ee/cs 450 spring 001 cpeg 419 introduction to networks znew homework is on the web. it is due in two...

EE/CS 450 Spring 00 1

CPEG 419 Introduction to Networks

New Homework is on the web. It is due in two weeks. No class on Thursday.Most likely, I will not miss any more classes in October.

University of Delaware CPEG 419 2

CRC Example – I was right!

Frame M 1010001101 = x9+x7+x3+x2+x0.

Pattern G 110101.Dividing (frame*25) by pattern results

in 01110.Thus [M|V] = 1010001101 | 01110.Receiver can detect errors unless

received message Tr is divisible by G.


Fragmentation

A packet is broken up into smaller pieces. Each fragment is sent as a frame. Link layer has an upper bound on the size of frame. E.g.,

ethernet frames must be smaller than 1500 bytes. ATM must be 53 bytes, cable modem use 204 bytes.

P1 = (1-Pb)^F. So the smaller the F, the smaller P1. But the smaller the F, the lower the throughput. The

frame has overhead, e.g., the destination address, source address, etc. So a frame of 64 byte Ethernet frame will only contain 38 bytes of data.


Example Data Link Layer Protocol

High-Level Data Link Control (HDLC) Widely-used (ISO standard). Single frame format. Synchronous transmission.


HDLC: Frame Format

Flag: frame delimiters (01111110). Address field for multipoint links. 16-bit or 32-bit CRC. Refer to book (pages 214-221) for more

details.

8bits

8ext.

8 or16

variable 16 or32

8

flag address control data FCS flag


Other DLL Protocols 2

LLC: Logical Link Control 802.2 Part of the 802 protocol family for LANs. Link control functions divided between

the MAC layer and the LLC layer. LLC layer operates on top of MAC layer.

Dst.MACaddr

Src.MACaddr

FCSDst.LLCaddr

Src.LLCaddr

LLCctl. DataMAC

control


Other DLL Protocols 3

SLIP: Serial Line IP Dial-up protocol. No error control. Not standardized.

PPP: Point-to-Point Protocol Internet standard for dial-up

connections. Provides framing similar to HDLC.


Multiplexing

Sharing a link/channel among multiple source-destination pairs.

Example: high-capacity long-distance trunks (fiber, microwave links) carry multiple connections at the same time.

MU

X

...

DE

MU

X ...


Multiplexing Techniques

3 basic types: Frequency-Division Multiplexing (FDM). Time-Division Multiplexing (TDM). Statistical Time-Division Multiplexing

(STDM).


FDM 1

High bandwidth medium when compared to signals to be transmitted.

Widely used (e.g., TV, radio).Various signals carried simultaneously

where each one modulated onto different carrier frequency, or channel.

Channels separated by guard bands (unused) to prevent interference.


FDM 2

Time

Frequency

1 2 N


TDM 1

TDM or synchronous TDM.High data rate medium when

compared to signals to be transmitted.

Time

Frequency

12

N


TDM 2

Time divided into time slots.Frame consists of cycle of time slots.In each frame, 1 or more slots

assigned to a data source.

1 2 N... 1 2 ... N

frame Time

U1 U2 ... UN


TDM 3

No control info at this level.Flow and error control?

To be provided on a per-channel basis. Use DLL protocol such as HDLC.

Examples: SONET (Synchronous Optical Network) for optical fiber.

+’s: simple, fair.-’s: inefficient.


Statistical TDM 1Or asynchronous TDM.Dynamically allocates time slots on

demand.N input lines in statistical multiplexer, but

only k slots on TDM frame, where k < n.Multiplexer scans input lines collecting

data until frame is filled.Demultiplexer receives frame and

distributes data accordingly.


STDM 2

Data rate on mux’ed line < sum of data rates from all input lines.

Can support more devices than TDM using same link.

Problem: peak periods. Solution: multiplexers have some buffering

capacity to hold excess data. Tradeoff data rate and buffer size (response time). When two sources transmit at the same time, a

decision must eb made as to who gets to go first (fairness is an issue).

EE/CS 450 Spring 00 17

LAN Systems - Chapter 14

Examples of Link Layer MAC Layer – Medium Access

Control LLC – Logical Link Layer


Local Area Networks 1

Interconnect devices over short distances. Within same floor, Building, Campus.

Characterized by low delays.


LANs 2

Typically use common broadcast medium. Hosts share same communication medium. Also called multiple-access networks.

LANs are characterized by: Topology – how nodes are hooked together. Transmission medium. Medium access control mechanism.


LAN Protocol Architecture

LAN protocol standards collectively known as IEEE 802 reference model.

Physical

Data link

Network

Transport

Session

Presentation

Application

PhysicalMACLLC

OSI

IEEE802

Upper layerprotocols


LAN Protocols

MAC sublayer: performs functions that control access to shared medium.

