1. CPEG512 Advanced Computer Networks
University of Nizwa
College of Engineering and Architecture
Electrical and computer Engineering
CPEG512 Advanced Computer Networks
Week7
Fall 2014/2015
Atef Abu Salim

2. Code Division Multiplexing
Also known as code division multiple access
An advanced technique that allows multiple devices to transmit on the same frequencies at the same time.

3. CDM cont. Each mobile device is assigned a unique 64-bit code
To send a binary 1, mobile device transmits the unique code
To send a binary 0, mobile device transmits the inverse of code

4. Transmission Mode
The transmission of binary data across a link can be accomplished in either parallel or serial mode.

5. Transmission Modes cont.
Data Transmission
Serial
Asynchronous
synchronous
Isochronous
Parallel

6. Transmission Modes cont.
Parallel Transmission:
uses n wires to transmit n bits at one time.
Ex. Computer to printer

7. Transmission Modes cont.
Serial Transmission:
In serial Transmission one bit follows another, so we need one communication channel to transmit data between two communication devices.
Serial transmission occurs in one of the three way: Asynchronous, Synchronous and Isynchronous.
Ex. computer to modem

8. Serial Transmission
In asynchronous Transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits or gaps at the end of each byte.
…..
Stop Bit
Byte N
Start Bit
Stop Bit
Byte2
Start Bit
Stop Bit
Byte1
Start Bit

9. Serial Transmission cont.
In synchronous Transmission, we send one bit after another without start or stop bits.
It is the responsibility of the receiver to group the bits.
Synchronous transfer (self-clocking), uses sync bytes in the frame. Frame has multiple bytes. (global or synchronized clocks). …
Byte 1…ByteN=Block
Sync Byte
Byte N
Byte 2
Byte 1
Sync Byte

10. Serial Transmission cont.
The isochronous mode provide synchronized for the entire stream of bit must.
It guarantees that data arrive at the fixed rate.

11. Ethernet
Ethernet Technology (IEEE 802.3) : It is the most important technology of LAN.
Definition
Ethernet standards define Physical and MAC layers of the OSI model only.
Physical layer
Electrical characteristics of cable connectors, including maximum lengths
Bit encoding (e.g., Manchester coding) and transmission rate (10/100/1000 Mbps)
Medium Access Control (MAC) layer
Ethernet frame format and addressing
Access protocol = Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

12. Ethernet cont Easy to understand, implement, manage, and maintain
Most popular industry use and acceptance.
Easy to understand, implement, manage, and maintain
Low-cost network implementations
Extensive topological flexibility for network installation.
Interoperability and operation of standards- compliant products, regardless of manufacturer.

13. Ethernet Standards IEEE 802.3 Committee standards 10Base5 (1985)
Compatible with DIX Ethernet Small difference in frame format (Frame Type vs. Length field) and signal naming.
10Base2 (1988)
Uses less expensive, thinner coaxial cable. Eliminates external transceiver (MAU) and puts transceiver onto Network Interface Card. Also called ThinNet and CheaperNet.
1Base5 (1988)
1Mbps StarLAN star topology over unshielded twisted pair. 250m max distance from node to hub.

14. Ethernet Standards cont.
10BaseT (1990)
10Mbps star topology over unshielded twisted pair meters max. distance from node to hub
10BaseF (1993)
10Mbps star topology over optical fiber meters max. distance from NIC to hub. Replaces previous FOIRL (Fiber Optic Inter-Repeater Link) specification for inter-connecting repeaters or hubs.
100BaseT / 100BaseF (1995)
100Mbps Ethernet using CSMA/CD over twisted pair or fiber.

15. Ethernet Standards cont.
1000BaseX / 1000BaseT (1998/1999)
Gigabit Ethernet over coaxial cable and fiber / over 4 - Category 5 twisted pair.
10GB Ethernet – 802.3ae (2002)

16. Multiple-access protocols

17. CSMA/CD Carrier Sense Multiple Access with Collision Detection:
Carrier sense: listing to physical medium if other station is transmitting.
Collision detection: During transmission, station listens if no other station is corrupting its message.

18. CSMA/CD cont. Basic Idea
When a station has frame to transmit:
Listen for Data Transmission (Carrier Sense)
When no other station is transmitting
Start to transmit frame, listening for collision.
If collision is heard, stop transmitting, wait random time, and transmit again.
If channel is in-use, then the station backs off.

19. Station is ready to send
CSMA/CD Procedure
Station is ready to send
try again
Wait
Sense
Channel
channel
busy
Send
jam signal
Transmit data and
sense channel
collision detected
successful
transmission

20. Collision in CSMA
t1 – A sense medium idle and transmits. t2 – Z senses medium idle
and transmits. t3 – signals collide.

