CSC 335 Data Communications and Networking Lecture 7: Local Area Networking Dr. Cheer-Sun Yang Fall 2000.

CSC 335 Data Communications

and Networking

Lecture 7: Local Area Networking

Dr. Cheer-Sun Yang

Fall 2000

Motivation

• Up to this point, we’ve talked about point-to-point communication.

• We many need to connect many computers together.

• Local Area Network(LAN): if they are located in a relatively close geographic area.

• Metropolitan Area Network (MAN) : extends over entire city

• Wide Area Network (WAN) : extends across public switching network.

LAN Applications (1)

• Personal computer LANs– Low cost– Limited data rate

• Back end networks and storage area networks– Interconnecting large systems (mainframes and large

storage devices)• High data rate• High speed interface• Distributed access• Limited distance• Limited number of devices

LAN Applications (2)

• High speed office networks– Desktop image processing– High capacity local storage

• Backbone LANs– Interconnect low speed local LANs– Reliability– Capacity– Cost

LAN Architecture

• Topologies

• Protocol architecture– Physical Layer– Media access control– Logical Link Control

Topologies

• Bus: A single communication line, typically a twisted pair, coaxial cable, or optical fiber, represents the primary medium.

• Ring: packets can only be passed from one node to it’s neighbor.

• Star: A hub or a computer is used to connect to all other computers.

• Tree: no loop exists (logical connection).

LAN Topologies

Frame Transmission - Bus LAN

Ring Topology

• Repeaters joined by point to point links in closed loop– Receive data on one link and retransmit on another– Links unidirectional– Stations attach to repeaters

• Data in frames– Circulate past all stations– Destination recognizes address and copies frame– Frame circulates back to source where it is removed

• Media access control determines when station can insert frame

Frame Transmission

Ring LAN

Star Topology

• Each station connected directly to central node– Usually via two point to point links

• Central node can broadcast– Physical star, logical bus– Only one station can transmit at a time

• Central node can act as frame switch

Bus and Tree• Multipoint medium• Transmission propagates throughout medium • Heard by all stations

– Need to identify target station• Each station has unique address

• Full duplex connection between station and tap– Allows for transmission and reception

• Need to regulate transmission– To avoid collisions– To avoid hogging

• Data in small blocks - frames

• Terminator absorbs frames at end of medium

Protocol Architecture

• Protocol layering (IEEE 802.X)– Physical Layer– Media access control (MAC) Sublayer– Logical link control (LLC)

IEEE 802.X

• IEEE 802.3 : Ethernet LAN

• IEEE 802.4 : Token Bus LAN

• IEEE 802.5 : Token Ring LAN

• Other Ring Networks: FDDI, Slotted Rings.

• IEEE 802.6 : Distributed Queue Dual Bus (DQDB) MAN standard.

IEEE 802 vs. OSI

Fig 6.4

LAN Protocols in Context

802 Physical Layer Design Issues

• Encoding/decoding

• Preamble generation/removal

• Bit transmission/reception

• Transmission medium and topology

802 Physical Layer

• Required hardware for connecting a PC to Ethernet directly:– Transceiver

– Attachment Unit Interface (AUI) cable

– Network Interface Card (NIC) also known as Network Adapter

• Required hardware for connecting a PC to a remote computer: modem (with the help of PPP)

Bus LANs

• Signal balancing– Signal must be strong enough to meet receiver’s

minimum signal strength requirements

– Give adequate signal to noise ration

– Not so strong that it overloads transmitter

– Must satisfy these for all combinations of sending and receiving station on bus

– Usual to divide network into small segments

– Link segments with amplifies or repeaters

Transmission Media• Twisted pair– Not practical in shared bus at higher data rates

• Baseband coaxial cable– Used by Ethernet

• Broadband coaxial cable– Included in 802.3 specification but no longer made

• Optical fiber– Expensive– Difficulty with availability– Not used

• Few new installations– Replaced by star based twisted pair and optical fiber

Baseband Coaxial Cable

• Uses digital signaling• Manchester or Differential Manchester encoding• Entire frequency spectrum of cable used• Single channel on cable• Bi-directional• Few kilometer range• Ethernet (basis for 802.3) at 10Mbps• 50 ohm cable

10Base5

• Ethernet and 802.3 originally used 0.4 inch diameter cable at 10Mbps

• Max cable length 500m• Distance between taps a multiple of 2.5m

– Ensures that reflections from taps do not add in phase

• Max 100 taps• 10Base5

10Base2• Cheapernet• 0.25 inch cable

– More flexible

– Easier to bring to workstation

– Cheaper electronics

– Greater attenuation

– Lower noise resistance

– Fewer taps (30)

– Shorter distance (200m)