LLC – logical link control (the past two lectures): performs flow and error control and provides services to upper layer.


802 standards 1

LLC: IEEE 802.2 connectionless and connection oriented

services. Reliable and unreliable. Ethernet (802.3) includes 802.2, but

typically does not use the reliable features. So what why is 802.2 included? Just in case you wanted to make the link reliable.


802 standards 2MAC + physical layers

802.3 802.5Bus/tree/star topologies. Ring topology.CSMA/CD. Token ring.E.g., Ethernet

802.4 FDDIBus/tree/star topologies. Dual bus

(optical).Token bus. Token ring.

802.11 Wireless.CSMA.


Encapsulation of FrameApplication data

header

header

header

header

TCP

IP

LLC

MAC MACtrailer

TCP segmentIP datagram

LLC PDUMAC frame


LLC for LANs

Similar functions as general LLCs.But it has to interface with MAC

sublayer.LLC functions:

Addressing: source and destination.LLC address versus MAC address.

Control data exchange between 2 users.User as higher-layer protocol in the station.


LLC Services3 different services:

Unacknowledged connectionless (type 1).No error or flow control.No delivery guarantees.

Connection-mode (type 2).Logical connection established.Flow and congestion control provided.

Acknowledged connectionless (type 3).No logical connection.Flow and error control.


LLC (802.2) Protocol

Similar to HDLC (ISO standard).LLC PDU:

DSAP SSAP LLC control Information

1 byte 1 byte 1 or 2 bytes variable


MAC Frame Format

Dst.MACaddr

Src.MACaddr

CRCDst.LLCaddr

Src.LLCaddr

LLC PDUMACcontrol

MAC control: protocol information (protocol type, version #).Destination MAC address: physical address of LAN destination.Source MAC address: physical address of the LAN source.


LAN Topologies

Star

Central node

Ring

Bus

Tree


Bus Topology

Use of multipoint medium.Stations attach to bus through tap.

Full-duplex communication allows data to be sent to/received from bus.

Transmission from any station propagates in both directions and is received by all. Media Access Control is required to gain

control of the bus.


Tree Topology

Tree is generalization of bus.Headend: start of 1 or more cables

(branches).Transmission from one station

propagates to all others.


Issues

Inherently, broadcast. Frames to transmit data. Need for specifying the destination. Addresses.

Multi-access. Need for controlling access to medium.

Avoid collisions.MAC protocol.


Ring Topology 1

Stations attach to repeaters.Repeaters are linked to each other

by point-to-point links forming a closed loop.

Links are unidirectional.Repeaters: receive data from one

link and repeat it on the other with no buffering.


Ring 2

Stations transmit/receive via repeater.

Frames circulate past all stations; destination copies frame as it goes by; source removes frame.

Ring shared by multiple stations. Need MAC protocol to determine when

each station may insert frame.


Star Topology

Each station directly connected to central node via point-to-point link.

Central node’s modes of operation: Broadcast mode: node broadcasts received

frame on all other links; logically works like bus. Switching mode: node sends frame out only on

the link to the destination. MAC is easy.

Central node as single-point of failure.


Medium Access Control

Control access to shared medium.Where and how?Where: centralized versus

decentralized.How: synchronous versus

asynchronous.


Centralized versus Distributed MAC

Centralized approaches: Controller grants access to medium. Simple, greater control: priorities, QoS. But, single point of failure and

performance bottleneck. Decentralized schemes:

All stations collectively run MAC to decide when to transmit.


Synchronous versus Asynchronous

Synchronous approaches: Static channel allocation. Examples: FDM, TDM. Simple but inefficient.

Asynchronous or dynamic: Example: STDM. 3 categories: round-robin, reservation,

and contention.


Round-Robin MAC

Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time).

Centralized (e.g., polling) or distributed control of who is next to transmit.

When done, station relinquishes and right to transmit goes to next station.

Efficient when many stations have data to transmit over extended period (stream).


Reservation

Time divided into slots.Station reserves slots in the future.Multiple slots for extended

transmissions.Suited to stream traffic.


Contention

No control.Stations try to acquire the medium.Distributed in nature.Perform well for bursty traffic.Can get very inefficient under heavy load.

NOTE: round-robin and contention are the most common.


Standardized MACs

TopologiesBus Ring

Round robin

Reservation

Contention

Token bus(802.4)Polling (802.11)

DQDB(802.6)

CSMA/CD(802.3)CSMA(802.11)

Token ring(802.5; FDDI)

Techniques


Wireless LANs

Use wireless transmission media. Infrared (IR): limited to indoors and

single room (IR light doesn’t penetrate walls).

RadioNarrowband microwave.Spread Spectrum LANs.


Wireless LAN Applications

Nomadic access (e.g., users roaming around campus).

LAN interconnection (e.g., across buildings).

Ad Hoc Networks (e.g., disaster relief crew).


