Download Lecture2: Physical and data link layer

Physical Media
 physical link:
transmitted data bit
propagates across link
 guided media:

signals propagate in
solid media: copper,
fiber
 unguided media:
 signals propagate
freely, e.g., radio
Twisted Pair (TP)
 two insulated copper
wires


Category 3: traditional
phone wires, 10 Mbps
ethernet
Category 5 TP:
100Mbps ethernet
1
Physical Media: coax, fiber
Coaxial cable:
 wire (signal carrier)
within a wire (shield)


baseband: single channel
on cable
broadband: multiple
channel on cable
 bidirectional
 common use in 10Mbs
Fiber optic cable:
 glass fiber carrying
light pulses
 high-speed operation:


100Mbps Ethernet
high-speed point-to-point
transmission (e.g., 5 Gps)
 very low error rate
Ethernet
2
Physical media: radio
 signal carried in
electromagnetic
spectrum
 no physical “wire”
 bidirectional
 propagation
environment effects:



reflection
obstruction by objects
interference
Radio link types:
 microwave
 e.g. up to 45 Mbps channels
 LAN (e.g., 802.11b/g)
 11/54 Mbps
 wide-area (e.g., cellular)
 e.g. CDPD, 10’s Kbps
 satellite
 up to 50Mbps channel (or
multiple smaller channels)
 270 Msec end-end delay
 geosynchronous versus
LEOS (low earth orbit)
3
The Data Link Layer
Our goals:
Overview:
 understand principles
 link layer services
behind data link layer
services:



error detection,
correction
sharing a broadcast
channel: multiple access
link layer addressing
 error detection, correction
 multiple access protocols and
LANs
 link layer addressing
 specific link layer technologies:

Ethernet
 instantiation and
implementation of various
link layer technologies
4
Link Layer: setting the context
5
Recap: The Hourglass Architecture of the Internet
Telnet Email
TCP
FTP WWW
UDP
IP
Ethernet Wireless FDDI
6 6
Link Layer: setting the context
 two physically connected devices:
 host-router, router-router, host-host
 unit of data: frame
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
network
link
physical
Hl Hn Ht M
frame
adapter card
7
Link layer: Context
 Data-link layer has
responsibility of
transferring datagram
from one node to
another node over a link
 Datagram transferred by
different link protocols
over different links, e.g.,



Ethernet on first link,
frame relay on
intermediate links
802.11 on last link
transportation analogy
 trip from New Haven to
San Francisco
 taxi: home to union
station
 train: union station to
JFK
 plane: JFK to San
Francisco airport
 shuttle: airport to
hotel
8 8
Link Layer Services
 Framing, link access:



encapsulate datagram into frame, adding header, trailer
implement channel access if shared medium,
‘physical addresses’ used in frame headers to identify
source, destination
• different from IP address!
 Reliable delivery between two physically connected
devices:


seldom used on low bit error link (fiber, some twisted
pair)
wireless links: high error rates
• Q: why both link-level and end-end reliability?
9
Link Layer Services (more)
 Flow Control:

pacing between sender and receivers
 Error Detection:
errors caused by signal attenuation, noise.
 receiver detects presence of errors:

• signals sender for retransmission or drops frame
 Error Correction:

receiver identifies and corrects bit error(s)
without resorting to retransmission
10
Adaptors Communicating
datagram
sending
node
frame
adapter
 link layer
implemented in
“adaptor” (aka NIC)

Ethernet card,
modem, 802.11 card
 adapter is semi-
autonomous,
implementing link &
physical layers
receiving
node
link layer protocol
frame
adapter
 sending side:
 encapsulates datagram in
a frame
 adds error checking bits,
rdt, flow control, etc.
 receiving side
 looks for errors, rdt, flow
control, etc
 extracts datagram, passes
to receiving node
11
Link Layer: Implementation
 implemented in “adapter”
e.g., PCMCIA card, Ethernet card
 typically includes: RAM, DSP chips, host bus
interface, and link interface

