Medium Access Control

1. Medium Access Control Token Ring – 802.5 CSMA-CA

2. Let’s talk about token ring network….

3. History • Token Ring network was originally developed by IBM in the 1970s • It is IBM's primary local-area network (LAN) technology. • The related IEEE 802.5 specification is almost identical to and completely compatible with IBM's Token Ring network. • In fact, the IEEE 802.5 specification was modeled after IBM Token Ring, and it continues to shadow IBM's Token Ring development.

4. Specification

5. Spesification • Token-passing networks are deterministic, which means that it is possible to calculate the maximum time that will pass before any end station will be capable of transmitting. • Token Ring networks ideal for applications in which delay must be predictable and robust network operation is important. • Example : factory automation environments are examples of such applications.

6. Relay Token Ring Host / Node Topology

7. Host Aktif Host Tidak Aktif Relay Relay MSAU Host Physical Property • Relay: bypass unactive node • MSAU (Multistation Access Unit) : group of relay from some stations.

8. MSAU

9. Spesification Token Passing : • Token moving around the network • Data always flow to one direction • Every node receives and release the token. • Node which possesses token, allowed to transmit data • If node has no data to transmit, token passed to the next node

10. Medium Access Control • The destination host will copy the frame that sent. • The sender will discard the frame from ring after being copied by destination • Station can hold the token for a maximum period of time (Token Holding Time), default 10ms • There is no token on the network while the information frame is circling the ring • The information frame circulates the ring : • Copied when reaches the destination • Removed when reaches the sender • Amount of time needed by the token to circulate the ring  Token Rotation Time

11. Node 1 akan mengirim data ke node 4

12. Token Ring Management • Terdapat satu node berstatus monitor. • Monitor bertugas mengawasi ring agar tetap lancar. • Setiap node dalam ring bisa menjadi monitor. • Monitor dipilih pada saat pertama kali ring terhubung atau pada saat monitor sekarang gagal. • Monitor yang sehat akan mengirim control message secara terus menerus menunjukan keberadaannya. • Pada saat monitor gagal, maka node lain dapat mengajukan diri sebagai monitor dengan mengirim frame “Claim Token”. Jika frame tersebut diterima kembali oleh pengirim berarti node lain setuju.

13. Monitor • Monitor bertanggung jawab terhadap keberadaan token dalam ring. • Menghilangkan token jika terjadi bit error atau crash pada token. • Monitor menggunakan timer yang nilainya sama dengan maksimum waktu rotasi token (Token Rotation Time) : (Jumlah Stasion x THT) + Ring Latency • Jika timer expires dan token belum sampai monitor, maka monitor menerbitkan kembali token baru. • Monitor juga memeriksa frame yang rusak atau frame tidak bertuan.

14. Monitor • Monitor akan menghapus frame yang salah atau token yang salah dari ring sebelum menerbitkan token baru. • Frame “yatim” (orphaned frame) adalah frame yang dikirimkan oleh node kemudian node pengirim mati. • Frame “yatim” akan diketahui dari header monitor. Header monitor diset 1 pada saat frame tersebut melewati monitor. • Pada saat suatu node mati, maka relay pada MSAU akan berfungsi membuat jalur bypass sehingga jalur tidak putus. • Mekanisme Beaconing: saat node mendeteksi adanya kerusakan yg serius di jaringan, dikirim frame beacon yg berisi domain kerusakan

15. Access Protocol • IEEE 802.5 memiliki 2 bit status, yaitu bit A dan bit C • Bit A dan bit C pada awalnya di set 0 • Pada saat node yang dituju membaca frame maka bit A diset 1. • Pada saat node yang dituju menyalin frame maka bit C diset 1. • Jika bit A bernilai 0 artinya node yang menjadi tujuan tidak ada. • Jika bit A bernilai 1 dan bit C bernilai 0 artinya node yang dituju telah membaca frame tapi tidak menyalinnya – karena alasan tertentu, misal buffernya penuh.

16. 1 1 1 6 6 >= 0 4 1 1 SD AC FC DA SA DATA UNIT FCS ED FS SD = Starting Delimiter SA = Source Address AC = Access Control FCS = Frame Check Sequence FC = Frame Control ED = Ending Delimiter DA = Destination Address FS = Frame Status 1 1 1 SD AC ED Frame Format • Format frame umum (dalam byte) • Format frame token

17. Frame Format • General frame explanation • Starting Delimiter : alerts each station to the arrival of a token, or data /command frame • Access Control : P  priority field and R  reservation. M bit monitor bit. T distinguishes a token from a data/command frame • Frame Control : indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information. • Destination Address : identify the destination address • Source Address : identify the source address • Data unit : contain data unit. • Frame Check Sequence : Error control • End Delimiter : signals the end of the token or data/command frame. It contains bits that indicate a damaged frame, and a frame that is the last of a logical sequence • Frame Status : A menunjukan terbaca atau tidak oleh node tujuan dan C tanda apakah frame disalin oleh node tujuan.

18. Early Token Release Fakta : “Jika suatu node mengirim frame dengan panjang kurang dari ukuran ring maka akan ada kapasitas ring yang terbuang” Early Token Release : Node yang telah selesai mengirim frame akan langsung melepas token.

19. CSMA-CACarrier Sense Multiple Access – Collision Avoidance

20. Introduction • CSMA/CAcarrier sense – multiple access – collision avoidance • CSMA/CA is used in 802.11 based wireless LANs.

21. IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA sender: - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then performs backoff procedure (Eth like) 802.11 CSMA receiver: if received OK return ACK after SIFS DIFS : Distributed Inter Frame Space SIFS : Short Inter Frame Space

22. Hidden Terminal effect • hidden terminals: A, C cannot hear each other • obstacles, signal attenuation • collisions at B • goal: avoid collisions at B • CSMA/CA: CSMA with Collision Avoidance

23. Collision Avoidance: RTS-CTS exchange • CSMA/CA: explicit channel reservation • sender: send short RTS: request to send • receiver: reply with short CTS: clear to send • CTS reserves channel for sender, notifying (possibly hidden) stations • Avoid hidden station collisions

24. Collision Avoidance: RTS-CTS exchange • RTS and CTS short: • collisions less likely, of shorter duration • end result similar to collision detection