MAC Protocols

Contention-based ALOHA and Slotted ALOHA. CSMA. CSMA/CD.

Round-robin : token-based protocols. Token bus. Token ring.


The ALOHA Protocol

Developed @ U of Hawaii in early 70’s.Packet radio networks.“Free for all”: whenever station has a

frame to send, it does so. Station listens for maximum RTT for an ACK. If no ACK, re-sends frame for a number of

times and then gives up. Receivers check FCS and destination address

to ACK.


Collisions

Invalid frames may be caused by channel noise or

Because other station(s) transmitted at the same time: collision.

Collision happens even when the last bit of a frame overlaps with the first bit of the next frame.


ALOHA’s Performance 1

Timet0

t0+t t0+2t t0+3t

vulnerable


ALOHA’s Performance 2

S = G e-2G, where S is the throughput (rate of successful transmissions) and G is the offered load.

S = Smax = 1/2e = 0.184 for G=0.5.


Slotted Aloha

Doubles performance of ALOHA.Frames can only be transmitted at

beginning of slot: “discrete” ALOHA.Vulnerable period is halved.S = G e-G.S = Smax = 1/e = 0.368 for G = 1.


ALOHA Protocols

Poor utilization.Key property of LANs: propagation

delay between stations is small compared to frame transmission time.

Consequence: stations can sense the medium before transmitting.


Carrier-Sense Multiple Access (CSMA) 1

Station that wants to transmit first listens to check if another transmission is in progress (carrier sense).

If medium is in use, station waits; else, it transmits.

Collisions can still occur.Transmitter waits for ACK; if no ACKs,

retransmits.


CSMA 2Effective when average transmission

time >> propagation time.Collisions can occur only when 2 or more

stations begin transmitting within short time.

If station transmits and no collisions during the time leading edge of frame propagates to farthest station, then NO collisions.


CSMA 3

Maximum utilization is function of frame size and propagation time. Longer frames or shorter propagation

time, higher utilization.


CSMA Flavors

1-persistent CSMA (IEEE 802.3) If medium idle, transmit; if medium busy,

wait until idle; then transmit with p=1. If collision, waits random period to re-send.

Non-persistent CSMA: after collision, node waits a random time before retransmitting.

P-persistent: when channel idle detected, transmits packet in the first slot with p.


CSMA/CD 1

CSMA with collision detection.Problem: when frames collide, medium

is unusable for duration of both (damaged) frames.

For long frames (when compared to propagation time), considerable waste.

What if station listens while transmitting?


CSMA/CD Protocol

1. If medium idle, transmit; otherwise 2.2. If medium busy, wait until idle, then

transmit with p=1.3. If collision detected, transmit brief

jamming signal and abort transmission.4. After aborting, wait random time, try

again.


CSMA/CD Performance

Wasted capacity restricted to time to detect collision.

Time to detect collision < 2*maximum propagation delay.

Rule in CSMA/CD protocols: frames long enough to allow collision detection prior to end of transmission.


IEEE 802.3 LAN Standards

802.3: 10 Mbps Ethernet.802.3u: 100Mbps (Fast) Ethernet.802.3z: 1Gbps (Gigabit) Ethernet.


Ethernet

Most popular CSMA/CD protocol.1-persistent.Developed at Xerox Parc (1976).Different implementations (10Mbps):

Notation: <bps><signaling><max seg size (100’s of meters)>

Table page 409.


Ethernet Implementations

10Base5 (thick net): up to 500m segments and 100 stations; coaxial cable(10mm); baseband (Manchester); bus.

10Base2 (thin net): up to 200m segments and 30 stations; coaxial cable(5mm); baseband (Manchester); bus.

10BaseT: up to 100m segments; unshielded TP; baseband (Manchester); star.


Baseband and Broadband

Signaling techniques.Baseband: signals transmitted

without modulation; digital signals represented by different voltages (e.g., using Manchester encoding).

Broadband: analog signaling; if digital, modulation required.


Ethernet (cont’d)

Multiple segments can be connected using repeaters.

Repeater


Ethernet Frame Format

Preamble DA SA Type Data CRC Postamble

Type: identifies upper layer protocol (for demux’ing)Data: 0-1500 bytes (min. is 46 bytes).DA and SA: destination and source addresses. Example: 6:2b:3e:0:0:1d Broadcast: all 1’s. Multicast: first bit is 1. Promiscuous mode: stations accept all frames.

8 6 6 2 4 1


Ethernet TransmissionIf channel idle:

Send frame immediately (p=1). Waits 2t between back-to-back transmissions.

If channel busy: Wait till free, then transmit (p=1).

If collision: Jam for 512 bits (for both ends to detect

collision). Waits for 0-2t (1st try), 0-4t (2nd try),...

ee/cs 450 spring 001 cpeg 419 introduction to networks znew homework is on the web. it is due in two...

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