prof. younghee lee 1 1 computer networks u lecture 3: data link and lan prof. younghee lee * some...

1Prof. Younghee Lee1

Computer Networks

Lecture 3: Data Link and LAN

Prof. Younghee Lee

* Some part of this teaching materials are prepared referencing the lecture note made by F. Kurose, Keith W. Ross(U. of Massachusetts)


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, dest

» different from IP address!

Reliable delivery between two physically connected devices: – we learned how to do this already – 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?


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


The need for flow and error control Flow control

– protocol mechanism that enables a destination entity to regulate the flow of PDUs sent by a source entity» examples; different flow control characteristics

X.25 virtual circuit: network layer LAPB: data link layer TCP connections: transport layer


The need for flow and error control Flow control

– employed in various contexts» Hop Scope, Network interface, Entry to exit, End to end

Error control– used to recover from the loss or damage of PDUs in transit


Link control mechanism-Stop and Wait Simplest form of flow control CRC ACK0:

– acknowledges receipt of a frame numbered 1

– indicates that the receiver is ready for a frame numbered 0

– positive acknowledgement ARQ: Automatic Repeat reQuest Time out interval rarely used because of inefficiency

– Case: link with very small RTT?


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ARQ performance: Stop-and-wait ARQ Error-free Stop-and-wait

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Performance of stop-and-wait protocol(P=10-3)


Link control mechanism-Sliding window Sliding-window Techniques

– Receiver accept n frames, and A is allowed to send n frames without waiting for any acknowledgements

– 12~ 0 :number sequence k


Link control mechanism-Sliding window Sequence Number Space

– SeqNum field is finite; sequence numbers wrap around Sequence number space must be larger than number of outstanding frames

– SWS <= MaxSeqNum-1 is not sufficient» suppose 3-bit SeqNum field (0..7)» SWS=RWS=7» sender transmit frames 0..6» arrive successfully, but ACKs lost» sender retransmits 0..6» receiver expecting 7,0..5, but receives second incarnation of 0..5

– SWS < (MaxSeqNum+1)/2 is correct rule Intuitively, SeqNum slides between two halves of sequence number space


Link control mechanism-Sliding window Sliding-window protocol


Link control mechanism-Sliding window Go-back-N: http://wps.aw.com/aw_kurose_network_2/0,7240,227091-,00.html

– most commonly used– The form of error control based on sliding-window flow control– RR– REJ:negative acknowledgement– Damaged frame:

» Case A: errored frame» Case B: lost in transit» Case C:lost in transit. No additional frame to send soon. Sender time out

Sender sends RR with a P bit of 1 as a command Receiver acknowledges by sending a RR indicating the next frame that it expect Sender retransmit

– Damaged RR» damaged RR. Subsequent RR» damaged RR. A’s timer expires.

Sender send RR in Case C. set P-bit timer. If receiver fails to respond, then sender’s P-bit timer will expire Sender tries again multiple times. No ACK -> reset procedure

– Damaged REJ: equivalent to Case C


Link control mechanism-Sliding window Selective-reject ARQ

– retransmit only the frame that receive a negative ACK: SREJ– much less widely used than go-back-N– more efficient than go-back-N– receiver must maintain a buffer large enough to save post-

SREJ frames until the frame in error is retransmitted– transmitter, too, requires complex logic.


ARQ performance:Sliding-window ARQ

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Parity Checking

Single Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0


Cyclic Redundancy Check Add k bits of redundant data to an n-bit message. Represent n-bit message as an n-1 degree polynomial; e.g.,

MSG=10011010 corresponds to M(x) = x7+ x4 + x3 + x1. Let k be the degree of some divisor polynomial C(x); e.g.,

C(x) = x3+ x2 + 1. Transmit polynomial P(x) that is evenly divisible by C(x), and

receive polynomial P(x) + E(x); E(x)=0 implies no errors. Recipient divides (P(x) + E(x)) by C(x); the remainder will be

zero in only two cases: E(x) was zero (i.e. there was no error), or E(x) is exactly divisible by C(x). Choose C(x) to make second case extremely rare.


Cyclic Redundancy Check Sender:

– multiply M(x) by xk; for our example, we get x10 + x7 + x6 + x4 (10011010000);

– divide result by C(x) (1101);

– Send 10011010000 - 101 = 10011010101, since this must be exactly divisible by C(x);

– Modulo 2 arithmetic without carries or borrows» Coefficient are restricted to be binary and the arithmetic on coefficients is performed modulo 2» (1+1)modulo2=0, (0-1)modulo2=1» Exclusive OR; addition and subtraction are same


Cyclic Redundancy Check

Want to ensure that C(x) does not divide evenly into polynomial E(x).

