Mac
-
Upload
mahesh-attri -
Category
Education
-
view
1.233 -
download
2
description
Transcript of Mac
1
What are going to be discussed Introduction to broadcast networks Overview of LANs: frame format &
placement in OSI. Random access: ALOHA & CSMA-CD
(Carrier Sensing Multiple Access with Collision Detection ) i.e., Ethernet.
Scheduling: token-ring. LAN standards (brief view) LAN bridges: used to connect several
LANs.
OSIApplication
PresentationSession
Transport
Network
Data Link
Physical
Framing
Error control
Flow control
Transmission/reception of frames
MEDIA ACCESS sublayer
LOGICAL LINK sublayer
3
LAN & MAC (Medium Access Control) protocols
Two basic types of networks: Switched networks: transmission lines,
multiplexers, and switches; routing. Broadcast networks: a single shared medium,
simpler, no routing, messages received by all stations, flat address; however, when users try to transmit messages into the medium, potential conflict, so MAC is needed to orchestrate the transmission from various users.
LAN is a typical broadcast network.
Single communication channel that is shared by all the machines on the network
Broadcast Networks
1 2 3 4 5
computer
cable
Packets Short messages sent by any machine are received by all others
Fields
Address
General Rule: Smaller, geographically localized networks
Quick Review…
Packets 1 2 3 4 5
ALL machines receive it, but one processes it
Also possible to address a packet to ALL machines (special code in the address field)
Mode of operation: Broadcasting
Also possible to address a packet to a SUBSET of machines(group number code in the address field)
Mode of operation: MulticastingQuick Review…
3
BROADCAST NETWORKS AND THEIR PROTOCOLS
The Medium Access Sublayer
deals with
12
34
5M
Shared MultipleAccess Medium
1. Any transmission from any station can be heard by any other stations
2. If two or more stations transmit at the same time, collision occurs
Multiple access communications
Satellite Channel = fin
= fout
Satellite communication involves sharing of uplink and downlink frequency bands
9
Multidrop telephone lines
Inbound line
Outbound line
Figure 6.4
Multi-drop telephone line requires access control
Host
Terminals
10
Ring networks
Multitapped Bus
Figure 6.5
Ring networks and multi-tapped buses require MAC
11Figure 6.6
Wireless LAN: share wireless medium and require MAC
12
Medium Sharing Techniques
Static Channelization
Dynamic Medium Access Control
Scheduling Random Access
Figure 6.2
Approaches to sharing transmission medium
Partitioned channelsare dedicated to individual users, sono collision at all.Good for steady trafficand achieve efficient usage of channels
Good for bursty traffic.
Schedule a orderly accessof medium. Good for heaviertraffic.
Try and error. if no collision,that is good, otherwise wait a random time, try again. Good for light traffic.
The Channel Allocation Problem
How to allocate a single broadcast channel among competing users?
Static FDM /TDM (Frequency/Time Division Multiplexing)
FDM : Radio/TV broadcasts TDM : POTS (Plain Old Telephone System)
GSM uses both (Global System for Mobile Communications)
Dynamic Pure/ Slotted ALOHA Carrier Sense Multiple Access (CSMA) Protocols Collision free protocols
Dynamic Channel Allocation Technologies
1. Pure ALOHA
2. Slotted ALOHA
3. CSMA
4. CSMA/CD (old ETHERNET)
5. Switching (Fast ETHERNET)
6. Token passing (Token Ring )
ALOHA Protocols
Anyone may transmit whenever they want. (Continuous time model.)
Each radio detects collisions by listening to its own signal. A collision is detected when a sender doesn't receive the signal that it just sent.
After a collision, wait a random amount of time and transmit the same frame again. This technique is known as backoff.
