2019 term 3 tele3118: network technologies · network technologies 1c-15 n single shared broadcast...

2019 term 3TELE3118: Network Technologies

Week 1: Data Link Layer Framing, Error Control, Multiple Access

Some slides have been taken from:qComputer Networking: A Top Down Approach Featuring the Internet, 7th edition. Jim Kurose, Keith Ross. Addison-Wesley, 2016. All material copyright 1996-2016. J.F Kurose and K.W. Ross, All Rights Reserved.qComputer Networks, 5th edition. Andrew S. Tanenbaum. Prentice-Hall, 2010.

Link layer: introduction

1c-2

terminology:n hosts and routers: nodesn communication channels

that connect adjacent nodes along communication path: linksq wired linksq wireless linksq LANs

n layer-2 packet: frame, encapsulates datagram

Network Technologies

data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link

Where is the link layer implemented?

1c-3Network Technologies

n in each and every hostn link layer implemented in “adaptor” (aka network interface card NIC) or on a chipq Ethernet card, 802.11

card; Ethernet chipsetq implements link, physical

layern attaches into host’s system

busesn combination of hardware,

software, firmware

controller

physicaltransmission

cpu memory

host bus (e.g., PCI)

network adaptercard

applicationtransportnetwork

link

linkphysical

Link Layer Functions


n Framingn Error controln Addressing and Multiple-access controln Flow controln Reliable transfer

Framing


A character stream. (a) Without errors. (b) With one error.

Framing (2)


(a) A frame delimited by flag bytes.(b) Four examples of byte sequences before and after stuffing.

Framing (3)


Bit stuffing(a) The original data.(b) The data as they appear on the line.(c) The data as they are stored in receiver’s memory after destuffing.

Error detection


EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

n Error detection not 100% reliable!n protocol may miss some errors, but rarelyn 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 errors

Theory: Hamming Distances


n Hamming distance of two codewords:q The number of bit positions in which they differq Example: 10001001 and 10110001 have Hamming distance 3

n Hamming distance of a code:q Minimum distance between any two valid codewordsq E.g. the code: 0000000000, 0000011111, 1111100000,

1111111111 has Hamming distance 5n message size n = data bits m + redundant bits r

q Of 2n codewords, only 2m are validn How many redundant bits needed for 1-bit error

correction?q Should satisfy: (n+1)2m ≤2n : why?

Cyclic redundancy check


n more powerful error-detection codingn view data bits, D, as a binary numbern choose r+1 bit pattern (generator), Gn goal: choose r CRC bits, R, such that

q <D,R> exactly divisible by G (modulo 2) q receiver knows G, divides <D,R> by G. If non-zero remainder: error

detected!q can detect all burst errors less than r+1 bits

n widely used in practice (Ethernet, 802.11 WiFi, ATM)

CRC example


want:D.2r XOR R = nG

equivalently:D.2r = nG XOR R

equivalently:if we divide D.2r by G, want remainder R to satisfy:

R = remainder[ ]D.2r

G

CRC properties


n Can catch all errors with odd number of inverted bitsn Odd number of bits in error

q Represented by a polynomial with odd number of terms (e.g. 𝑥" + 𝑥$ + 1 )n No polynomial with odd number of terms has (x+1) as a factor in

modulo 2 n Add (x+1) as a factor in G(x) Jn Can catch all burst errors length ≤ r n Burst error of length k is represented 𝑥' 𝑥()* + ⋯+ 1

q there are k consecutive 1’s in your error

n If G(x) contains an 𝑥, term, it will never have 𝑥' as a factorq G(x) ends with +1 at the end! J

n G(x) = 𝑥-$ + 𝑥$. + 𝑥$- + 𝑥$$ + 𝑥*. + 𝑥*$ + 𝑥** + 𝑥*, + 𝑥/ + 𝑥0 +𝑥" + 𝑥1 + 𝑥$ + 𝑥* + 1

Multiple access links, protocols


two types of “links”:n point-to-point

q PPP for dial-up accessq point-to-point link between Ethernet switch, host

n broadcast (shared wire or medium)q old-fashioned Ethernetq upstream HFCq 802.11 wireless LAN

shared wire (e.g., cabled Ethernet)

shared RF(e.g., 802.11 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air, acoustical)

Multiple access protocols


n single shared broadcast channel n two or more simultaneous transmissions by nodes:

interference q collision if node receives two or more signals at the same time

multiple access protocoln distributed algorithm that determines how nodes

share channel, i.e., determine when node can transmitn communication about channel sharing must use channel

itself! q no out-of-band channel for coordination

An ideal multiple access protocol


given: broadcast channel of rate R bpsdesiderata:

1. when one node wants to transmit, it can send at rate R.2. when M nodes want to transmit, each can send at average rate

R/M3. fully decentralized:

n no special node to coordinate transmissionsn no synchronization of clocks, slots

