Algorithm: – Nodes transmit immediately whenever they have a frame to send – No synchronization among nodes • If collision, retransmit after random delay – random delay prevents the same frames from colliding over and over again • collision window or “vulnerable period”: – frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1]
Pure Aloha efficiency
• Assume that the aggregate frame arrival is Poisson Process • P [k arrivals in a time-interval] =
e
G G k k
!
• G: the mean number of aggregate arrivals (all nodes in network) in the time interval • time-interval = one frame transmission time • Conditional successful probability for one frame transmission attempt is – P 0 = P [0 other attempts in 2 time-intervals] = • The probability of successful transmission e -2G – S = GP 0 = Ge -2G • S is optimum at G=1/2 • S=1/2e = 0.184
Slotted Aloha
• Assumptions – all frames same size – time is divided into equal size slots, time to transmit 1 frame – nodes start to transmit frames only at beginning of slots – nodes are synchronized – if 2 or more nodes transmit in slot, detect collision – Feedback channel about whether packet is received or not (half duplex) • Operation – when node obtains fresh frame, it transmits at the beginning of next slot – – no collision, node can send new frame in next slot if collision, wait a random number of slots and try to send again
Efficiency of Slotted ALOHA
• Aggregate frame arrival is Poisson Process • P [k arrivals in a time-interval] =
e
G G k k
!
• G: the mean number of aggregate arrivals (for all nodes in network) in this interval • time-interval = slot (one frame transmission time) • Successful probability for each slot is : S= P [1 attempt in a slot] = Ge -G • S is optimum at G=1 • S=1/e = 0.368
(slotted aloha reduce the potential collision period from synchronization) 2t to t by node
Performance of ALOHA
• Throughput versus offered traffic for ALOHA systems • The main reason for poor channel utilization of ALOHA (pure or slotted) is that all stations can transmit at will, without paying attention to what the other stations are doing.
Carrier Sense Multiple Access (CSMA) • CSMA: listen before transmit: – If channel sensed idle: transmit – If channel sensed busy, defer transmission • Human analogy: don ’ t interrupt others!
• Can collisions occur in this scheme?
– Two nodes might attempt to transmit a frame at the same time – Propagation delay means two nodes may not hear each other ’ s transmission immediately • Several variants of CSMA protocols: – Non-Persistent CSMA – 1-Persistent CSMA – P-Persistent CSMA
Non-persistent CSMA
• To send data, a node first listens to the channel to see if anyone else is transmitting. • If so, the node waits a random period of time (instead of keeping sensing until the end of the transmission) and repeats the algorithm. Otherwise, it transmits a frame. • If a collision occurs, the node waits a random amount of time and starts all over again.
1-persistent CSMA
Algorithm: 1. To send data, a node first listens to the channel to see if anyone else is transmitting. 2. If so, the node waits (keeps sensing it) until the channel becomes idle. Otherwise, it transmits a frame. 3. If a collision occurs, the node waits a random amount of time and starts all over again. It is called 1-persistent because the station transmits with a probability of 1 whenever it starts sensing the channel and finds the channel idle. (Greedy)
P-persistent CSMA
• • Assume channels are slotted One slot = contention period (i.e., one round trip propagation delay) Algorithm: 1.
– – – Sense the channel • If channel is idle, transmit a packet with probability p if a packet was transmitted, go to step 2 • if a packet was not transmitted, wait one slot and go to step 1 If channel is busy, wait one slot and go to step 1.
In other words, wait until idle and then transmit with probability p 2.
– Detect collisions If a collision occurs, wait a random amount of time and go to step 1
Propagation Delay
A B C D • D only sense A ’ s transmission after a propagation delay τ • If τ is larger than packet transmission time, too much time wasted.
• CSMA in satellite communication? No.
• Distance & propagation delay determine collision probability
The size (length) of the network must be limited!