Basic idea:
Pure ALOHA
A Shared Medium Collision Domain
Slotted ALOHA
Time is divided into slots… can only transmit at start of slot
Vulnerable period halved => max. eff is doubled
Requires sync of clocks
Still poor at hi-loads
Carrier Sense, Multiple Access (CSMA)
We can improve the performance of our simple network greatly if we introduce carrier sensing (CS). With carrier sensing, each host listens to the data being transmitted over the cable. A host will only transmit its own frames when it cannot hear
any data being transmitted by other hosts. When a frame finishes, an interframe gap of about 9.6sec
is allowed to pass before another host starts transmitting its frame.
Communication Link
Carrier Sense Multiple Access (CSMA)
Improves performance when higher medium utilisation When a node has data to transmit, the node first listens
to the cable (using a transceiver) to see if a carrier (signal) is being transmitted by another node.
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols.
CSMA with Collision Detection
CSMA/CD can be in one of three states: contention, transmission, or idle.
Wireless LAN Protocols
Wireless LAN. (a) A transmitting.
(b) B transmitting.
Wireless LAN Protocols
The MACA protocol. (a)
A sending an RTS to B.
(B responding with a CTS to A.
IEEE 802.3: CSMA/CD Bus LAN The 802.3 standard describes the operation of the MAC
sub-layer in a bus LAN that uses carrier sense, multiple access with collision detection (CSMA/CD). Beside carrier sensing, collision detection and the binary
exponential back-off algorithm, the standard also describes the format of the frames and the type of encoding used for transmitting frames.
The minimum length of frames can be varied from network to network. This is important because, depending on the size of the network, the frames must be of a suitable minimum length.
The standard also makes some suggestions about the type of cabling that should be used for CSMA/CD bus LANs.
The CSMA/CD Bus LAN is also widely called Ethernet.
Ethernet MAC Sublayer Protocol
Frame formats. (a) DIX Ethernet,
(b) IEEE 802.3.
IEEE 802.3: MAC Addresses Every network card in the world has a unique 46-bit serial
number called a MAC address. The IEEE allocates these numbers to network card manufacturers who encode them into the firmware of their cards. The destination and source address fields of the MAC
frame have 48 bits set aside (the standard also allows for 16-bit addresses but these are rarely used).
The most significant bit is set to 0 to indicate an ordinary address and 1 to indicate a group address (this is for multicasting, which means that frames are sent to several hosts). If all 48 bits are set to 1 then frames are broadcast to all the hosts.
If the two most significant bits are both zero then the 46 least significant bits contain the MAC addresses of the source and destination hosts.
IEEE 802.3: Minimum Frame Length
When a host transmits a frame, there is a small chance that a collision will occur. The first host to detect a collision transmits a 48-bit jam sequence.
To ensure that any hosts involved with the collision realise that the jam sequence is associate with their frame, they must still be transmitting when the jam sequence arrives. This means that the frame must be of a minimum length.
The worse case scenario is if the two hosts are at far ends of the cable. If host A’s frame is just reaching host B when it begins transmitting, host B will detect the collision first and send a jam signal back to host A.
CSMA/CD Minimum Ethernet Frame Size
To ensure that no node may completely receive a frame before the transmitting node has finished sending it, Ethernet defines a minimum frame size (i.e. no frame may have less than 46 bytes of payload).
The minimum frame size is related to the distance which the network spans, the type of media being used and the number of repeaters which the signal may have to pass through to reach the furthest part of the LAN.
Together these define a value known as the Ethernet Slot Time, corresponding to 512 bit times at 10 Mbps.
IEEE 802.3: Minimum Frame Length
The longest time between starting to transmit a frame and receiving the first bit of a jam sequence is twice the propagation delay from one end of the cable to the other.
This means that a frame must have enough bits to last twice the propagation delay.
The 802.3 CSMA/CD Bus LAN transmits data at the standard rate of r = 10Mbps.
The speed of signal propagation is about v = 2108m/s.
A BPacket starts attime 0
A BPacket at time tp-
A BCollision occurs
at time tp
Jam sequence
A BJam sequence getsback to A at 2tp
Jam sequence
(a)
(c)
(b)
(d)
IEEE 802.3: Minimum Frame Length
In order to calculate the minimum frame length, we must first work out the propagation delay from one end of the cable to the other.
IEEE 802.3: Minimum Frame Length
Propagation delay time:sec2102
102
400 68
V
dtprop
The round-trip propagation delay is, of course, twice this. Thus the round trip delay is
sec42 propt
With a data rate of MbpsR 10 each bit has
duration
sec1.0000,000,10
11 R
tb
Example #1: Cable = 400m, transm. speed = 10 Mbit/sec, propagation speed = 2*10**8 m/sec
IEEE 802.3: Minimum Frame Length
The number of bits we can fit into a round-trip propagation delay is
bitst
tn
b
pb 40
1.0
42
The minimum frame length is thus 40 bits (5 bytes).
A margin of error is usually added to this (often to make it a power of 2) so we might use 64 bits (8 bytes).
Example #1 – cont.
Two nodes are communicating using CSMA/CD protocol.
Speed transmission is 100 Mbits/sec and frame size is 1500 bytes. The propagation speed is 3*10**8 m/sec.
Calculate the distance between the nodes such that the
time to transmit the frame = time to recognize that the collision have occurred.4
6102.1
10100
81500
R
Lt frame
propframetripround ttT 2_
54
1062
102.1
2
frameprop
tt
V
dtprop
kmVtd prop 181018103106 385
EEE 802.3: Minimum Frame LengthExample # 2
IEEE 802.3: Minimum Frame Length
The standard frame length is at least 512 bits (64 bytes) long, which is much longer than our minimum requirement of 64 bits (8 bytes). We only have to start worrying when the LAN reaches
lengths of more than 2.5km.
802.3 CSMA/CD bus LANs longer than 500m are usually composed of multiple segments joined by in-line passive repeaters, which output on one cable the signals received on another cable. When we work out the minimum frame length for these
longer LANs, we also have to take the delays caused by the passive repeaters (about 2.5sec each) into account as well.
Shortest Ethernet Frame
64 bytes sent at 10Mbps 51.2sec 500m/segment, 4 repeaters between nodes 2500m 25
sec propagation delay The frame should be longer enough for sender to detect
the collision(2x25 or about 50 sec )
Why specify a shortest frame of 64byte?
Node A Node BR1 R2 R3 R4
500m 25 sec propagation delay
IEEE 802.3: Non-Deterministic The 802.3 CSMA/CD bus LAN is said to be a non-
deterministic network. This means that no host is guaranteed to be able to send its frame within a reasonable time (just a good probability of doing so). When the network is busy, the number of collisions rises
dramatically and it may become very difficult for any hosts to transmit their frames.
A real-time computing application (such as an assembly line) will demand that data is transmitted within a specified time period. Since the 802.3 bus LAN cannot guarantee this, its use for
real-time applications may not only be undesirable but potentially dangerous in some situations.
