COSC 3213: Computer Networks IInstructor: Dr. Amir Asif

Department of Computer ScienceYork University

Section M

“Local Area Networks” Topics:LAN Standards: IEEE 802.3, IEEE 802.3u, IEEE 802.3z, IEEE 802.5

Garcia: Sections 6.7 – 6.8

2

Review of Number System

Consider four representations for integers:

1. Decimal, each digit lies between 0 and 9

2. Binary, each bit is either 0 or 1

3. Octal, ecah octal digit lies between 0 and 7

4. Hexadecimal, each hexadecimal digit lies between 0 and F

Activity 1: Convert the decimal number 1500 into representations (2) – (4).

3

IEEE 802.3 - History

1. Developed in the 1970s by Xerox

2. Dec, Intel, and Xerox completed the “DIX” standard for 10Mbps LAN based on coaxial cable. “DIX” standard is referred to as “DIX Ethernet Standard” or simply “Ethernet”.

3. IEEE 802.3 LAN standard was developed in 1985, very similar to DIX.

4. In 1995, the 100Mbps Fast Ethernet standard (IEEE 802.3u) was approved.

5. In 1998, the 1Gbps Gigabit Ethernet Standard (IEEE 802.3z) was approved.

6. Trend of faster and long range Ethernet continues …

4

IEEE 802.3 – Protocol (1)

1. IEEE 802.3 uses the 1-persistent CSMA/CD protocol in the MAC sublayer A station with a frame to transmit waits until the channel is free (or silent). When the channel goes silent, the frame is transmitted. If a collision is not detected for (2 × propagation time), frame is assumed delivered In case of a collision, the station aborts the transmission and reattempts after a randomly

scheduled time

2. Rescheduling Algorithm: is based on a truncated exponential backoff algorithm. For n’th transmission, the backoff period is selected by choosing a length at random

between (0, 2k – 1) minislots where k = min(n,10). Minislot: defined as a duration that is at least as long as (2 × propagation delay) 1st Retransmission: (0,1) minislots

2nd Retransmission: (0,1,2,3) minislots

3rd Retransmission: (0,1,2,3,4,5,6,7) minislots

10th and higher retransmissions: (0,1,2, …, 210 – 1) minislots

5

IEEE 802.3 – Protocol (2)

3. A total of 16 retransmission attempts are allowed.

Activity 2: Show that the total propagation delay for a 10Mbps LAN consisting of five segments, each 2500m long and connected to each other with 4 repeaters, is around 51.2 ms. What is the minimum length of an Ethernet frame designed for the 10Mbps LAN? (Assume a propagation speed of 2.5 × 108 m/s) Answer: 512 bits

4. Activity on the IEEE 802.3 LAN is sum of four components:

Idle: nearly 0 near saturation. Contention: multiple of (2tprop)

Transmission: L / R Propagation: tprop

5. The average number of minislots per contention period is e minislots.

6. The normalized throughput of IEEE 802.3 is given by:

contention transmission idle

2tprop

proptXaae

/with )12(1

1

6

IEEE 802.3 – Protocol (3)

7. Effect of a on performance of IEEE 802.3:

8. Transfer delays (multiples of slots) grow very large as load approaches throughput

a Normalized throughput

0.01 0.94

0.1 0.61

0.2 0.44

CSMA-CD

0

5

10

15

20

25

30

0

0.06

0.12

0.18

0.24 0.3

0.36

0.42

0.48

0.54 0.6

0.66

0.72

0.78

0.84 0.9

0.96

Load

Avg

. Tra

nsf

er D

elay

a = 0.01a = 0.1a = 0.2

7

IEEE 802.3 – Frame Structure (1)

Preamble SDDestination

AddressSource Address

Length Information Pad FCS

7 1 2 or 6 2 or 6 2 4 (bytes)

802.3 MAC Frame

Preamble: has 7 bytes of the bit pattern 10101010 …….

Used for synchronization

Starting Delimiter (SD): 10101011, indicates start of the frame

Source/Destination Address: 6 octets (or 48 bits long) is always used => 246 global addresses

Length: specifies the length of data (information) in bytes (or octets)

Max. frame length = 1518 bytes excluding preamble & SD

Information = 1518 – 18 = 1500 bytes

Length field = (05DC)16 < (0600)16

Min. frame length = 512 bits or 64 bytes

Pad field ensures that the frame is at least 64 bytes long

Length field = ?

8

IEEE 802.3 – Frame Structure (2)

Preamble SDDestination

AddressSource Address

Length Information Pad FCS

7 1 2 or 6 2 or 6 2 4 (bytes)

802.3 MAC Frame

Source/Destination Address: 6 octets (or 48 bytes) are always used

Types of Addresses:

0 Single address

1 47 bits

Unicast addressing based on NIC card

1 Group address

Multicast addressing to identify groups

111 … 1 = broadcast to all stations

1 47 bits

0 Local address

1 46 bits

Machine addressed is on the local network

1 Global address

1 46 bits

Machine addressed is outside the local network

9

IEEE 802.3 – Frame Structure (3)

Preamble SDDestination

AddressSource Address

Length Information Pad FCS

7 1 2 or 6 2 or 6 2 4 (bytes)

802.3 MAC Frame

Source/Destination Address: 6 octets (or 48 bytes) are always used

Types of Addresses:

FCS: Frame Check Sum

Based on CCITT 32-bit CRC code

Structure of DIX Ethernet Frame is same as IEEE 802.3 frame except for the length bytes.