21. Non-persistent CSMA When a station has a frame to transmit:
Listen for data transmission (Carrier)
a) When medium is quiet(idle), transmit.
When medium is busy, wait a random period of time and go
back to step 1.
The random wait gives a probability that stations will be sensing the channel at different periods when helps in avoiding collision.

22. p-persistent CSMA P stands for probability.
When a station has a frame to transmit:
Listen for data transmission (Carrier)
When medium is idle
Choose random value,  , between 0 and 1.
If  < p then
Transmit frame
Wait for Ack
If no Ack received within time T, go to step 1.
If  > p then
Wait for 1 contention slot time.
Go to step 1.

23. 1-persistent CSMA
A station transmits immediately upon sensing the channel is idle.
Upon collision, station waits a random period before sensing the channel again.
Called 1-persistent, because station transmits with probability of 1 when the channel is sensed idle.

24. Persistence strategies

25. CSMA/CD procedure

26. Token-passing network

28. Channelization
Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations.
There are three basic forms of channelization:
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)

29. In FDMA, the bandwidth is divided into channels.
In TDMA, the bandwidth is just one channel that is timeshared.
In CDMA, one channel carries all transmissions simultaneously.

30. IEEE Networking Specifications

31. Data Link - MAC Sub Layer
This is the first layer where data can be manipulated above the physical layer
In OSI, Data Unit is exchanged between two ends.
In LANs, this layer sends/receives IP packets.
The information is transmitted in “Frame”.
IP packets are encapsulated in Ethernet frames.

32. Data Link - MAC Sub Layer
Stations transmit when channel is idle.
If a collision occurs, a back-off mechanism is applied.
10Mbps LAN give about 33% true throughput (user information successfully exchanged taking out contention, collision and other overhead)

33. MAC Address Medium Access Control.
Ethernet address/MAC address is 48bits. XX XX XX XX XX XX
B-E1-50

34. Questions???????????????
Q1: does every workstation (PC) have a MAC address?
How do I find the MAC address of my PC (Windows)?
Q2: could one workstation have more than one MAC address?
Q3: who assign the MAC address?
Q4: Can I change the MAC address?
Q5: is MAC address local (to LAN) or global (like Internet)?

35. Data Link - MAC Sub Layer
Physical Layer
OSI Layers
LLC
LAN Sub
Layers
MAC
IEEE 802.x
IEEE 802.3
CSMA/CD
LLC:
Logical Link Control
MAC:
Medium Access Control

36. Data Link - MAC Sub Layer cont.
LLC
Physical Layer
Station B
MAC
Station A
P-P

37. Data Link - MAC Sub Layer cont.
LLC hides the details of the different IEEE 802.x(4=token bus, 5=token ring, 3=Ethernet) from the network layer.

38. A network with and without a bridge

39. Switched Ethernet

40. Full-duplex switched Ethernet

41. 802.3 MAC frame
Preamble (PRE) (for synchronization): Consists of 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.
The preamble is actually added at the physical layer and is not (Formally) part of the frame.

42. 802.3 MAC frame cont.
Start-of-frame delimiter (SFD): the second filed, (1 byte : ) signals the beginning of the frame .
The SFD warns the station that this is the last chances for synchronization.
The last 2 bits is 11 and alert the receiver that the next field is the destination address.
Destination address (DA): Consists of 6 bytes(48-bits) , MAC Station HW address.
Source addresses (SA): Consists of 6 bytes(48-bits) , MAC Station HW address.

43. 802.3 MAC frame cont.
Length or type : this field is defined as a type field or length field (used it as the length field to define the number of bytes in the data field).
the original Ethernet used this filed as the type field to define the upper layer protocol using the byte.
Data : this field carries data encapsulated from the upper layer protocol (min 64 byte , max 1518 bytes).

44. 802.3 MAC frame cont.
CRC(Cyclic Redundancy Check ): the last field contains error detection information.

45. 802.3 MAC frame and Ethernet Frame16 56 8
48*
48*
Variable 32
Preamble
SFD
Dest Add
Src Add
TYPE
Data
CRC
ETHERNET Frame 16 56 8
48*
48*
Variable 32
Preamble
SFD
Dest Addr
Src Addr
DATALEN
Data
CRC
IEEE Frame
LLC
Data
PAD
LLC: Link Logical Control
Data: Encapsulated information , (User data, protocol-ARP…)
PAD: Packet Assembly/De-assembly, to make the data field to a minimum of 38 bytes.

46. MAC Sub Layer 8 8 8 DSAP SSAP Control LLC Field
DSAP: Destination Service Access Point = 170 (0xaa)
SSAP: Source Service Access Point = 170 (0xaa)
170=SNAP=Network Layer is not determined (since it is determined in the type field)
IP was not defined to a have a code as part of the SAP definition. The Type defines
the protocol.
Control: 3, LLC frame type, Information Frame