Ethernet Performance
Ethernet Physical Layer standards
10Base5 10 Mbps, Baseband transmission, 500m cable length
10Base2 10 Mbps, Baseband transmission, ~200m cable length
10Base-T 10 Mbps, Baseband transmission, UTP cable
100Base-TX 100 Mbps, Baseband transmission, UTP cable
Ethernet 10Base-T & 100Base-TX
Wiring
Unshielded Twisted Pair (UTP)
Category 5 wiring is best Cat 3 and Cat 4 in some older
installations
Bundle of eight wires (only uses four)
Terminates in RJ-45 connector
10Base-T & 100Base-TX hubs
UTP-based networks use hubs to interconnect NICs each UTP cable runs directly from a NIC to a hub
10Base-T & 100Base-TX hubsHubs have many ports, each of which has one incoming network cable
Hubs are usually located in computer rooms, or network distribution cupboards a patch panel (or patch bay) is used to connect
between hubs and the wall sockets throughout a building
10Base-T & 100Base-TX wiring Wiring
100 meters maximum distance hub-to-station
Can use multiple hubs (max 4) to increase the distance between any two stations
100 m 100 m
200 m
10Base-T to 100Base-TX
Upgrading from 10Base-T to 100Base-TX
Need new hub May have some 10 Mbps ports to handle 10Base-T
NICs May have autosensing 10/100 ports that handle either
Need new NICs Only for stations that need more speed
No need to rewire This would be expensive
Multiple Hubs in 10Base-T
Farthest stations in 10Base-T can be five segments (500 metres apart) 100 metres per segment
Separated by four hubs
100m
100m
100m
100m
100m
500m, 4 hubs
10Base-T hubs
Multiple Hubs in 100Base-TX Limit of Two Hubs in 100Base-TX
Must be within a few metres of each other Maximum span ~200 metres Shorter distance span than 10Base-T
100m
100m2 Co-located
Hubs
100Base-TXHubs
Latency and Congestion with hubs Ethernet is a shared media LAN
Only one station can transmit at a time Even in multi-hub LANs Others must wait This causes delay
One Station Sends
All OtherStationsMust Wait
Fast EthernetThe original fast Ethernet cabling.
Gigabit Ethernet Gigabit Ethernet cabling.
IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
Repeaters
Regenerate the signal Provide more flexibility in network design Extend the distance over which a signal may
travel down a cable Example Ethernet HUB
Ethernet Repeaters and Hubs
Connect together one or more Ethernet cable segments of any media type
If an Ethernet segment were allowed to exceed the maximum length or the maximum number of attached systems to the segment, the signal quality would deteriorate.
Ethernet Repeaters and Hubs Used between a pair of segments
Provide signal amplification and regeneration to restore a good signal level before sending it from one cable segment to another
Ethernet Bridge
Join two LAN segments (A,B), constructing a larger LAN
Filter traffic passing between the two LANs and may enforce a security policy separating different work groups located on each of the LANs.
Local Internetworking
A configuration with four LANs and two bridges.
Ethernet Bridges Simplest and most frequently used Transparent Bridge
(meaning that the nodes using a bridge are unaware of its presence).
Bridge could forward all frames, but then it would behave rather like a repeater
Bridges are smarter than repeaters!
Ethernet Bridges
A bridge stores the hardware addresses observed from frames received by each interface and uses this information to learn which frames need to be forwarded by the bridge.
Ethernet Switch Modern LANs
Fundamentally similar to a bridge Supports a larger number of connected LAN segments Richer management capability. Logically partition the traffic to travel only over the
network segments on the path between the source and the destination (reduces the wastage of bandwidth)
Ethernet Switch Benefits
Improved security users are less able to tap-in into other user's data
Better management control who receives what information (i.e. Virtual
LANs) limit the impact of network problems
Full duplex rather than half duplex required for shared access
Switched LAN• Hub and Switched LAN
– hub simulates a single shared medium– switch simulates a bridged LAN with one computer per
segment
Ethernet Switches
Highly Scalable10Base-T switches Competitive with 100Base-TX hubs in
both cost and throughput Increasingly used to desktops
100Base-TX switches Higher performance (and price)
Gigabit Ethernet switches Very expensive
Ethernet Switches No limit on number of Ethernet
switches between farthest stations So no distance
limit on size ofswitched networks
Ethernet Switches Ethernet Switches must be Arranged in a
Hierarchy (or daisy chain) Only one possible path between any two stations,
switches
4
5 6
2 3
1
Path=4,5,2,1,3
Repeaters, Hubs, Bridges, Switches, Routers and Gateways
(a) Which device is in which layer.
(b) Frames, packets, and headers.
Repeaters, Hubs, Bridges, Switches, Routers and Gateways
(a) A hub. (b) A bridge. (c) a switch.