M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
adapter card
network
link
physical
Hl Hn Ht M
frame
12
Error Detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
• Error detection not 100% reliable! Q: why?
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction
13
Parity Checking
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
Parity bit=1 iff
Number of 1’s even
0
0
14
Internet checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment (note: used at transport layer only)
Sender:
 treat segment contents
as sequence of 16-bit
integers
 checksum: addition (1’s
complement sum) of
segment contents
 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 nonetheless?
15
Checksumming: Cyclic Redundancy Check
 view data bits, D, as a binary number
 choose r+1 bit pattern (generator), G
 goal: choose r CRC bits, R, such that



<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
can detect all burst errors less than r+1 bits
 widely used in practice (ATM, HDCL)
16
CRC Example
Want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want reminder R
R = remainder[
D.2r
G
]
17
Example G(x)
 16 bits CRC:
 CRC-16: x16+x15+x2+1,
CRC-CCITT: x16+x12+x5+1
 both can catch
• all single or double bit errors
• all odd number of bit errors
• all burst errors of length 16
or less
• >99.99% of the 17 or 18 bits
burst errors
CRC-16 hardware implementation
Using shift and XOR registers
http://en.wikipedia.org/wiki/CRC-32#Implementation
18 18
Multiple Access Links and Protocols
Three types of “links”:
 point-to-point (single wire, e.g. PPP, SLIP)
 broadcast (shared wire or medium; e.g, Ethernet,
Wavelan, etc.)
 switched (e.g., switched Ethernet, ATM etc)
19
Multiple Access protocols
 single shared communication channel
 two or more simultaneous transmissions by nodes:
interference

only one node can send successfully at a time
 multiple access protocol:
 distributed algorithm that determines how stations share
channel, i.e., determine when station can transmit
 communication about channel sharing must use channel itself!
 what to look for in multiple access protocols:
• synchronous or asynchronous
• information needed about other stations
• robustness (e.g., to channel errors)
• performance
20
Multiple Access protocols
 claim: humans use multiple access protocols
all the time
 class can "guess" multiple access protocols
multiaccess protocol
 multiaccess protocol
 multiaccess protocol
 multiaccess protocol

1:
2:
3:
4:
21
MAC Protocols: a taxonomy
Three broad classes:
 Channel Partitioning


divide channel into smaller “pieces” (time slots,
frequency)
allocate piece to node for exclusive use
 Random Access
 allow
collisions
 “recover” from collisions
 “Taking turns”

tightly coordinate shared access to avoid collisions
Goal: efficient, fair, simple, decentralized
22
MAC Protocols: Measures
 Channel Rate = R bps
 Efficient:
 Single
user: Throughput R
 Fairness
N
users
 Min. user throughput R/N
 Decentralized

Fault tolerance
 Simple
23
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
 access to channel in "rounds"
 each station gets fixed length slot (length = pkt
trans time) in each round
 unused slots go idle
 example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6
idle
24
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
 channel spectrum divided into frequency bands
 each station assigned fixed frequency band
 unused transmission time in frequency bands go idle
 example: 6-station LAN, 1,3,4 have pkt, frequency
frequency bands
bands 2,5,6 idle
25
TDMA & FDMA: Performance
 Channel Rate = R bps
 Single user
 Throughput
R/N
 Fairness
 Each
user gets the same allocation
 Depends on maximum number of users
 Decentralized

Requires resource division
 Simple
26
Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access)
 unique “code” assigned to each user; ie, code set partitioning
 used mostly in wireless broadcast channels (cellular,




satellite, etc)
all users share same frequency, but each user has own
“chipping” sequence (ie, code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding: inner-product of encoded signal and chipping
sequence
allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes are
almost “orthogonal”)
27
CDMA - Basics
 Orthonormal codes:


<ci,cj> =0 i≠j
<ci,ci> =1
 Encoding at user i:


Bit 1 send +ci
Bit 0 send -ci
 Decoding (at user i):




Receive a vector ri
Compute t=<ri,ci>
If t=1 THEN bit=1
If t=-1 THEN bit=0
 Correctness of decoding


Single user
Multiple users
• Assume additive channel.
• R = c1 – c2
• Output <R,c1> = <c1,c1> + <-c2,c1> = 1 + 0 = 1
28
CDMA Encode/Decode
29
CDMA: two-sender interference
30
Random Access protocols
 When node has packet to send
 transmit at full channel data rate R.
 no a priori coordination among nodes
 two or more transmitting nodes -> “collision”,
 random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed
retransmissions)
 Examples of random access MAC protocols:
 slotted ALOHA
 ALOHA
 CSMA and CSMA/CD
31
Slotted Aloha [Norm Abramson]
 time is divided into equal size slots (= pkt trans. time)
 node with new arriving pkt: transmit at beginning of
next slot
 if collision: retransmit pkt in future slots with
probability p, until successful.
Success (S), Collision (C), Empty (E) slots
32
Slotted Aloha efficiency
Q: what is max fraction slots successful?
A: Suppose N stations have packets to send
 each transmits in slot with probability p
 prob. successful transmission S is:
by single node:
S= p (1-p)(N-1)
by any of N nodes
S = Prob (only one transmits)
= N p (1-p)(N-1)
… choosing optimum p =1/N
as N -> infty ...
S≈ 1/e = .37 as N -> infty
At best: channel
use for useful
transmissions 37%
of time!
33
Goodput vs. Offered Load
Slotted Aloha
0.5
1.0
1.5
2.0
G = offered load = np
 when p n < 1, as p (or n) increases
 probability of empty slots reduces
 probability of collision is still low, thus goodput increases
 when p n > 1, as p (or n) increases,
 probability of empty slots does not reduce much, but
 probability of collision increases, thus goodput decreases
 goodput is optimal when p n = 1
34
Maximum Efficiency vs. n
0.4
1/e = 0.37
maximum efficiency
0.35
0.3
0.25
0.2
At best: channel
use for useful
transmissions 37%
of time!
0.15
0.1
0.05
0
2
7
12
17
n
35 35
Pure (unslotted) ALOHA
 unslotted Aloha: simpler, no synchronization
 pkt needs transmission:
 send without awaiting for beginning of slot
 collision probability increases:
 pkt sent at t0 collide with other pkts sent in [t0-1, t0+1]
36
Pure Aloha (cont.)
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0,t0+1]
= p . (1-p)N-1 . (1-p)N-1
P(success by any of N nodes) = N p . (1-p)N-1 . (1-p)N-1
… choosing optimum p=1/(2N-1)
as N -> infty ... S≈ 1/(2e) = .18
0.4
0.3
Slotted Aloha
0.2
0.1
protocol constrains
effective channel
throughput!
Pure Aloha
0.5
1.0
1.5
2.0
G = offered load = Np
37
Aloha: Performance
 Channel Rate = R bps
 Single user
 Throughput
R!
 Fairness
 Multiple
users
 Combined throughput only 0.37*R
 Decentralized

Slotted needs slot synchronization
 Simple
38
CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit:
 If channel sensed idle: transmit entire pkt
 If channel sensed busy, defer transmission
 Persistent CSMA: retry immediately with
probability p when channel becomes idle
 Non-persistent CSMA: retry after random interval
 human analogy: don’t interrupt others!
39
CSMA collisions
spatial layout of nodes along ethernet
collisions can occur:
propagation delay means
two nodes may not yet
hear each other’s
transmission
collision:
entire packet transmission
time wasted
note:
role of distance and
propagation delay in
determining collision prob.
40
CSMA/CD: Collision Detection
spatial layout of nodes along Ethernet
spatial layout of nodes along Ethernet
C
D
A
t0
t0
time
B
time
A
B
C
B detects
collision,
aborts
D
D detects
collision,
aborts
instead of wasting the whole packet
transmission time, abort after detection.
41 41
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
 colliding transmissions aborted, reducing channel
wastage
 persistent or non-persistent retransmission

 collision detection:
 easy in wired LANs: measure signal strengths,
compare transmitted, received signals
 difficult in wireless LANs: receiver shut off while
transmitting
 human analogy: the polite conversationalist
42
CSMA/CD collision detection
43
Efficiency of CSMA/CD
 Given collision detection, instead of wasting the
whole packet transmission time (a slot), we waste
only the time needed to detect collision.
P/C
P: packet size, e.g. 1000 bits
C: link capacity, e.g. 10Mbps
 Use a contention slot of 2 T, where T is one-way
propagation delay (why 2 T ?)
 When the transmission probability p is approximately
optimal (p = 1/N), we try approximately e times
before each successful transmission
44
44
Efficiency of CSMA/CD
 The efficiency (the percentage of useful time) is
approximately
P
C
P  e 2T
C