All single-bit errors, as long as the xk and x0 terms have non-zero coefficients.

All double-bit errors, as long as C(x) has a factor with at least three terms.

Any odd number of errors, as long as C(x) contains the factor (x + 1).

Any burst error (i.e sequence of consecutive errored bits) for which the length of the burst is less than k bits.

Most burst errors of larger than k bits can also be detected.


Cyclic Redundancy Check Common polynomials for C(x):

CRC

CRC-8

CRC-10

CRC-12

CRC-16

CRC-CCITT

CRC-32

C(x)

x8+x2+x1+1

x10+x9+x5+x4+x1+1

x12+x11+x3+x2+x1+1

x16+x15+x2+1

x16+x12+x5+1

x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1


Comparison L2, L4 Flow control + Error control

– RTT» Delay variance

– Timeout value

Throughput– Error control at each L2’s + L4; Loss at L3 buffer– Error control only at L4;– TCP throughput in case of many loss due to intensive errors at L2?

» Satellite link, wireless link…


HDLC

High Level Data Link Control : ISO Service

– Connectionless service» Acknowledged service

» Unacknowledged service: most common

– Connection-oriented service

Balanced mode, unbalanced mode


LLC

Logical Link Control Data link layer for LAN consists of MAC & LLC Service

– Type 1: unacknowledged connectionless service» most common in LAN

– Type2: reliable connection-oriented service:» End system without TCP

– Type3: acknowledged connectionless service» The receiving node will send acknowledgments for individual frames.

No virtual circuit


LANs


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, Wa

velan, etc.)

switched (e.g., switched Ethernet, ATM etc)


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 bands 2,5,6 idle

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.

frequ

ency

bands time


Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) unique “code” assigned to each user; ie, code set partition

ing used mostly in wireless broadcast channels (cellular, satel

lite,etc) all users share same frequency, but each user has own “c

hipping” sequence (ie, code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping s

equence allows multiple users to “coexist” and transmit simultaneo

usly with minimal interference (if codes are “orthogonal”)


CDMA Encode/Decode


CDMA: two-sender interference


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


Slotted Aloha

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


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 as n -> infty ...

= 1/e = .37 as N -> infty

At best: channeluse for useful transmissions 37%of time!


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]


Pure Aloha (cont.)P(success by given node) = P(node transmits) .

P(no other node transmits in [p0-1, p0] .

P(no other node transmits in [p0, p0+1]

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

P(success by any of N nodes) = N p . (1-p)2(N-1)

… choosing optimum p as n -> infty ...

= 1/(2e) = .18

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Slotted Alohaprotocol constrainseffective channelthroughput!


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 (may cause instability)

– Non-persistent CSMA: retry after random interval

human analogy: don’t interrupt others!


CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

note:role of distance & propagation delay in determining collision probability


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 http://wps.aw.com/

aw_kurose_network_2/0,7240,227091-,00.html


CSMA/CD collision detection


“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!


“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 n

ode to next sequentially. token message concerns:

– token overhead – latency– single point of failure (token)

Good thing:– Fairness control– QoS control– Throughput– Use of Fiber optic links


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


Ethernet LAN 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

Bandwidth: 10Mbps and 100Mbps Problem: Distributed algorithm that provides fair access to a shared medium CDPD: Cellular Digital Packet Data

– Mac protocol for CDPD: Digital sense Multiple access; a variation of CSMA/CD


Physical Properties(Ethernet) Classical Ethernet (thick-net)

– maximum segment of 500m– transceiver taps at least 2.5m apart– connect multiple segments with repeaters– no more than 2 repeaters between any pair of nodes (1500m total)– maximum of 1024 hosts– also called 10Base5

Alternative technologies– 10Base2 (thin-net): 200m; daisy-chain configuration– 10BaseT (twisted-pair): 100m; star configuration


Ethernet LAN

Addresses: Unique, 48-bit unicast address assigned to each adaptor Example: 8:0:2b:e4:b1:2 Broadcast: all 1s Multicast: first bit is 1


LAN Addresses and ARP

32-bit IP address: network-layer address used to get datagram to destination network (recall IP

network definition)

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


LAN Addresses and ARP

Each adapter on LAN has unique LAN address


LAN Address (more)

MAC address allocation administered by IEEE manufacturer buys portion of MAC address space

(to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address 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: 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>

– TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bknowing B’s IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237.196.7.23

237.196.7.78

237.196.7.14

237.196.7.88


ARP protocol: Same LAN (network) A wants to send datagram to

B, and B’s MAC address not in A’s ARP table.

