Ch 4- Medium Access Control sub layer

• Channel Allocation problem
• Media Access Protocol – ALOHA
• CSMA, CSMA/CD Protocols
• Collision free protocols
• Contention protocols, MACAW (RTS,CTS)
• IEEE 802 Subgroups and Responsibilities
• Ethernet- types of Ethernet
• Bus-token ring,
• DQDB
• FDDI, Bridges

Computer Networks

IEEE 802 Subgroups and their Responsibilities

• 802.1 Internetworking
• 802.2 Logical Link Control (LLC)
• 802.3 CSMA/CD
• 802.4 Token Bus LAN
• 802.5 Token Ring LAN
• 802.6 Metropolitan Area Network
• 802.7 Broadband Technical Advisory Group
• 802.8 Fiber-Optic Technical Advisory Group
• 802.9 Integrated Voice/Data Networks
• 802.10 Network Security
• 802.11 Wireless Networks
• 802.12 Demand Priority Access LANs
• 802.15 Bluetooth Networks
• 802.16 Wireless MAN

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Ethernet

• IEEE has standardized a number of LANs and MANs under the name of IEEE 802. A few have survived but many have not.
• The most important of the survivors are 802.3 (Ethernet) and 802.11 (wireless LAN)
• For 802.15 (Bluetooth) and 802.16 (wireless MAN), it is too early to tell.
• Both 802.3 and 802.11 have different physical layers and different MAC sublayers but converge on the same logical link control sublayer (defined in 802.2), so they have the same interface to the network layer.

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Ethernet Cabling

Ethernet Topologies

Ethernet Connectivity

10Base5 – ThickNet

< 500m

Controller

Vampire Tap

Transceiver

Bus Topology

Ethernet Connectivity

10Base2 – ThinNet

< 200m

185 mtr

Controller

BNC T-Junction

Transceiver

Bus Topology

Ethernet Connectivity

10BaseT

< 100m

Controller

Star Topology

10Base F

< 2000 mtr & more.

1024 nodes can connect

Example Networks. Cont..

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Ethernet LAN

-Co-axial cable- up to 2.5 km

repeater at every 500 meters

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-max. 256 machines connected via Transceiver

-multi drop cable used

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- speed 2.94 Mbps

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Ethernet card, cable, connector, & network

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Classic Ethernet

“dominant” wired LAN technology:

• cheap $20 for NIC
• first widely used LAN technology
• simpler, cheaper than token LANs and ATM
• kept up with speed race: 10 Mbps – 10 Gbps

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Metcalfe’s Ethernet

sketch

Ethernet -Star topology

• bus topology popular through mid 90s
• all nodes in same collision domain (can collide with each other)
• today: star topology prevails
• active switch in center
• each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)

switch

bus: coaxial cable

star

Ethernet Frame Structure

• Addresses: 6 bytes
• if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol
• otherwise, adapter discards frame
• Type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk)
• CRC: checked at receiver, if error is detected, frame is dropped

802.3 Ethernet Standards: Link & Physical Layers

• many different Ethernet standards
• common MAC protocol and frame format
• different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps
• different physical layer media: fiber, cable

MAC protocol

and frame format

100BASE-TX

100BASE-T4

100BASE-FX

100BASE-T2

100BASE-SX

100BASE-BX

CSMA - Ethernet

• Ethernet uses following mechanisms to handle collisions:
• Carrier sense
• Collision detection

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Ethernet Technologies: 10Base2

• 10: 10Mbps; 2: under 200 meters max cable length
• thin coaxial cable in a bus topology

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• repeaters used to connect up to multiple segments
• repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!
• has become a legacy technology

10BaseT and 100BaseT

• 10/100 Mbps rate; latter called “fast ethernet”
• T stands for Twisted Pair
• Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub

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• Hubs are essentially physical-layer repeaters:
• bits coming in one link go out all other links
• no frame buffering
• no CSMA/CD at hub: adapters detect collisions
• provides net management functionality

Manchester encoding

• Used in 10BaseT, 10Base2
• Each bit has a transition
• Allows clocks in sending and receiving nodes to synchronize to each other
• no need for a centralized, global clock among nodes!
• Hey, this is physical-layer stuff!

Gbit Ethernet

• use standard Ethernet frame format
• allows for point-to-point links and shared broadcast channels
• in shared mode, CSMA/CD is used; short distances between nodes to be efficient
• uses hubs, called here “Buffered Distributors”
• Full-Duplex at 1 Gbps for point-to-point links
• 10 Gbps now !

Token Ring – IEEE 802.5

• A ring toplogy network developed in the late 1960s. Supported mainly by IBM.�Pushed into the background by Ethernet in the 1990s.
• a LAN protocol which resides at the data link layer (DLL) of the OSI model.

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Token Bus – IEEE 802.4

• A network which implements the modified Token Ring protocol over a "virtual ring" on a coaxial cable with a bus topology.
• It is mainly used for industrial applications (GM^®).

DQDB

• Distributed queue dual Bus
• IEEE 802.3,5 LAN
• IEEE 802.6 ? (MAN)
• Cover entire city(up to 16 km at 44 mbps)
• Data stream 53 byte cell (5 header+ 48 payload)
• Each Cell bit (B,R)

if cell occupied it’s B bit is 1

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CH 4

DQDB Architecture

CH 4

• Each bus supports traffic in only one direction
• Beginning of bus is denoted by a square .
• Bus B traffic moves from right to left and Bus A traffic from left to right
• Each bus connects to stations directly through input and output ports

What is DQDB?

• Distributed Queue Dual Bus
• Protocol for MAN
• DQDB is an IEEE standard: 802.6
• Designed for both voice & video
• Topology used: Dual Bus - uses 2 unidirectional logical buses
• Extend up to 30 miles at 34-55 Mbps
• Uses optical fibre links
• Queued-packet distributed switch (QPSX) algo.

CH 4

About DQDB?

• Works on Data-link layer

(specially in MAC sub-layer)

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• Used in data, voice and video transmissions

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• Based on Cell Relay Technology (like ATM)
• Provides
• connection-oriented services
• connection less services
• asynchronous services

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CH 4

Technical fact and Medium

• Medium: Copper or Fiber

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• up to 160 KM approx speed is 44 Mbps (Copper)

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• up to 100 KM approx speed is 150 Mbps (Fiber)

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• Transmission Rate: 34 Mbps to 150 Mbps

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• Working: head end generate fixed size cells in both direction (Cell Generator)

CH 4

DQDB

• User’s queue up, they became ready to send & transmit in FIFO order
• Achieved without central queue control called distributed queue
• MAC Protocol:
• Be polite to your data stream station,

& let them have to go first,

if they requested before yours.

CH 4

FDDI: [Fiber Distributed Data Interface]

• FDDI (Fiber Distributed Data Interface) is a set of ANSIFDDI (Fiber Distributed Data Interface) is a set of ANSI and ISOFDDI (Fiber Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber optic lines in a local area network (LAN) that can extend in range up to 200 km.
• The FDDI protocol is based on the token ring protocol. In addition to being large geographically.
• FDDI local area network can support thousands of users. FDDI is frequently used on the backboneFDDI local area network can support thousands of users. FDDI is frequently used on the backbone for a wide area network (WAN).

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Computer Networks

FDDI: [Fiber Distributed Data Interface]

• FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km.

Computer Networks

FDDI: [Fiber Distributed Data Interface]

Summery:

• 100 Mbps Dual ring
• Multiple-token
• Self-Healing Ring
• Target Token Rotation Time (TTRT)
• 4B/5B Encoder

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Computer Networks

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End of Part-4 (Ch-4 MAC layer)

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Thanks.

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