Medium Access Control

Medium Access Control Sublayer

• Networks are divided into point-to-point links or broadcast channels
• With broadcast networks, the core challenge is to determine channel usage during competition

Static Channel Allocation

• In Frequency Division Multiplexing (FDM) with N users, bandwidth is divided into N equal bands, each assigned to a user
• FDM works when the user base N is small, constant with a high traffic load per user
• FDM demonstrates inefficiency with larger user numbers because when the spectrum is divided into N bands, and there are more than N users seeking communication, some are excluded even if assigned users are not transmitting
• Even holding the user number constant at N, FDM proves inefficient, as idle users' frequency bands are wasted
• Time Division Multiplexing (TDM) divides time into N slots, assigning each to a user
• TDM is also inefficient if a user has nothing to transmit during their assigned slot
• A solution exists in Dynamic Channel Allocation through methods such as ALOHA and CSMA

ALOHA

• Developed by Norman Abramson and colleagues at the University of Hawaii
• There are two versions of ALOHA: pure and slotted

Pure ALOHA

• Users transmit whenever they have a frame to send
• On collision, the sender waits randomly before retransmission
• Collisions can be detected by listening to the channel or by using acknowledgement

Performance of pure ALOHA

• Offered load (G) is the average number of generated frames (new and retransmissions) per frame time
• Throughput(S) is the average number of frames successfully delivered per frame time
• Throughput in relation to offered load is S = Ge−2G
• The maximum throughput occurs at G = 0.5, with S = approximately 0.184
• Consequently, the best channel utilization achievable with ALOHA is 18%

Slotted ALOHA

• Time is divided into discrete slots
• Users are only allowed to transmit at the beginning of a time slot
• This is the sender algorithm for slotted ALOHA:
  • First, when a user has a frame to send, it waits until the start of the next time slot
  • Next, If there is a collision, the user will wait a random amount of time before sending the same frame again
• Throughput where S = Ge-G, for slotted ALHOA is better than Pure ALOHA
• Maximum throughput happens when G = 1, with S = around 0.368, doubling pure ALOHA's capacity

Carrier Sense Multiple Access (CSMA)

• Best channel utilization that can be achieved with ALOHA is 36%
• This is because users transmit without considering the actions of others
• An improvement is achieved by listening for ongoing transmissions for ongoing transmissions and acting accordingly

1-persistent CSMA

• A station with data first listens to the channel to see if anyone is transmitting:
  • If the channel is idle, the station transmits
  • If the channel is busy, the station waits until the channel is idle, then transmits
• If there is a collision, the station waits a random amount of time and iterates again
• Intuitively, this protocol performs better than ALOHA
• Collisions occur if 2 stations are waiting and both see the channel become idle at the same time

Nonpersistent CSMA

• A station with data first listens to the channel
  • When the channel is idle, the station transmits
  • When the channel is busy, the station does not continuously sense, but waits a random period and repeats the process
• This method results in better channel use but longer delays than 1-persistent CSMA

P-persistent CSMA

• Applies to slotted channels and involves the station sensing the channel when it becomes ready
• If the channel is idle, the station transmits with probability p or defers to the next slot with probability (1-p), repeating until transmission or another station transmits
• If the channel is busy, the station waits until the next time slot before retrying

CSMA/CD (Collision Detection)

• In CSMA, stations may transmit simultaneously, so an advancement is to employ:
  • Collision detection, which causes a station to sense the channel while transmitting and stop transmitting as soon as congestion is detected

Wireless LAN protocols

• Mobile devices can communicate using a shared radio channel
• the configurations is known as wireless LAN
• Two users can communicate within radio range of each other
• Collision is when a receiver is in range of 2 active transmitters

The hidden station problem

• CSMA cannot be used in wireless LANs
• A and B are within each other range, and they can potentially interfere with one another.
• C can also interfere with B and D, but not with A.
• A is transmitting to B.
• If C senses the medium but cannot hear A as A is out of range, and C may falsely conclude it can transmit to B, but Collision will transpire.
• The problem of a station not being able to detect a competitor for the medium because they are too far away is called the hidden station problem