A broadcasts ARP query packet, containing B's IP address

– Dest MAC address = FF-FF-FF-FF-FF-FF

– all machines on LAN receive ARP query

B receives ARP packet, replies to A with its (B's) MAC address– frame sent to A’s MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – soft state: information that

times out (goes away) unless refreshed

ARP is “plug-and-play”:– nodes create their ARP

tables without intervention from net administrator


Routing to another LANwalkthrough: send datagram from A to B via R

assume A know’s B IP address

Two ARP tables in router R, one for each IP network (LAN)

In routing table at source Host, find router 111.111.111.110 In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc

A

RB


A creates datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, frame contains A-to-B

IP datagram A’s adapter sends frame R’s adapter receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B

A

RB


Transmitter AlgorithmIf line is idle: Send immediately Upper bound message size of 1500 bytes Must wait 51s between back-to-back framesIf line is busy: Wait until idle and transmit immediately Called 1-persistent (special case of p-persistent)If collision: jam for 512 bits, then stop transmitting frame minimum frame is 64 bytes (header + 46 bytes of data) delay and try again

– 1st time: uniformly distributed between 0 and 51.2s– 2nd time: uniformly distributed between 0 and 102.4s– 3rd time: uniformly distributed between 0 and 204.8s– give up after several tries (usually 16)– exponential backoff


ExperiencesObserve in Practice 10-200 hosts (not 1024) Length shorter than 1500m (RTT closer to 5 than 51) Packet length is bimodal High-level flow control and host performance limit load

Recommendations Do not overload (30% utilization is about max) Implement controllers correctly Use large packets Get the rest of the system right (broadcast, retransmission)


Ethernet Notation of Ethernet: <data rate> <signaling method> <Max. segment length in hundreds of meters> 10Base5: original 802.3, 50, 500M

– Max. 4 repeater => 2.5 Km 10Base-T: unshielded twisted pair

– good in the case of using prewired lines in office buildings as excess telephone cable– 100m– optical fiber link: 500m– HHUB: header hub– IHUB: intermediate hub, wiring closet on each floor


Hub and Switch Hub:

– Shared medium bus – Shared medium hub– Switching hub

» to achieve greater performance» throughput on the LAN in figure is

20Mbps» advantages

No change is required aggregated capacity scales easily.

» Types Store-and-forward switch: buffers

input frame, and then routes to output line

Cut-through switch: repeating the incoming frame onto the output line after recognizing the destination address at the beginning of MAC frame. Highest possible throughput but at some risk of propagating bad frames(CRC)

N x 10M


Hubs (more)

Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may collide

with any node residing at any segment in LAN Hub Advantages:

– simple, inexpensive device– Multi-tier provides graceful degradation: portions of

the LAN continue to operate if one hub malfunctions– extends maximum distance between node pairs

(100m per Hub)


Hub limitations

single collision domain results in no increase in max throughput– multi-tier throughput same as single segment

throughput individual LAN restrictions pose limits on number of

nodes in same collision domain and on total allowed geographical coverage

cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)


IEEE 802.11 Wireless LAN wireless LANs: untethered (often mobile) networ

king IEEE 802.11 standard:

– MAC protocol– unlicensed frequency spectrum: 900Mhz, 2.4Ghz

Basic Service Set (BSS) (a.k.a. “cell”) contains:– wireless hosts– access point (AP): base statio

n BSS’s combined to form distribu

tion system (DS)


Characteristics of selected wireless link standards

384 Kbps384 Kbps

56 Kbps56 Kbps

54 Mbps54 Mbps

5-11 Mbps5-11 Mbps

1 Mbps1 Mbps

802.15

802.11b

802.11{a,g}

IS-95 CDMA, GSM

UMTS/WCDMA, CDMA2000

.11 p-to-p link

2G

3G

Indoor

10 – 30m

Outdoor

50 – 200m

Mid rangeoutdoor

200m – 4Km

Long rangeoutdoor

5Km – 20Km


MAC MAC Problems in wireless network: to use CSMA/CD

– Collision Detection(CD) does not work– CS might not work in some case( if a terminal is “hidden”)