CSMA Performance Analysis • Assumptions
– Constant length packets – No errors, except those caused by collisions – Collision: entire packet transmission time wasted – Each host can sense the transmissions of all other hosts – The propagation delay is small compared to the transmission time
Analysis of Non-persistent CSMA
Unsuccessful transmission period Successful transmission period
Normalized Time
Y a 1
Busy period
a
Idle period
1
Busy period
a a
: the ratio of propagation delay to packet transmission time • Poisson arrival, P(k arrivals in time duration t) = • Prob. of success transmission S= U x I/(B+I) • Mean B = Y + 1 + a , mean I = 1/G • U = G e -Ga • F Y (y)=P{no packet occur in an duration of a-y } = e -G(a-y) (CDF) 1
E
(
Y
)
a
( 1
e
aG
)
G S e
Gt G kt k
!
G
( 1
Ge
aG
2
a
)
e
aG
Comparison of the channel utilization versus load for various random access protocols
CSMA with Collision Detection
CSMA/CD (Carrier Sense Multiple Access with Collision
Detection) protocol further improves ALOHA by aborting transmissions as soon as a collision is detected . Operation: • To send data, a node first listens to the channel to see if anyone else is transmitting. • • • – If not, it transmits a frame If channel busy, deferral as in CSMA the node wait a random period of time and repeats the algorithm (non-persistent), or waits until the end of the transmission (1 persistent) The node will detect the collision , if collision detected, abort its transmission (reducing channel wastage), waits a random amount of time, and starts all over again.
How to Detect Collision
• Prerequisite: A node can listen while talking • Easy in wired LANs: measure signal strength, compare Tx and Rx signals • Difficult in wireless LANs: receiver shut off while transmitting Tx Rx
CSMA/CA
• Wireless LANs • How can a node detect collision if it cannot listen while talking?
• Collision Avoidance – Random Backoff (instead of 1-persistent) – Request-to-send (RTS)/clear-to-send (CTS) • CS no longer works well – Rules: • carrier ==> do not transmit • no carrier ==> OK to transmit – But the above rules do not always apply to wireless.
Problems with carrier sensing
Hidden terminal problem
Z Y W
W finds that medium is free and it transmits a packet to Z
Problems with carrier sensing
Exposed terminal problem
W
Z is transmitting to W
Z X Y
Y will not transmit to X even though it cannot interfere
Solving Hidden Node problem with RTS/CTS CTS listen RTS - wait long enough for the requested station to respond with CTS - if (timeout) then ready to transmit RTS
X Y Z W
- listen CTS - wait long enough for the transmitter to send its data listen RTS ==> transmitter is close listen CTS ==> receiver is close Note: RTS/CTS does not solve exposed terminal problem. In the example above, X can send RTS, but CTS from the responder will collide with Y’s data.
Transmitter
RTS/CTS exchange example
SIFS DIFS
Frame RTS
Receiver
CTS
352 µs 10 µs 304 µs 10 µs 8192 s
ACK
10 µs 304 µs Other
NAV (RTS) NAV (CTS)
• RTS + CTS + Frame + ACK exchange invoked when frame size is large • NAV (Network Allocation Vector) – NAV maintains prediction of future traffic on the medium based on duration information that is announced in RTS/CTS frames prior to actual exchange of data
“Taking Turns” MAC protocols
Polling: • master node “invites” slave nodes to transmit in turn • concerns: – polling overhead Token passing: • control token passed from one node to next sequentially.
• token message • concerns: – token overhead – complexity – single point of failure (master) – single point of failure (token)
TDMA
• Time Division Multiple Access (TDMA)
Fixed TDMA
• access to channel in "rounds" • each station gets fixed length slot (length = packet transmission time) in each round • unused slots go idle – Not efficient • example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
Dynamic TDMA
• In dynamic TDMA, a reserves user data stream. scheduling algorithm dynamically a variable number of timeslots in each frame to variable user data streams, based on the traffic demand of each • Negotiations (beforehand) to determine how to allocate slots dynamically.
