1. Overview Jaringan Komputer (3)
Kuliah 4
Overview Jaringan Komputer (3)

2. ARQ Protocols Automatic Repeat Request.
Protocols that wait for ACK before sending more data.
ACKs now are used for flow AND error control.
What can happen?
At receiver: frame arrives correctly, frame arrives damaged, frame does not arrive.
At sender: ACK arrives correctly, ACK arrives damaged, ACK does not arrive.

3. ARQ Protocols Sender: Receiver: Send frame 0. Start timer.
If ACK 0, arrives, send frame 1.
If timeout, re-send frame 0.
Receiver:
**Waits for frame.
If frame arrives, check if correct sequence number.
Then send ACK for that frame.
Go to (**)

4. Sliding Window Protocols
Window: number of “outstanding” frames at any given point in time.
Every ACK received, window slides…

5. Sliding Window Protocols
A sliding window of size 1, with a 3-bit sequence number.
(a) Initially.
(b) After the first frame has been sent.
(c) After the first frame has been received.
(d) After the first acknowledgement has been received.

6. A One-Bit Sliding Window Protocol
Normal case. (b) Abnormal case.
The notation is (seq, ack, packet number). An asterisk indicates where a network layer accepts a packet.

7. A Protocol Using Go Back N
Pipelining and error recovery. Effect on an error when
(a) Receiver’s window size is 1.
(b) Receiver’s window size is large.

8. A Sliding Window Protocol Using Selective Repeat
(a) Initial situation with a window size seven.
(b) After seven frames sent and received, but not acknowledged.
(c) Initial situation with a window size of four.
(d) After four frames sent and received, but not acknowledged.

9. Multiple Access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes: interference
collision if node receives two or more signals at the same time
multiple access protocol
distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself!
no out-of-band channel for coordination

10. MAC Protocols: a taxonomy
Three broad classes:
Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency, code)
allocate piece to node for exclusive use
Random Access
channel not divided, allow collisions
“recover” from collisions
“Taking turns”
Nodes take turns, but nodes with more to send can take longer turns

11. Random Access Protocols
When node has packet to send
transmit at full channel data rate R.
no a priori coordination among nodes
two or more transmitting nodes ➜ “collision”,
random access MAC protocol specifies:
how to detect collisions
how to recover from collisions (e.g., via delayed retransmissions)
Examples of random access MAC protocols:
slotted ALOHA
ALOHA
CSMA, CSMA/CD, CSMA/CA

12. Pure (unslotted) ALOHA
unslotted Aloha: simpler, no synchronization
when frame first arrives
transmit immediately
collision probability increases:
frame sent at t0 collides with other frames sent in [t0-1,t0+1]

13. Slotted ALOHA Assumptions all frames same size
time is divided into equal size slots, time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot, all nodes detect collision
Operation
when node obtains fresh frame, it transmits in next slot
no collision, node can send new frame in next slot
if collision, node retransmits frame in each subsequent slot with prob. p until success

14. Slotted ALOHA
Pros
single active node can continuously transmit at full rate of channel
highly decentralized: only slots in nodes need to be in sync
simple
Cons
collisions, wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization

15. Throughput versus offered traffic for ALOHA systems.
Pure ALOHA (3)
Throughput versus offered traffic for ALOHA systems.

16. Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access protocols.

17. CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
If channel sensed busy, defer transmission
Human analogy: don’t interrupt others!

18. CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
colliding transmissions aborted, reducing channel wastage
collision detection:
easy in wired LANs: measure signal strengths, compare transmitted, received signals
difficult in wireless LANs: receiver shut off while transmitting
human analogy: the polite conversationalist

19. CSMA with Collision Detection
CSMA/CD can be in one of three states: contention, transmission, or idle.

20. Collision-Free Protocols
The basic bit-map protocol.

21. Collision-Free Protocols (2)
The binary countdown protocol. A dash indicates silence.

22. Adaptive Tree Walk Protocol
The tree for eight stations.

23. IP ADRESSING & ROUTER

24. IP Addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host, router and physical link
router’s typically have multiple interfaces
host may have multiple interfaces
IP addresses associated with interface, not host, router =
223 1 1 1

25. IP Addressing IP address:
network part (high order bits)
host part (low order bits)
What’s a network ? (from IP address perspective)
device interfaces with same network part of IP address
can physically reach each other without intervening router
LAN
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)

26. IP Addressing How to find the networks?
Detach each interface from router, host
create “islands of isolated networks
Interconnected
system consisting
of six networks

27. IP Addresses class A B C D 32 bits network host 10 network host 110to A
network
host B to 10
network
host to C
110
network
host to D
1110
multicast address
32 bits

28. IP addressing: Subnetting
The technique used to allow a single network address to
span multiple physical networks is called subnet addressing,
subnet routing, or subnetting.
Rest of
Internet R
All traffic to
A single Class B network with two
physical networks

29. IP addressing: Subnetting
Internet part
Local part
Subnet scheme
Physical
network
Internet part
Host

30. IP addressing: CIDR classful addressing:
inefficient use of address space, address space exhaustion
e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network
CIDR: Classless InterDomain Routing
network portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in network portion of address
network
part
host
/23

31. Tugas Baca & Presentasi
Bab
Nrp 9
01,04 10
05,09, 11
12,13,14,15 12
16,17,19 13
20,21,26 14
28,29,30,701 15
31,32,33
Referensi:
William Stallings: Wireless Communications & Networks (second edition)