Hidden terminal problem– Nodes A and C cannot hear each other

» Node A : currently transmitting to B» Node C : wants to transmit to B» Transmissions by nodes A and C can collide at node B

• Nodes A and C are hidden from each other

A B C


MAC Exposed terminal problem

– Node C cannot send to D due to carrier of B sense » Node B : currently transmitting to A» Node C : wants to transmit to D» Carrier of C doesn’t interfere A’s reception, Carrier of B doesn’t interfere D’s

reception Waiting is not necessary

» But C is waiting since it sense carrier of B

– C is exposed to B

A B C D


IEEE 802.11 Wireless LAN

802.11b– 2.4-5 GHz unlicensed

radio spectrum– up to 11 Mbps– direct sequence spread

spectrum (DSSS) in physical layer

» all hosts use same chipping code

– widely deployed, using base stations

802.11a – 5-6 GHz range– up to 54 Mbps

802.11g – 2.4-5 GHz range– up to 54 Mbps

All use CSMA/CA for multiple access

All have base-station and ad-hoc network versions


802.11 LAN architecture

wireless host communicates with base station– base station = access poi

nt (AP) Basic Service Set (BSS) (aka

“cell”) in infrastructure mode contains:– wireless hosts– access point (AP): base st

ation– ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP


802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at

different frequencies; 3 non-overlapping– AP admin chooses frequency for AP– interference possible: channel can be same as that chosen by

neighboring AP!

host: must associate with an AP– scans channels, listening for beacon frames containing AP’s

name (SSID) and MAC address– selects AP to associate with; initiates association protocol– may perform authentication [Chapter 8]– will typically run DHCP to get IP address in AP’s subnet


IEEE 802.11: multiple access Like Ethernet, uses CSMA:

– random access– carrier sense: don’t collide with ongoing transmission

Unlike Ethernet:– no collision detection – transmit all frames to completion– acknowledgment – because without collision detection, you don’

t know if your transmission collided or not

Why no collision detection?– difficult to receive (sense collisions) when transmitting due to we

ak received signals (fading)– can’t sense all collisions in any case: hidden terminal, fading

Goal: avoid collisions: CSMA/C(ollision)A(voidance)


MAC IFS: interframe space: depend on the type of frame to transmit

– SIFS: Short IFS» High priority frame before contending for channel» ACK frame, CTS frame, data transfer of a segmented MSDU, frames from station that are responding to a poll

from an AP, any frame from an AP during CFP

– PIFS: PCF-IFS» Used by PCF to gain priority access to the medium at the start of a CFP(Contention Free Period)

– DIFS: DCF-IFS» Used by the DCF to transmit data and management MPDU

DCF : CSMA/CA– Initial MAC PDU: medium idle for a period DIFS or greater -> transmit

» Medium busy -> wait random backoff time to schedule a reattempt

– Reattempt: decrement a counter each time an idle contention slot transpire – After successful frame transmission -> backoff procedure to transmit next frame

Busy Medium SIFS

PIFS

DIFSContentionWIndow

Next

Frame


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender

1 if sense channel idle for DIFS then

- transmit entire frame (no CD)

2 if sense channel busy then

- start random backoff time

- timer counts down while channel idle

- transmit when timer expires

- if no ACK, increase random backoff interval, repeat 2

802.11 receiverif frame received OK

- return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK


RTS/CTS

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

optional; not typically used sender first transmits small request-to-send (RTS) packets to A

P using CSMA– RTSs may still collide with each other (but they’re short)

AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes

– sender transmits data frame– other stations defer transmissions

Avoid data frame collisions completely using small reservation packets!


Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer


hub or switch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

router

H1 remains in same IP subnet: IP address can remain same

switch: which AP is associated with H1?– self-learning (Ch. 5):

switch will see frame from H1 and “remember” which switch port can be used to reach H1


Mradius ofcoverage

S

SS

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive)P

802.15: personal area network

less than 10 m diameter replacement for cables

(mouse, keyboard, headphones)

ad hoc: no infrastructure master/slaves:

– slaves request permission to send (to master)

– master grants requests

802.15: evolved from Bluetooth specification– 2.4-2.5 GHz radio band– up to 721 kbps


Mobile Switching

Center

Public telephonenetwork, andInternet

Mobile Switching

Center

Components of cellular network architecture

connects cells to wide area net manages call setup (more later!) handles mobility (more later!)