TDD-TDMA Frame TDM Downlink D-TDMA Uplink
Modem preamble
S-ALOHA control Burst from Access Point -> Mobiles Burst from User A To Access Point
Frame header and schedule
User B User C
FDMA
• FDMA: frequency division multiple access – channel spectrum divided into frequency bands – each station assigned a frequency band • unused transmission time in frequency bands go idle if assignment fixed – Inefficient => make it dynamically assigned to different stations based on traffic demand – OFDMA
Spread Spectrum and CDMA
• What if we don ’ y not divide up the channel by time (as in TDMA), or frequency (as in FDMA)? Is collision inevitable?
• Not if collision is no longer damaging!
– Is there any way to decode bits garbled by other overlapping frames?
Code Division Multiple Access (CDMA) based on Spread Spectrum • Another perspective to solve multiple access problems • Spread Spectrum is a PHY innovation, not a MAC technique.
• CDMA encodes data with a special code associated with each user and uses the constructive interference properties of the special codes to perform the multiplexing.
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
1 0 1 0 1 0
Spread Spectrum (cont)
• Direct Sequence – for each bit, send XOR of that bit and random bits – random sequence known to both sender and receiver – called n -bit chipping code n Data stream: 1010 Random sequence: 0100101101011001 XOR of the two: 1011101110101001
Code Division Multiple Access (CDMA)
• Multiplexing Technique used with spread spectrum • Start with data signal rate – Called bit data rate • Break each bit into specific to each user – User’s code • New channel has chip data rate • E.g. k k D chips according to fixed pattern kD chips per second =6, three users (A,B,C) communicating with base station R • Code for A = <1,-1,-1,1,-1,1> • Code for B = <1,1,-1,-1,1,1> • Code for C = <1,1,-1,1,1,-1>
LAN technologies
Ethernet Token Ring Wireless LAN
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 • Evolution: Bus topology (90’s)
Star topology (now)
• Most successful access network technology Hub or switch Advance
Ethernet Frame
• Preamble: 8 bytes – 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 – used to synchronize receiver, sender clock rates • Addresses:6 bytes – if adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to net-layer protocol, otherwise, adapter discards frame • Type: 2 bytes – indicates the higher layer protocol (mostly IP but others also supported) • CRC: 4 bytes – checked at receiver, if error is detected, the frame is simply dropped • Body: 46-1500 bytes – Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Octets 8 Preamble 6 Dest addr 6 Src addr 2 Type 64-1518 46-1500 Body 4 CRC 72-1526
MAC Address
• MAC Addresses – unique, 48-bit unicast address assigned to each adapter • example: 3
8:10:2b:e4:b1:02
– broadcast: all
1
s, ff:ff:ff:ff:ff:ff – multicast: multicast flag (the lowest bit of the 1 st octet)= 1 • 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for IP multicast – IP multicast group address mapped to the lower order 23 bits of MAC address (not one-to-one mapping) – Unique MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space • the first three octets as vendor-specific
MAC Address vs. IP Address
• 48-bit MAC address – Layer 2 – Used to get packet from one interface to another within the same LAN/subnet (Ethernet, token ring…) – Flat – Unique – No change when moving • 32-bit IP address – Network layer – Used to get packet to destination IP subnet – Hierarchical – Change when moving • Depending on IP subnet to which node is attached • IP to MAC address translation: ARP (more later)
Different Flavors of Ethernet Format
• Ethernet version II Octets 8 Preamble 6 6 Dest MAC Src MAC Data link header 2
Type
46-1500 Body 4 FCS Data & CRC (FCS) • IEEE 802.3
Octets 8 Preamble 6 Dest MAC 6 Src MAC Datalink Header 2 1 1
Length
DSAP SSAP 1 Control Logical Link Control 43-1497 Body 4 FCS Data & CRC (FCS) • • • • • Length: the length of the data in the frame (excluding preamble, CRC, DLC addresses, and the Length field itself) Destination Service Access Point (DSAP): a pointer to a memory buffer in the receiving station. It tells the receiving NIC in which buffer to put this information. useful in situations where users are running multiple protocol stacks, etc... Source Service Access Point (SSAP) Control: the type of LLC frame Distinguish Ethertypes and Control field – Ethertypes value > 0x05DC (1500), Length <= 1500
Unreliable, connectionless service • Connectionless: No handshaking between sending and receiving adapter.
• Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter
– stream of datagrams passed to network layer can have gaps – gaps will be filled if app is using TCP – otherwise, app will see the gaps
Ethernet CSMA/CD
1. If sender senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits (1-persistent CSMA) • Inter-frame gap: time to send 96 bits (9.6 frame !
transmitting, aborts and sends 32-bit s for 10Mbps) 2. If adapter transmits entire frame without detecting another transmission, the adapter is done with 3. If adapter detects another transmission while jam signal (collision detection) 4. After aborting, sender enters exponential backoff – after the mth collision, adapter chooses a K at random from {0,1,2,…,2 m -1}. Then waits K·512 bit times (k x 51.2 us in 10 Mbps Ethernet) and returns to Step 1 – give up after several tries (usually 16)
Ethernet CSMA/CD (Cont)
Jam Signal: • make sure all other transmitters are aware of collision; • 32 bits • Frame: 64 (preamble) + 32 (jamming sequence) = 96 bits – Runt Frame Exponential Backoff: • Goal: adapt retransmission attempts to the estimated current # of active stations or load – heavy load: random wait will be longer • first collision: choose K from {0,1}; delay is K·512 bit (51.2 s in 10 Mbps) transmission times • after second collision: choose K from {0,1,2,3}… • after ten collisions, choose K from {0,1,2,3,4,…,1023}
A A A A
Collisions
B B B B The longer the propagation delay, the higher probability of collision.
Worst case: • A sends at t, A’s frame arrives B at t+d • B begins transmitting at t+d and collides with A’s frame • B sends runt frame, the runt frame arrives A at t+2d • To detect collision, A must continue transmit until t+2d. A must transmit for 2d. • Round-trip delay about 51.2 us for 2500m long Ethernet with 4 repeater – Corresponds to 512 bits for 10 Mbps Ethernet – So min frame size 512 bits
10BaseT and 100BaseT
• 10/100 Mbps rate; latter called “fast ethernet” • T stands for Twisted Pair • Star toplogy, max 100m between node and hub • Hubs: physical-layer repeaters – bits coming from one link go out all other links at the same rate – no frame buffering – no CSMA/CD at hub: adapters detect collisions – provides net management functionality Hub
Legacy Ethernet
• 10Base5 – Bus topology with coaxial cable – 10 Mbps, Up to 500m each segment – No more than 4 repeaters between any pair of stations – Max 2500 m – Max 1024 hosts • 10Base2 – Daisy chain – Up to 200m Repeater Terminator Transceiver Terminator Adaptor 10 Base5 Ethernet
Gbit Ethernet
• uses standard Ethernet frame format • allows for point-to-point links and shared broadcast channels • in shared mode, CSMA/CD is used; short distances between nodes required for efficiency • uses hubs • Full-Duplex at 1 Gbps for point-to-point links • 10 Gbps now
Ethernet Performance
• Max throughput <1 as a function of span – As propagation delay increases, efficiency decreases – instability can occur unless load is reduced under congestion conditions – retransmission backoff policy for stability ~0.8
Thru Capacity Limit Traffic margin Overload region Normal operating point stable policy (backoff too high) Offered Traffic stable policy (retx backoff) unstable policy (no backoff) load lines
Wireless LANs
• 802.11 a/b/g different Phy technologies – 802.11 b/g: 20 MHz channel in 2.4 GHz, up to 11 Mbps (802.11b), 54 Mbps (802.11g) phy data rate – 802.11a: 20 MHz channel in 5GHz, up to 54 Mbps phy data rate • 802.11n: – 130 Mbps phy data rate on 20 MHz channel (2 x 2 MIMO) – 300 Mbps phy data rate on 40 MHz channel (channel bonding with 2 x 2 MIMO)
See supplementary WLAN tutorial slides
Today’s Homework
• Peterson & Davie, Chap 2, 4th ed 2.6
2.18
2.23
2.33
2.44
2.42
Download and review Ethernet and 802.11 MAC specs, and study IEEE 802.11 Wireless LAN Overview slides Due 2/8 74