1. COSC 3213: Computer Networks I Instructor: Dr. Amir Asif
Department of Computer Science
York University
Section B
“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:
Decimal, each digit lies between 0 and 9
Binary, each bit is either 0 or 1
Octal, ecah octal digit lies between 0 and 7
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 Developed in the 1970s by Xerox
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”.
IEEE LAN standard was developed in 1985, very similar to DIX.
In 1995, the 100Mbps Fast Ethernet standard (IEEE 802.3u) was approved.
In 1998, the 1Gbps Gigabit Ethernet Standard (IEEE 802.3z) was approved.
Trend of faster and long range Ethernet continues …

4. IEEE – Protocol (1)
IEEE 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 based on the the following rescheduling algorithm.
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 – Protocol (2)
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
Activity on the IEEE LAN is sum of four components:
Idle: nearly 0 near saturation.
Contention: multiple of (2tprop)
Transmission: L / R
Propagation: tprop
The average number of minislots per contention period is e minislots.
The normalized throughput of IEEE is given by:
contention
transmission
idle
2tprop

6. Normalized throughput
IEEE – Protocol (3)
Effect of a on performance of IEEE 802.3:
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
a = 0.2
a = 0.1
a = 0.01 30 25 20
Avg. Transfer Delay 15 10 5
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

7. IEEE 802.3 – Frame Structure (1)
Preamble SD
Destination
Address
Source
Length
Information
Pad
FCS 7 1
2 or 6 2
4 (bytes)
802.3 MAC Frame
Preamble: has 7 bytes of the bit pattern …….
Used for synchronization
Starting Delimiter (SD): , 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 SD
Destination
Address
Source
Length
Information
Pad
FCS 7 1
2 or 6 2
4 (bytes)
802.3 MAC Frame
Source/Destination Address: 6 octets (or 48 bits) are always used
Types of Addresses:
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
47 bits
Local address 1
46 bits
Machine addressed is on the local network 1
Global address
46 bits
Machine addressed is outside the local network

9. IEEE 802.3 – Frame Structure (3)
Preamble SD
Destination
Address
Source
Length
Information
Pad
FCS 7 1
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 frame except for the length bytes.
In DIX Ethernet Frame, length field is replaced by type field
Value of type field > (0600)16 1 1
24 bits
22 bits
First 24 bits are specified by the vendor
CISCO = (00000C)16; 3Comm = (02608C)16
Address
Vendor

10. IEEE 802.3 – Physical Layer (1)
Physical layers defined by the IEEE Standard
10Base5
10Base2
10BaseT
10BaseF
Medium
Thick Coaxial
Thin Coaxial
Twisted Pair
Optical Fiber
Segment length (max)
500m
185m
100m
2km
Topology
Bus
Star
Point to Point
Data Rate
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)
10 Base T
Read more about Fast and Gigabit Ethernet ….
     
Single collision domain
   
High-Speed Backplane or Interconnection fabric

13. IEEE History
Set of protocols at the physical and data link layer (MAC sublayer)
Developed by IBM in 1980s
IEEE standard modeled after IBM Token Ring in 1990s
IBM and IEEE specifications differ in minor ways:
IBM’s Token Ring specifies a star; IEEE does not specify a topology but most IEEE implementations are based on a star
IBM’s Token Ring uses twisted-pair wire; IEEE 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
transmit mode
input
from
ring
output to
ring
delay
delay
to device
from device

15. MAC Scheduling Approaches: Token Ring (2)
Interface operates in two modes
listen mode
transmit mode
input
from
ring
output to
ring
delay
delay
to device
from device
1. Each bit is reproduced on the ring with a delay 2. 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 ring 3. 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
Token Frame Format SD AC ED
Starting delimiter
J, K non-data symbols (line code)
J K J K 0
P P P T M
R R R
Access control
PPP Priority; T Token bit
M Monitor bit; RRR Reservation
Ending delimiter
I intermediate-frame bit
E error-detection bit I E
J K 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) SD
Destination
Address
Source
Information
FCS 1 4 ED FC
2 or 6 AC FS
Data Frame Format SD AC ED
Token Frame Format
P P P T M
R R R
Access control
PPP Priority; T Token bit
M 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 (t’) in seconds = t + Mb/R
Ring Latency in bits = (t + Mb/R)R
where t 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:
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.
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
Cost : Ethernet is generally less expensive and easier to install than Token Ring .
Stability : Token Ring is generally more secure and more stable than Ethernet.
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.
QoS : Ethernet uses CSMA/ DC 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.