4. MAC protocols and LANs - University of...
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Computer and Data Networks, 4. MAC protocols and LANs 1 ©Dr.Z.Sun
4. MAC protocols and LANs
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Computer and Data Networks, 4. MAC protocols and LANs 2 ©Dr.Z.Sun
Outline
� MAC protocols and sublayers, � LANs: Ethernet, Token ring and Token bus � Logic Link Control (LLC) sublayer protocol� Bridges: transparent (spanning tree), source routing and
remote bridges
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Computer and Data Networks, 4. MAC protocols and LANs 3 ©Dr.Z.Sun
Multiple Access Control (MAC) Protocols
� There are two categories of networks:• One uses point to point connections: computer dial-up
links, using SLIP, PPP, HDLC• The other uses broadcast channels: more than one
stations share the same channel, such as LANs using random access or token
� MAC protocols is used to determine which station goes next to access the shared channels
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Computer and Data Networks, 4. MAC protocols and LANs 4 ©Dr.Z.Sun
Random Access: Aloha and Slotted Aloha
• User transmit whenever they have data
• Listen to the channel to see if the frame is OK
• Contention system
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Computer and Data Networks, 4. MAC protocols and LANs 5 ©Dr.Z.Sun
Channel efficiency
� Throughput S = GP0
� Poisson distribution:P[k] = Gk e-G / k!
In 2 frame interval, the number of frames generated is 2G, thus P0 = e -2G
=> S = G e -2G
� Max. throughputS = 1/2e, when G=1/2
� For slotted Alohathe vulnerable period is 1 frame period
(halved), thus P0 = e -G
=> S = G e -G
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Computer and Data Networks, 4. MAC protocols and LANs 6 ©Dr.Z.Sun
Carrier Sense Multiple Access (CSMA)
� 1-persistent: the station listens before sending. if the channel is busy, it waits until it idle. Transmit when the channel is idle. if collision, the station waits a random amount of time and start all over again
� non-persistent: If busy, the station does not continually sense. Instead, waiting for a random period, then repeating the algorithm
� p-persistent: It applies to slotted channel. If it is idle, it transmits with probability of p.
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Computer and Data Networks, 4. MAC protocols and LANs 7 ©Dr.Z.Sun
CSMA/CD
� Further improvement than persistent and non-persistent over Aloha, by aborting transmission as soon as stations detect a collision
� Contention period is 2τ where τ is propagation delay
� Example: for a 1 km cable, the τ is about 5 µseconds
� Ethernet is one of this version� No MAC-sublayer protocol
guarantees reliable delivery
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Computer and Data Networks, 4. MAC protocols and LANs 8 ©Dr.Z.Sun
Collision free protocols
� A bit map protocol: it is also called reservation protocol. Each contention period consists of exactly N slots for N stations. Efficiency = d/(d+1)
� Binary countdown: each station has a binary address, start to broadcast with the high order bit. It stops as soon as a high order position 0 is overwritten with a 1. Efficiency = d/(d+log2N)
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Computer and Data Networks, 4. MAC protocols and LANs 9 ©Dr.Z.Sun
Limited contention protocols
� Two important performance measures: delay at low load and channel efficiency at high load. At low load, contention is preferable due to low delay. At high load, reservation is preferable due to high efficiency.
� The adaptive tree walk protocol: dynamically allocate time slots. If a collision occurs during slot 1, the entire tree is searched, depth first to allocate all the ready stations.
1/e = 0.368
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Computer and Data Networks, 4. MAC protocols and LANs 10 ©Dr.Z.Sun
IEEE 802.3 and Ethernet
� Bus Topology � Whole family of 1-persistent
CSMA/CD� From 1 - 10 Mbit/s on different
media� Switched Ethernet
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Computer and Data Networks, 4. MAC protocols and LANs 11 ©Dr.Z.Sun
Ethernet MAC Frame
For a 10-Mbit/s LAN� Max. 2500 meters and 4 repeaters (500 m/segment)� Min. allowed frame must take 51.2 µ seconds
(corresponds to 64 bytes)If operating at 1 Gbit/s� Max. 250 meters, and Min. frame size 640 bytes
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Computer and Data Networks, 4. MAC protocols and LANs 12 ©Dr.Z.Sun
IEEE 802.5 Token Ring
• Ring topology, suitable for real time• Token holding time is 10 ms• Speed 1 and 4 Mbit/s• Delimiter, access control, frame
control • sources and destination address,
checksum are the same as the 802.3
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Computer and Data Networks, 4. MAC protocols and LANs 13 ©Dr.Z.Sun
IEEE 802.4 Token Bus
• Logical ring, suitable for real time• For priorities: 0, 2, 4, 6• Speed 1, 5, and 10 Mbit/s• Preamble, delimiter, control, • Sources and destination address,
checksum are the same as the 802.3
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Computer and Data Networks, 4. MAC protocols and LANs 14 ©Dr.Z.Sun
IEEE 802.2 Logical link control
� Hide difference between the various 802 networks by providing a single format and interface to the network layer
� Based on HDLC, provide 3 service options as the link layer
� Error control using acknowledgment
� Flow control using a slide window
� All 802 LANs and MAN offer best-efforts service
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Bridges
• Why using bridges?1. Different department
have different LANs initially;
2. geographical separated; 3. Accommodate the load; 4. Physical distance limit; 5. Reliability, 6. Security
• A bridge connecting k different LANs will have k different MAC layers and k different physical layers, one for each type.