4. simple

MAC protocols: taxonomy


three broad classes:n channel partitioning

q divide channel into smaller “pieces” (time slots, frequency, code)

q allocate piece to node for exclusive use

n random accessq channel not divided, allow collisionsq “recover” from collisions

n “taking turns”q nodes take turns, but nodes with more to send can take

longer turns

Channel partitioning MAC protocols: TDMA


TDMA: time division multiple access

n access to channel in "rounds" n each station gets fixed length slot (length = packet

transmission time) in each round n unused slots go idle n example: 6-station LAN, 1,3,4 have packets to send,

slots 2,5,6 idle

1 3 4 1 3 4

6-slotframe

6-slotframe

Channel partitioning MAC protocols: FDMA


FDMA: frequency division multiple access n channel spectrum divided into frequency bandsn each station assigned fixed frequency bandn unused transmission time in frequency bands go idle n example: 6-station LAN, 1,3,4 have packet to send,

frequency bands 2,5,6 idle

freq

uenc

y ba

nds

time

FDM cable

Random access protocols


n when node has packet to sendq transmit at full channel data rate R.q no a priori coordination among nodes

n two or more transmitting nodes ➜ “collision”,n random access MAC protocol specifies:

q how to detect collisionsq how to recover from collisions (e.g., via delayed

retransmissions)n examples of random access MAC protocols:

q slotted ALOHAq ALOHAq CSMA, CSMA/CD, CSMA/CA

Slotted ALOHA


assumptions:n all frames same sizen time divided into equal

size slots (time to transmit 1 frame)

n nodes start to transmit only slot beginning

n nodes are synchronizedn if 2 or more nodes

transmit in slot, all nodes detect collision

operation:n when node obtains fresh

frame, transmits in next slotq if no collision: node can

send new frame in next slot

q if collision: node retransmits frame in each subsequent slot with prob. p until success

Slotted ALOHA


Pros:n single active node can

continuously transmit at full rate of channel

n highly decentralized: only slots in nodes need to be in sync

n simple

Cons:n collisions, wasting slotsn idle slotsn nodes may be able to

detect collision in less than time to transmit packet

n clock synchronization

1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C CS S SE E E

Slotted ALOHA: efficiency


n max efficiency: find p* that maximizes Np(1-p)N-1

n for many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives:max efficiency = 1/e = 0.37

efficiency: long-run fraction of successful slots (many nodes, all with many frames to send)

at best: channelused for useful transmissions 37%of time! !

n suppose: N nodes with many frames to send, each transmits in slot with probability p

n prob that given node has success in a slot = p(1-p)N-1

n prob that any node has a success = Np(1-p)N-1

Pure (unslotted) ALOHA


n unslotted Aloha: simpler, no synchronizationn when frame first arrives

q transmit immediately n collision probability increases:

q frame sent at t0 collides with other frames sent in [t0-1,t0+1]

Pure ALOHA efficiency


P(success by given node) = P(node transmits) .P(no other node transmits in [t0-1,t0] .P(no other node transmits in [t0-1,t0]

= p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)

… choosing optimum p and then letting n

= 1/(2e) = 0.18

even worse than slotted Aloha!

CSMA (carrier sense multiple access)


CSMA: listen before transmit:if channel sensed idle: transmit entire framen if channel sensed busy, defer transmission

n human analogy: don’t interrupt others!

CSMA collisions


n collisions can still occur: propagation delay means two nodes may not hear each other’s transmission

n collision: entire packet transmission time wastedq distance & propagation

delay play role in determining collision probability

spatial layout of nodes

CSMA/CD (collision detection)


CSMA/CD: carrier sensing, deferral as in CSMAq collisions detected within short timeq colliding transmissions aborted, reducing channel wastage

n collision detection: q easy in wired LANs: measure signal strengths, compare

transmitted, received signalsq difficult in wireless LANs: received signal strength

overwhelmed by local transmission strength

n human analogy: the polite conversationalist

CSMA/CD (collision detection)


spatial layout of nodes

“Taking turns” MAC protocols


channel partitioning MAC protocols:§ share channel efficiently and fairly 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!



polling:n master node “invites”

slave nodes to transmit in turn

n typically used with “dumb” slave devices

n concerns:q polling overhead q latencyq single point of failure

(master)

master

slaves

poll

data

data



token passing:§ control token passed from

one node to next sequentially.

n token messagen concerns:

q token overhead q latencyq single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocols


n channel partitioning, by time, frequency or codeq Time Division, Frequency Division

n random access (dynamic), q ALOHA, S-ALOHA, CSMA, CSMA/CDq carrier sensing: easy in some technologies (wire), hard in

others (wireless)q CSMA/CD used in Ethernetq CSMA/CA used in 802.11

n taking turnsq polling from central site, token passingq Bluetooth, FDDI, token ring

2019 term 3 tele3118: network technologies · network technologies 1c-15 n single shared broadcast...

Documents