MSC

covers geographical region base station (BS) analogous to 802.11 AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BS

cell

wired network


Cellular networks: the first hop

Two techniques for sharing mobile-to-BS radio spectrum

combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots

CDMA: code division multiple access frequency

bands

time slots


Cellular standards: brief survey

2G systems: voice channels IS-136 TDMA: combined FDMA/TDMA (north america) GSM (global system for mobile communications): combi

ned FDMA/TDMA – most widely deployed

IS-95 CDMA: code division multiple access

IS-136 GSM IS-95GPRS EDGECDMA-2000

UMTS

TDMA/FDMADon’t drown in a bowlof alphabet soup: use thisoor reference only



2.5 G systems: voice and data channels for those who can’t wait for 3G service: 2G extensions general packet radio service (GPRS)

– evolved from GSM – data sent on multiple channels (if available)

enhanced data rates for global evolution (EDGE)– also evolved from GSM, using enhanced modulation – Date rates up to 384K

CDMA-2000 (phase 1)– data rates up to 144K– evolved from IS-95



3G systems: voice/data Universal Mobile Telecommunications Service (UMTS)

– GSM next step, but using CDMA CDMA-2000

….. more (and more interesting) cellular topics due to mobility (stay tuned for details)


Ad Hoc Networks

Ad hoc network: IEEE 802.11 stations can dynamically form network without AP

Applications:– “laptop” meeting in conference room, car– interconnection of “personal” devices– battlefield

IETF MANET (Mobile Ad hoc Networks) working group


Bridges

Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination

Bridge isolates collision domains since it buffers frames

When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit


Bridges (more)

Bridge advantages:– Isolates collision domains resulting in higher total

max throughput, and does not limit the number of nodes nor geographical coverage

– Can connect different type Ethernet since it is a store and forward device

– Transparent: no need for any change to hosts LAN adapters


Bridges: frame filtering, forwarding

bridges filter packets – same-LAN -segment frames not forwarded onto other

LAN segments

forwarding: – how to know which LAN segment on which to forward

frame?– looks like a routing problem (more shortly!)


Backbone Bridge


Interconnection Without Backbone

Not recommended for two reasons:- single point of failure at Computer Science hub- all traffic between EE and SE must path over CS segment


Bridge Filtering

bridges learn which hosts can be reached through which interfaces: maintain filtering tables– when frame received, bridge “learns” location of

sender: incoming LAN segment– records sender location in filtering table

filtering table entry: – (Node LAN Address, Bridge Interface, Time Stamp)– stale entries in Filtering Table dropped (TTL can be 60

minutes)


Bridge Learning: example

Suppose C sends frame to D and D replies back with frame to C

C sends frame, bridge has no info about D, so floods to both LANs

– bridge notes that C is on port 1 – frame ignored on upper LAN – frame received by D


Bridge Learning: example

D generates reply to C, sends – bridge sees frame from D – bridge notes that D is on interface 2 – bridge knows C on interface 1, so selectively

forwards frame out via interface 1


Bridges vs. Routers both store-and-forward devices

– routers: network layer devices (examine network layer headers)– bridges are Link Layer devices

routers maintain routing tables, implement routing algorithms bridges maintain filtering tables, implement filtering, learning

and spanning tree algorithms


Routers vs. Bridges

Bridges + and -

+ Bridge operation is simpler requiring less processing bandwidth

- Topologies are restricted with bridges: a spanning tree must be built to avoid cycles

- Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)


Routers vs. Bridges

Routers + and -

+ arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)

+ provide firewall protection against broadcast storms

- require IP address configuration (not plug and play)

- require higher processing bandwidth bridges do well in small (few hundred hosts) while

routers used in large networks (thousands of hosts)


High Speed LANs Most important high speed LANs

– Fast Ethernet and Gigabit Ethernet– ATM LAN


Fast Ethernet 100BASE-T 100BASE-X: two physical links between nodes, one for reception and one for transmission 100BASE-T4: the use of four twisted pair lines making use of 3 pairs in one direction at a time

– category 3: voice grade UTP, standard telephone wire, – category 5: more tightly twisted for higher quality


Fast Ethernet 100BASE-X:

– unidirectional data rate of 100Mbps, 4B/5B-NRZI 100BASE-T4

– 3 separate data streams, each with data rate of 33 Mbps Configuration and Operation

– star-wire topology– the repeater: responsible for detecting collisions rather than the attached stations