Repeater HUBs
Switches
Switches
Repeater HUBs
Ethernet Switches and Multicast Traffic
Multicast Traffic from F is delivered to all output interfaces (ports) which asks for it
Virtual LANs (VLANs)
Cisco Systems
Cisco Systems
Virtual LANs (VLANs)
Cisco Systems
Virtual LANs (VLANs)
Cisco Systems
Switches Versus Routers
Switches Fast Inexpensive No benefits of alternative
routing No hierarchical
addressing
Routers Slow Expensive Benefits of alternative
routing Hierarchical addressing
“Switch where you can; route where you must”
Where Does Wireless RF Live?ISM Band: Industrial, Scientific, Medical
902-928 MHz 2400-2483.5 MHz 5725-5850 MHz
802.11/802.11b 802.11a
Bluetooth
Cordless Phones
Home RFBaby Monitors
Microwave Ovens
Old Wireless
IEEE 802.11 – Wireless Ethernet
Two configurations:
Ad-hoc. No central control, no
connection to the outside world
Infrastructure. Uses fixed network
Access Point to connect to the
outside world
IEEE 802.11 – Wireless Ethernet
Uses CSMA/CA protocol. CSMA part is
the same as in 802.3 Ethernet
CA stands for Collision Avoidance and works as follows:
If the carrier is present for a specific time period, transmitter sends a frame
If no collision receiver send ack
Transmitter can also reserve the channel by sending Request to Send (RTS)
IEEE 802.11 – Wireless Ethernet
IEEE 802.11 does not implement
Collision Detection because it cannot
detect collisions at the receiver end
(hidden terminal problem)
To avoid collisions the frame contains
field indicating the length of transmission
Other stations defer transmission
The 802.11 Protocol Stack
Where does 802.11 live in the OSI?
Telnet, FTP, Email, Web, etc.
IP, ICMP, IPX
TCP, UDP
Logical Link Control - 802.2(Interface to the upper layer protocols)
MAC
802.3, 802.5, 802.11
LAN: 10BaseT, 10Base2, 10BaseFLWLAN: FHSS, DSSS, IR
Application
PresentationSession
Transport
Network
Data Link
Physical
Wireless lives at Layers 1 & 2 only!
The 802.11 MAC Sublayer Protocol
(a) The hidden station problem.
(b) The exposed station problem.
CSMA-CA + Acknowledgement
Carrier Sense Multiple Access with Collision Avoidance
• Device wanting to transmit senses the medium (Air)
• If medium is busy - defers• If medium is free for certain period (DIFS) - transmits frame
How CSMA-CA works:
Latency can increase if “air” is very busy! Device has hard time finding “open air” to send frame!
* DIFS - Distributed Inter-Frame Space *(approx 128 µs)
The 802.11 MAC Sublayer Protocol
The use of virtual channel sensing using CSMA/CA.
The 802.11 Frame Structure
The 802.11 data frame.
Summary
IEEE 802.11b (WiFi) is a wireless LAN technology that is rapidly growing in popularity
Convenient, inexpensive, easy to use Growing number of “hot spots”
everywhere airports, hotels, bookstores,
Starbucks, etc Estimates: 70% of WLANs are
insecure!
IEEE 802.5 and Token Ring
FDDI Fiber Distributed Data Interface
– data rate 100Mbps, use as a backbone– With multi-mode fiber any given ring segment can be up
to 200 km in length. A total of 500 stations can be connected with a maximum separation of 2 km.
– two complete rings to overcome failures
High Speed LANs
• FDDI: Fiber Distributed Data Interface• 100Mbps, distance up to 200km, 100 hosts mainly used
as a backbone
1000900800700600500400300200100
0
Mbps
FE
EthernetEthernetEthernetEthernet
Gigabit Ethernet(Switched)
ATM OC-12(Switched)
ATM OC-3(Switched)
Fast Ethernet(Switched)
FDDI(Switched)
Token Ring(Switched)
Ethernet(Switched)
Switched LAN Type
Bandwidth Scaling
Cisco Systems