1
1 5PT

1
15 a
, where a 
TC
P
C
 The value of a plays a fundamental role in the
efficiency of CSMA/CD protocols.
 Question: you want to increase the capacity of a link
layer technology (e.g., , 10 Mbps Ethernet to 100
Mbps, but still want to maintain the same efficiency,
what do you do?
45 45
CDMA/CD
 Channel Rate = R bps
 Single user
 Throughput
 Fairness
R
 Multiple
users
 Depends on Detection Time
 Decentralized

Completely
 Simple
 Needs collision detection hardware
46
“Taking Turns” MAC protocols
channel partitioning MAC protocols:
 share channel efficiently at high load
 inefficient at low load: delay in channel access,
1/N bandwidth allocated even if only 1 active
node!
Random access MAC protocols
 efficient at low load: single node can fully
utilize channel
 high load: collision overhead
“taking turns” protocols
look for best of both worlds!
47
“Taking Turns” MAC protocols
Polling:
 master node
“invites” slave nodes
to transmit in turn
 Request to Send,
Clear to Send msgs
 concerns:



polling overhead
latency
single point of
failure (master)
Token passing:
 control token passed from
one node to next
sequentially.
 token message
 concerns:



token overhead
latency
single point of failure (token)
48
Reservation-based protocols
Distributed Polling:
 time divided into slots
 begins with N short reservation slots
reservation slot time equal to channel end-end propagation
delay
 station with message to send posts reservation
 reservation seen by all stations
 after reservation slots, message transmissions ordered by

known priority
49
Summary of MAC protocols
 What do you do with a shared media?
 Channel Partitioning, by time, frequency or code
• Time Division,Code Division, Frequency Division

Random partitioning (dynamic),
• ALOHA, S-ALOHA, CSMA, CSMA/CD
• carrier sensing: easy in some technologies (wire), hard
in others (wireless)
• CSMA/CD used in Ethernet

Taking Turns
• polling from a central cite, token passing
• Popular in cellular 3G/4G networks where
base station is the master
50
LAN technologies
Data link layer so far:

services, error detection/correction, multiple
access
Next: LAN technologies
addressing
 Ethernet
 hubs, bridges, switches
 802.11
 PPP
 ATM

51
LAN Addresses
32-bit IP address:
 network-layer address
 used to get datagram to destination network
LAN (or MAC or physical) address:
 used to get datagram from one interface to
another physically-connected interface (same
network)
 48 bit MAC address (for most LANs)
burned in the adapter ROM
52
LAN Addresses
Each adapter on LAN has unique LAN address
53
LAN Address (more)
 MAC address allocation administered by IEEE
 manufacturer buys portion of MAC address space
(to assure uniqueness)
 Analogy:
(a) MAC address: like ID number ‫תעודת זהות‬
(b) IP address: like postal address ‫כתובת מגורים‬
 MAC flat address => portability

can move LAN card from one LAN to another
 IP hierarchical address NOT portable
 depends on network to which one attaches
 ARP protocol translates IP address to MAC address
54
Comparison of IP address and MAC Address
 IP address is
hierarchical for
routing scalability
 IP address needs to be
globally unique (if no
NAT)
 MAC address is flat
 MAC address does not
need to be globally
unique, but the current
assignment ensures
uniqueness
 IP address depends on
IP network to which an
interface is attached

NOT portable
 MAC address is
assigned to a device

portable
55
ARP: Address Resolution Protocol
 Each IP node (Host, Router)
on LAN has ARP table
 ARP Table: IP/MAC address
mappings for some LAN
nodes
< IP address; MAC address; TTL>