In DIX Ethernet Frame, length field is replaced by type field

Value of type field > (0600)16

1 22 bits First 24 bits are specified by the vendor

CISCO = (00000C)16; 3Comm = (02608C)16AddressVendor

1 24 bits

10

IEEE 802.3 – Physical Layer (1)

Physical layers defined by the IEEE 802.3 Standard

10Base5 10Base2 10BaseT 10BaseF

Medium Thick Coaxial Thin Coaxial Twisted Pair Optical Fiber

Segment length (max)

500m 185m 100m 2km

Topology Bus Bus Star Point to Point

Data Rate 10Mbps 10Mbps 10Mbps 10Mbps

Miscellaneous

Transceiver needed to attach NIC card to coax

T-shaped BNC junctions used

Twisted pair connects NIC card to hub responsible for comm.

11

IEEE 802.3 – Physical Layer (2)

10Base5

10Base2

transceiver

12

IEEE 802.3 – Physical Layer (3)

High-Speed Backplane or Interconnection fabric

Single collision domain

10 Base T

Read more about Fast and Gigabit Ethernet ….

13

IEEE 802.5 - History

Set of protocols at the physical and data link layer (MAC sublayer)

Developed by IBM in 1980s

IEEE 802.5 standard modeled after IBM Token Ring in 1990s

IBM and IEEE specifications differ in minor ways: IBM’s Token Ring specifies a star; IEEE 802.5 does not specify a topology but most IEEE

802.5 implementations are based on a star IBM’s Token Ring uses twisted-pair wire; IEEE 802.5 does not specify a media type

Speed: 4 Mbps and 16 Mbps

Signalling: Differential Manchester

Size: max 250 stations

14

IEEE 802.5: Token Ring

Token Passing Systems: decenteralized approach with no central controller In ring topology, each station is connected in a ring using an interface

Interface operates in two modes

listen mode

delay

transmit mode

delay

to device from device

inputfromring

outputtoring

15

MAC Scheduling Approaches: Token Ring (2)

Interface operates in two modes

listen mode

delay

transmit mode

delay

to device from device

inputfromring

outputtoring

1. Each bit is reproduced on the ring with a delay2. Delay is a multiple of (one bit duration)3. Delay allows to check for certain bit patterns

1. Station transmits a message bit by bit on ring

2. Station receives a message bit by bit from ring3. No forwarding of bits is done

16

MAC Scheduling Approaches: Token Ring (3)

When no station is transmitting, there is a free token floating on the ring

When a free token is received (T = 0), the interface changes the passing token bit (T = 1) and starts transmitting

SD AC EDToken Frame Format

Starting delimiter J, K non-data symbols (line code)0 0J K 0 J K 0

P P P T M R R RAccess controlPPP Priority; T Token bitM Monitor bit; RRR Reservation

Ending delimiterI intermediate-frame bitE error-detection bit

I EJ K 1 J K 1

17

MAC Scheduling Approaches: Token Ring (4)

When a free token is received (T = 0), the interface changes the passing token bit (T = 1) and starts transmitting

Each transmitted bit is removed by the destination station or by the source station After the transmission is complete, the source station inserts the free token back onto the ring

with (T = 0)

SDDestination

AddressSource Address

Information FCS

1 4

EDFC

2 or 6 2 or 61 1

AC

1

FS

1Data Frame Format

SD AC EDToken Frame Format

P P P T M R R RAccess controlPPP Priority; T Token bitM Monitor bit; RRR Reservation

18

MAC Scheduling Approaches: Token Ring (5)

Ring Latency: Maximum number of bits in transition around the ring If frame size > ring latency, a complete frame cannot be present on the ring at one time If frame size < ring latency, complete frame is on transition in the ring.

Ring Latency (’ in seconds = + Mb/R

Ring Latency in bits = ( + Mb/R)R

where is total propagation delay around the ring, M is the number of stations in the ring, b is the number of bit-delays in an interface.

Approaches to Token Reinsertion:

1. Single token operation (delayed token release): in which the token is released only after a complete frame is received by the transmitting station. Suitable when frame size is nearly equal to ring latency.

2. Multiple token operation (early token release): in which token is released after the transmission of a frame is completed by the transmitting station. Suitable when frame size is less than ring latency.

19

MAC Scheduling Approaches: Token Ring vs Token Bus

1. Cost : Ethernet is generally less expensive and easier to install than Token Ring .

2. Stability : Token Ring is generally more secure and more stable than Ethernet.

3. Scalability : It is usually more difficult to add more computers on a Token Ring LAN than it is to an Ethernet LAN. However, as additional computers are added, performance degradation will be less pronounced on the Token Ring LAN than it will be on the Ethernet LAN.

4. QoS : Ethernet uses CSMA/CD media access control and Token Ring uses token passing. This makes Ethernet better suited in a situation where there are a large number of computers sending fewer, larger data frames. Token Ring is better suited for small to medium size LANs sending many, smaller data frames.