Cable Specifications for 802.3

• 10BaseT: 10 Mbps, baseband, unshield twisted

• 10Base2: 10Mbps, Cat. 2 coaxial• 10Base5: 10 Mbps, Cat. 5, Cat. 5e coaxial• 100BaseTX: 100 Mbps, twisted cable (Fast

Ethernet)• 10Broad36: maximum segment length 3600

meters

Gigabit Ethernet

• 1000Base-SX– Short wavelength, multimode fiber

• 1000Base-LX– Long wavelength, Multi or single mode fiber

• 1000Base-CX– Copper jumpers <25m, shielded twisted pair

• 1000Base-T– 4 pairs, cat 5 UTP

• Signaling - 8B/10B

Connectors

• T-connector: used to form a bus topology

• RJ-45 connectors: for connecting a PC to another PC, Ethernet, or hub.– Cross-over: a direct connection to another PC– Straight-through: connection with the Ethernet

jack or hub.

Repeaters

• Transmits in both directions• Joins two segments of cable• No buffering• No logical isolation of segments• If two stations on different segments send at

the same time, packets will collide• Only one path of segments and repeaters

between any two stations

Media Access Control Sublayer

• Assembly of data into frame with address and error detection fields

• Disassembly of frame– Address recognition

– Error detection

• Govern access to transmission medium– Not found in traditional layer 2 data link control

– Also known as Contention protocols (section 3.4)

Collision vs. Contention

• When the communication link is used by one station to transmit a frame, another station connecting to the same link tries to send a packet– collision

• Contention: accessing the medium with the consideration that a collision may occur.

• Contention Protocols: the protocol is designed to deal with collision using contention.

• Collision-free Protocols: the protocol is designed so that collision will not occur.

Contention Protocols

• Pure ALOHA

• Slotted ALOHA

• Carrier Sense Multiple Access (CSMA)

• Persistent and non-persistent CSMA

• CSMA with Collision Detection (CSMA/CD)

Collision-Free Protocols

• A Bit-Map Protocol: reservation protocol

• Binary Countdown

Pure Aloha• Packet Radio• When station has frame, it sends• Station listens (for max round trip time)plus small

increment• If ACK, fine. If not, retransmit• If no ACK after repeated transmissions, give up• Frame check sequence (as in HDLC)

Pure Aloha(cont’d)

• If frame OK and address matches receiver, send ACK

• Frame may be damaged by noise or by another station transmitting at the same time (collision)

• Any overlap of frames causes collision• Max utilization 18% (WHY?)

The Efficiency of Pure Aloha

G = the traffic measured as the average number of frames generated per slot

S = the success rate, success frame / slot

= G e –2G (pure Aloha)

S = G e –G (slotted Aloha)

S = Pr[no frame is generated]= e -G

Pr[k frames are generated] = G k e –G / k !

This is called a probability distribution function(pdf) forPoisson distribution. (e = 2.7818…)

The Efficiency of Pure Aloha

= G e –2G (pure Aloha)S = G * P 0

If there is no negative acknowledgement frame received after sending out one frame, the transmission is successful. SoP0 = Pr[no frames are generated in 2 time slots] = e -G * e –G

= e –2G

The Efficiency of Pure Aloha

S = G * P 0= G e –2G (pure Aloha)

We need to find the value of G such that S is maximized.S’ = G (-2) e –2G + e –2G = (1 – 2G) * e –2G

Let S’ = 0 => G = ½ When G = ½, S = 1/ 2e = 0.184 = 18%

Slotted ALOHA

• A computer is not allowed to send until the beginning of the next slot.

• Time in uniform slots equal to frame transmission time

• When a frame is allowed to be transmitted, there is no collision.

• Need central clock (or other sync mechanism)• Transmission begins at slot boundary• Max utilization 37% (WHY?)

The Efficiency of Slotted Aloha

= G e –G (slotted Aloha)S = G * P 0

If there is no other frame received after sending out one frame, the transmission is successful. SoP0 = Pr[no frames are generated in one time slots] = e -G

The Efficiency of Slotted Aloha

S = G * P 0= G e –G (slotted Aloha)

We need to find the value of G such that S is maximized.S’ = G (-1) e –G + e –G = (1 –G) * e –G

Let S’ = 0 => G = 1 When G = 1, S = 1/ e = 0.368 = 37%

Carrier Sense Multiple Access (CSMA) Protocols

• Protocols in which stations listen for a carrier (i.e., a transmission) and act accordingly are called carrier sense protocols.– 1-persistent CSMA– Non-persistent CSMA– p-persistent CSMA

CSMA

• Propagation time is much less than transmission time

• All stations know that a transmission has started almost immediately

• First listen for clear medium (carrier sense)

If Busy?

• If medium is idle, transmit

• If busy, listen for idle then transmit immediately

• No ACK then retransmit

• If two stations are waiting, it is called a collision.