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Bridges from 802.x to 802.y
Operation of a LAN bridge from 802.11 to 802.3.
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Computer and Data Networks, 4. MAC protocols and LANs 17 ©Dr.Z.Sun
Bridges from 802.x to 802.y (frame format)
• Problems with bridging different LANs:1. Different frame format, 2. Different rate, 3. Different maximum frame length
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Computer and Data Networks, 4. MAC protocols and LANs 18 ©Dr.Z.Sun
Transparent (spanning tree) bridgeForwarding (Filtering)• If destination & source address is the
same, discard the frame• If different, forward the frame• Destination unknown, use flooding• Spanning tree solve loop problemAddress learning• Initialise the forwarding database to
empty • Update the entry in the routing table
(make one if it does not exit) with the frame address and arrival time
• Periodically scan the routing table and purge all old entries (more than a few minutes.
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Computer and Data Networks, 4. MAC protocols and LANs 19 ©Dr.Z.Sun
Spanning tree algorithm
� Exchange bridge protocol data units (BPDUs) to elect root bridge with highest priority and smallest bridge identifier
� Each bridge selects root portwith minimum root path cost(RPC) from the root to the port
� Bridges connected to the same segment elect a designated bridge with a port having minimum path cost from the port to the root
� Port identifiers are used as tie-breakers
Root bridge
RPC = 1, 4 RPC = 2, 5
X
X
X
RPC = 1, 4 RPC = 2, 3 RPC = 3, 4RPC = 2, 3
RPC = 2, 5
RPC = 3, 4
RPC = 4,3,4
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Computer and Data Networks, 4. MAC protocols and LANs 20 ©Dr.Z.Sun
Source routeing bridges
� Transparent bridges use only subset of the topology (the tree)
� Source routeing assumes that the sender knows whether or not the destination is on its own LAN.
� Implicit in that every machine can find the best path to the other machine
� Discovery frame flooding is used if destination unknown
� Frame explosion is solved by flooding along the spanning tree
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Computer and Data Networks, 4. MAC protocols and LANs 21 ©Dr.Z.Sun
Comparison of 802 bridges
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Remote bridges
� It connect two or more distant LANs
� Put a bridge in each LAN and connect the bridge pairwise with point to point lines (such as leased telephone line)
� Various protocols can be used on the point to point lines (such as data link protocol) putting complete MAC frame in the payload
� Or strip off the MAC header and trailer at the sources and put it back at the destination
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Repeaters, Hubs, Bridges and Switches (1/2)
(a) Which device is in which layer.(b) Frames, packets, and headers.
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Computer and Data Networks, 4. MAC protocols and LANs 24 ©Dr.Z.Sun
Repeaters, Hubs, Bridges, Switches (2/2)
(a) A hub. (b) A bridge. (c) a switch.
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Virtual LANs
(a) Four physical LANs organized into two VLANs, by two bridges. (b) The same LANs organized into two VLANs by switches.
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The IEEE 802.1Q Standard
Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.
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The IEEE 802.1Q Standard (2)
The 802.3 (legacy) and 802.1Q Ethernet frame formats.
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Computer and Data Networks, 4. MAC protocols and LANs 28 ©Dr.Z.Sun
Summary
� MAC protocols and sublayers, � LANs:
• Ethernet, • Token ring• Token bus
� Logic Link Control (LLC)� LAN interconnections:
• Transparent (spanning tree) bridge• Source routing bridge• VLAN