[yry3@cicada yry3]$ /sbin/arp
Address
HWtype
zoo-gatew.cs.yale.edu
ether
artemis.zoo.cs.yale.edu ether
lab.zoo.cs.yale.edu
ether
Try
proc/net/arp
HWaddress
AA:00:04:00:20:D4
00:06:5B:3F:6E:21
00:B0:D0:F3:C7:A5
TTL (Time To Live): time
after which address
mapping will be forgotten
(typically 20 min)
Flags Mask
C
C
C
Iface
eth0
eth0
eth0
56
ARP Protocol
 ARP is “plug-and-play”:

nodes create their ARP tables without
intervention from net administrator
 A broadcast protocol:
 A broadcasts query frame, containing queried
IP address
• all machines on LAN receive ARP query

destination D receives ARP frame, replies
• frame sent to A’s MAC address (unicast)
57
Ethernet
“dominant” LAN technology:
 cheap $20 for 10/100/1000 Mbs!
 first widely used LAN technology
 Simpler, cheaper than token LANs and ATM
 Kept up with speed race: 1, 10, 100, 1000 Mbps
Metcalfe’s Etheret
sketch
58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble:
 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
 used to synchronize receiver, sender clock rates
59
Ethernet Frame Structure
(more)
 Addresses: 6 bytes, frame is received by all
adapters on a LAN and dropped if address does
not match
 Type: indicates the higher layer protocol, mostly
IP but others may be supported such as Novell
IPX and AppleTalk)
 CRC: checked at receiver, if error is detected, the
frame is simply dropped
60
Ethernet: uses CSMA/CD
A: sense channel, if idle
then {
transmit and monitor the channel;
If detect another transmission
then {
abort and send jam signal;
update # collisions;
delay as required by exponential backoff algorithm;
goto A
}
else {done with the frame; set collisions to zero}
}
else {wait until ongoing transmission is over and goto A}
61
Ethernet’s CSMA/CD (more)
Jam Signal: make sure all other transmitters are
aware of collision; 48 bits;
Exponential Backoff:
 Goal: adapt retransmission attempts to estimated
current load

heavy load: random wait will be longer
 first collision: choose K from {0,1}; delay is K x 512
bit transmission times
 after n-th collision: choose K from {0,1,…, 2n-1}
 after ten or more collisions, choose K from
{0,1,2,3,4,…,1023}
62
Exponential Backoff (simplified)
 N users
 Interval of size 2n
 Prob Node/slot is 1/2n
 Prob of success N(1/2n)(1 – 1/2n)N-1
 Average slot success N(1 – 1/2n)N-1
 Intervals size: 1, 2, 4, 8, 16 …
 Fraction (out of N) of success:
 2n = N/8 -> 0.03 %
2n = N/4 -> 2%
 2n = N/2 -> 15%
2n = N -> 37 %
 2n = 2N -> 60%
63
Ethernet Technologies: 10Base2
 10: 10Mbps; 2: under 200 meters max cable length
 thin coaxial cable in a bus topology
 repeaters used to connect up to multiple segments
 repeater repeats bits it hears on one interface to
its other interfaces: physical layer device only!
64
10BaseT and 100BaseT
 10/100 Mbps rate; latter called “fast ethernet”
 T stands for Twisted Pair
 Hub to which nodes are connected by twisted pair,
thus “star topology”
 CSMA/CD implemented at hub
65
10BaseT and 100BaseT (more)
 Max distance from node to Hub is 100 meters
 Hub can disconnect “jabbering” adapter
 Hub can gather monitoring information, statistics
for display to LAN administrators
66
Gbit Ethernet
 use 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 to be efficient
 uses hubs, called here “Buffered Distributors”
 Full-Duplex at 1 Gbps for point-to-point links
67
Token Rings (IEEE 802.5)
 A ring topology is a single unidirectional
loop connecting a series of stations in
sequence
 Each bit is stored and forwarded by each
station’s network interface
68
Token Ring: IEEE802.5 standard
 4 Mbps (also 16 Mbps)
 max token holding time: 10 ms, limiting frame length
 SD, ED mark start, end of packet
 AC: access control byte:
 token bit: value 0 means token can be seized, value 1 means
data follows FC
 priority bits: priority of packet
 reservation bits: station can write these bits to prevent
stations with lower priority packet from seizing token
after token becomes free
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Token Ring: IEEE802.5 standard
 FC: frame control used for monitoring and




maintenance
source, destination address: 48 bit physical
address, as in Ethernet
data: packet from network layer
checksum: CRC
FS: frame status: set by dest., read by sender


set to indicate destination up, frame copied OK from ring
DLC-level ACKing
70