1-persistent CSMA

• When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment.

• If the channel is busy, the station waits until it becomes idle.

• The station retransmits with a probability of 1 when it finds that the channel is idle.

Non-persistent CSMA

• When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment.

• If the channel is busy, the station waits until it becomes idle.

• The station does not keep trying. It waits for a random number of time and retries.

Non-persistent CSMA

• This applies to slotted channels. When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment.

• If the channel is idle, it transmits with a probability p. With a probability of 1-p, it defers until the next slot.

• If the next slot is also idle, it transmits or defers again with probability p and q.

CSMA

• Max utilization depends on propagation time (medium length) and frame length. Longer frame and shorter propagation gives better utilization.

• Collisions still can be a problem, especially with p-persistent CSMA.

• One way to reduce the frequency of collision with CSMA is to lower the probability that a station will send when a previous is done. (Fig. 3.26)

• Smaller values of p => fewer collision.

Any Other Way?

• Is there another way to improve the successful rate?

• Yes if there is a way to detect collision prior to transmission.

• Why is this faster?

Collisions with and without Detection

• Without collision detection, a station must send and then wait for 2 time slots before another attempt to send.

• With collision detection, a station can stop transmission if collision detection requires less time than sending a frame.

Collision Detection

• On baseband bus, collision produces much higher signal voltage than signal

• Collision detected if cable signal greater than single station signal

• Signal attenuated over distance• Limit distance to 500m (10Base5) or 200m

(10Base2)• For twisted pair (star-topology) activity on more

than one port is collision• Special collision presence signal

CSMA/CD

• With CSMA, collision occupies medium for duration of transmission

• Stations listen while transmitting

• If medium idle, transmit• If busy, listen for idle, then transmit• If collision detected, jam then ease transmission• After jam, wait random time then start again

– Binary exponential back off

CSMA/CDOperation

Binary Exponential Back Off

• If a station’s frame collides for the first time, wait 0 or 1 time slot (chosen randomly) before trying again.

• If it collides a second time, wait 0, 1, 2, or 3 slots (again, chosen randomly).

• After a third collision, wait anywhere from 0 to 2 n –1 slots if n <= 10, if n > 10, wait between 0 to 1024 (2 10) slots.

• After 16 collisions, give up. Further recovery is up to the upper layer, such as a user.

IEEE 802.3 Frame Format

Frame Format (802.3)

• Start of frame delimiter: 10101011• Destination address• Source address• Data length field• Data field• Pad field: the data field must be at least 46 octets.• Frame check sequence: using 32-bit CRC.

Efficiency of 802.3(p.375)

P: the probability that a frame is sent without a collisionPs: the probability that a station sendsThe probability of a collision = 1 – P.The probability of a transmission requiring exactly N attempts == N-1 collisions followed by a success= N * Ps * ( 1- Ps) N - 1

Efficiency of 802.3(p.376-377)

We would like to know under what conditions the largest number of frames are sent successfully.

Efficiency of 802.3(p.375)

The probability of a transmission requiring exactly N attempts =P= N * Ps * ( 1- Ps) N – 1

dP/dPs = N(1-Ps)N-1 + N Ps (N-1)(1-Ps)N-2

= N (1-Ps) N-2 [1- Ps – Ps (N –1) ]Let dP/dPs = 0 => Ps = 1/N

Efficiency of 802.3

How many time slots has passed before a frame is sent successfully?

The Efficiency of 802.3

The contention period = the number of time slots passed before a successful transmission

The Efficiency of 802.3

Assume that the probability of a success in each attempt = p.The probability of a collision = 1 – p.The probability of a transmission requiring exactly i+1 attempts = P(i+1)

= i collisions followed by a success= p (1- p) i

The Efficiency of 802.3

The contention period

=

= (1-p)/p

0

2)1/(*i

i xxxiGiven

0

2/)1(**)1(*i

i pppppi

The Efficiency of 802.3

The contention period (C)

= (1/p) –1

0 <= p <= 1 (p: the successful rate)If p -> 1 C -> 0If p -> 0, C = largeWe’ve found that when Ps=1/N, p is maximized.So, C = (1-1/N)1-N -1 when p is maximized. If N->large, C = 2.718 – 1 = 1.718 (close to 2).

The Utilization Rate of Ethernet

The percent utilization (U) does not depend on the number of stations in practice. A station will try to send regardless of how many other stations there are. The previous result often is used as benchmarks against which measures are made to estimate efficiency.

The Utilization Rate of Ethernet

The percent utilization (U) is defined as the amount of time spent on transmitting a frame as a percentage of the total time spent on contending and transmitting. Assume:R = transmission rateF = number of bits in a frameT = slot timeSo U = %100*

*CTRF

RF

Required Reading

• Section 6.1, 3.4, 6.2