The exposed station problem

• Consider a reverse situation in which B is transmitting to A
• If C senses the medium and hears a transmission and falsely concludes it cannot send to D.
• A transfer, however, from C to D wont interfere with the transmission from B to A, because A wont sense transission from C direction
• In Wireless LAN what matters is the interference at the receiver, not the sender

Collision Avoidance (MACA)

• Transmitting station A first sends a RTS (request to send) frame to the destination station B:
  • This frame tells the length of the data frame that will be sent soon
  • B replies with a CTS(clear to send frame that contains the length of data copied from the RTS frame)
  • Once A gets CTS framne it begins transmisson.
• If a station hears the RTS is near station A therefore, must remain quite enough for CTS to be transferred back to A without conflict
• Should any station hear the CTS is likely close to be and must remain quite throughout data transmission, it could obtain data transmission length to examined frame, length

CSMA/CA collision avoidance

• If a station hears an RTS but not a CTS, then it is near the sender but far from the receiver, so it will not collide with the CTS, and can transmit itself

Example use of (MACA)

• C is close to station A but is not in the range of station B, so C only hears RTS but doesn’t hear CTS. Therefore C transmit.
• D is close to station B, yet is not in range of Station A. Thus D will hear CTS with nothing else.
• E is close to both A and B, so E is unable to transmit

Collision still can happen

• Station B and Station C could both transmit at same time, thus be wasted.
• The transmitter will be aware of the RTS if it doesn’t get a CTS.
• If there is collision, the transmitter waits random amounts of time, and will keep trying again

Ethernet

• Ethernet (now IEEE 802.3 standard) employs CSMA
• Ethernet was introduced and it is an important networking technology

Classic Ethernet

• In Classic Ethernet (1978 standardized a medium of coaxial cable (the ether, up to 2.5 km long, with repeaters every 500 m.
• up to 256 machines connected
• Transmission rate 2.94 Mbps
• In 1978 Dec, intel and Xerox standardized 10 m bps Ethernet. Now 100 m bps and 1 gbps Ethernets exist.

Switched Ethernet

• Can Be built by using hubs or switches

Repeaters

• Are analog devices.
• Connect two cable segments, a signal appearing on one cable is amplified and put out on the other one.
• Are physical layer devices, meaning they do not look at any address.
• They Dont understand frames, packets, or headers, or volts

Hubs

• Each machine is connected to hub by a separate twisted pair cable.
• Hubs all have lines are sent out without buffering or looking at the address
• If two frames arrive at the , the entire hub forms a single collision domain

Switches

• Switches: where frame coming on port line are forwarded to the right output port, by reading the MAC addrrss in the Ethernet frame header, where to send the fram.
• Bridges switches used to connect several LANS (Ethernets, and create expandedlans

Ethernet Frame format

• The two frame format styles are:
  • (a)DIX (DEC, Intel, Xerox) Ethernet.
  • (b) IEEE 802.3.

DIX Ethernet frame format

• Each element listed below in order as it appears:
  • Preamble Is 8 bytes. The pattern 10101010 (repeated)8 times, used for extracting clock with manychester coding
  • Source and destination Addressed 48 bit data link layer MAC address. Any Ethernet board has 48 MAC address:
  • Type tells the receiver which net work protocol to hand frame over to:
  • Data is up to 1500 bytes of data:
  • The pad is when frames with fewer that sixty four are packed out. It does not
  • checksum is for eroor detection.

Bridges and LAN switches

• Switches can be used to forward packets between shared-media LANs such as Ethernets.
• These switches are LAN switches, sometimes referred to bridges:
• If the frame arrives at port it will be sent through other ports:

Learning bridges

• A gets sent port , but does not forward a frame to other lan:
• The forwarding able determines which host is on each port, and the destination and the forwarding table determine whether the frame is forwarded:

Learning bridge (created dynamically by the bridge)

• When the bridge boots, the table is empty:
• Bridge table includes source of all frames.
• If a frame is received at port, the , bridge will make the first entry of the table

Learning bridges (contd)

• With learning bridges is that there is a, every associated entry has a timeout, after a timeout the entry gets removed
• The table is not always completed
• The complete table is not as important as the operations: