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Computer and Data Networks, 4. MAC protocols and LANs 1 ©Dr.Z.Sun
4. MAC protocols and LANs
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
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
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
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
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.
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
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)
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
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
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
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
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
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
Computer and Data Networks, 4. MAC protocols and LANs 15 ©Dr.Z.Sun
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.
Computer and Data Networks, 4. MAC protocols and LANs 16 ©Dr.Z.Sun
Bridges from 802.x to 802.y
Operation of a LAN bridge from 802.11 to 802.3.
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
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.
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
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
Computer and Data Networks, 4. MAC protocols and LANs 21 ©Dr.Z.Sun
Comparison of 802 bridges
Computer and Data Networks, 4. MAC protocols and LANs 22 ©Dr.Z.Sun
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
Computer and Data Networks, 4. MAC protocols and LANs 23 ©Dr.Z.Sun
Repeaters, Hubs, Bridges and Switches (1/2)
(a) Which device is in which layer.(b) Frames, packets, and headers.
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.
Computer and Data Networks, 4. MAC protocols and LANs 25 ©Dr.Z.Sun
Virtual LANs
(a) Four physical LANs organized into two VLANs, by two bridges. (b) The same LANs organized into two VLANs by switches.
Computer and Data Networks, 4. MAC protocols and LANs 26 ©Dr.Z.Sun
The IEEE 802.1Q Standard
Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.
Computer and Data Networks, 4. MAC protocols and LANs 27 ©Dr.Z.Sun
The IEEE 802.1Q Standard (2)
The 802.3 (legacy) and 802.1Q Ethernet frame formats.
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