COSC 3213: Computer Networks I Instructor: Dr. Amir Asif Department of Computer Science

1. COSC 3213: Computer Networks I Instructor: Dr. Amir Asif Department of Computer Science York 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: 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 802.3 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 802.3 – Protocol (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 • 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. 2tprop contention transmission idle IEEE 802.3 – 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 802.3 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 802.3 is given by:

6. IEEE 802.3 – Protocol (3) • Effect of a on performance of IEEE 802.3: • Transfer delays (multiples of slots) grow very large as load approaches throughput CSMA-CD a = 0.1 a = 0.01 a = 0.2 30 25 20 Avg. Transfer Delay 15 10 5 0 0 0.3 0.6 0.9 0.06 0.12 0.18 0.24 0.36 0.42 0.48 0.54 0.66 0.72 0.96 0.78 0.84 Load

7. 802.3 MAC Frame 7 1 2 or 6 2 or 6 2 4 (bytes) Destination Address Source Address Preamble SD Length Information Pad FCS IEEE 802.3 – Frame Structure (1) 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. 802.3 MAC Frame 7 1 2 or 6 2 or 6 2 4 (bytes) Destination Address Source Address Preamble SD Length Information Pad FCS 1 47 bits Unicast addressing based on NIC card Single address 0 Multicast addressing to identify groups 111 … 1 = broadcast to all stations 1 47 bits Group address 1 Local address Global address 1 0 1 46 bits Machine addressed is on the local network 1 46 bits Machine addressed is outside the local network IEEE 802.3 – Frame Structure (2) Source/Destination Address: 6 octets (or 48 bytes) are always used Types of Addresses:

9. 802.3 MAC Frame 7 1 2 or 6 2 or 6 2 4 (bytes) Destination Address Source Address Preamble SD Length Information Pad FCS Vendor Address IEEE 802.3 – Frame Structure (3) 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 1 24 bits 22 bits First 24 bits are specified by the vendor CISCO = (00000C)16; 3Comm = (02608C)16

10. IEEE 802.3 – Physical Layer (1) • Physical layers defined by the IEEE 802.3 Standard

11. transceiver IEEE 802.3 – Physical Layer (2) • 10Base5 • 10Base2

12. Single collision domain       High-Speed Backplane or Interconnection fabric     IEEE 802.3 – Physical Layer (3) • 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 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 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. J K 0 J K 0 0 0 Starting delimiter J, K non-data symbols (line code) PPP Priority; T Token bit M Monitor bit; RRR Reservation Access control P P P T M R R R I intermediate-frame bit E error-detection bit Ending delimiter J K 1 J K 1 I E 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 ED SD AC

17. Token Frame Format Data Frame Format ED SD AC 1 1 2 or 6 4 1 1 2 or 6 1 Destination Address Source Address AC Information FS SD FC FCS ED PPP Priority; T Token bit M Monitor bit; RRR Reservation Access control P P P T M R R R 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)

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/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.