» A valid signal appearing on any single input is repeated on all output link» If 2 inputs occur at the same time, a jam signal is transmitted on all links

Full- duplex Operation: traditional Ethernet is half duplex– theoretical transfer rate would become 200 Mbps– attached stations must have full-duplex rather than half duplex adapter card– central point in the star can be a switched hub

» each station constitutes a separate collision domain» no collision» continue to execute the CSMA/CD algorithm for compatibility


Fast Ethernet Collision domain: used to define a single CSMA/CD network The bridge operates in a store-and-forward fashion 2 types of Repeater

– Class I repeater: 100BASE-T4, 100BASE-TX, used in a collision domain» Increased internal delay to handle the conversion from one signaling scheme to

another(the term signaling here has different meaning with previous lesson)– Class II repeater is limited to a single media type,

» there may be two Class II repeaters used in a single collision domain


Fast Ethernet Mixed Configuration

– readily supports a mixture of existing 10-Mbps LANs and newer 100-Mbps LANs

3

1


Gigabit Ethernet Gigabit Ethernet

– compatible with 100Base-T, 10Base-T– use of optical fiber over relatively short distance, although UTP,

STP, and coaxial cable configurations are also allowed.


Gigabit Ethernet Application of Gigabit Ether

net

Discussion: ATM LAN vs Gigabit Ethernet


Gbit Ethernet

uses standard Ethernet frame format allows for point-to-point links and shared broadcast chan

nels in shared mode, CSMA/CD is used; short distances bet

ween nodes required for efficiency uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now !


ATM LANs Typical requirements for a third generation LAN

– support multiple, guaranteed classes of services– provide scalable throughput– facilitate the internetworking between LAN and WAN technology

ATM ideally suited to these requirements The Internet: connectionless, ATM: connection oriented possible types of ATM LANs:

– Gateway to ATM WAN– Backbone ATM switch: interconnect other LANs– Workgroup ATM: high performance MM WSs, connect directly to switch


ATM LANs Need perform some sort of protocol conversion from the MAC

protocol to the ATM cell stream


Metropolitan Ethernet Ethernet services for metro transport network Carrier-class Ethernet-based transport technologies

Metro Ethernet Forum(MEF)– Forum to accelerate the adoption of optical Ethernet as the technology of choice in metro network world wide– Move beyond LAN origin to become a full duplex, switched technology capable of meeting the long-range transport, bandwidth, geographical and capacity requirements of metro networking

» A compelling combination of speed, scalability, operational simplicity, and economics

– Deliverables» Implementation agreements, Test procedure, positioning statements, technical specifications, Marketing

– Services» Ethernet virtual private line service(EVPLS), Ethernet virtual private LAN services(EVPLns) and Circuit emulation services(CES)


Metropolitan Ethernet IEEE 802.3 Ethernet in the First Mile(EFM)

– The optimal subscriber access network» For faster, simpler, better and more profitable

– Point to point on fiber(P2P), Point to multipoint fiber(EPON), Point to point copper(Copper)

– Provide a family of physical layer specifications– OAM– To expand the application of Ethernet to include subscriber access

networks in order to provide a significant increase in performance while maintaining equipment, operation, and maintenance cost


ATM LANs ATM hub includes a number of ports that operate at different

data rates and use different protocols pure ATM LAN?


ATM LANs: LANE Need Connectionless service for TCP/IP software implementation over a connection-oriented ATM

network– connectionless server approach

» Every host initially sets up a connection to this server

– ATM LAN emulation» Every hosts has a ATM virtual circuit to every other host» These VC can be established and released dynamically as needed or can be permanent virtual circuit» LES(LAN Emulation Server): ATM ARP server» BUS(Broadcast/Unknown server): broadcasting, multicasting


High Speed LANs Token Ring MAN:

– DQDB(802.6): Distributed Queue Dual Bus» high speed broadcast bus» slotted access

FDDI II: packet/circuit switched traffic FFOL: FDDI Follow - on LAN

– scalable beyond one Gbps– support cell-based ATM traffic– connect to SONET/SDH link

S-NET, Expressnet, Datakit HIPPI: High Performance Parallel Interface FC: Fiber Channel Broadband LAN, Baseband LAN, data PABX 100VG-AnyLAN: IEEE 802.12

– demand priority: » hub polls each computer in turn» computer transmit only when the hub grants» support a simple priority scheme» support frame format other than Ethernet

prof. younghee lee 1 1 computer networks u lecture 3: data link and lan prof. younghee lee * some...

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