Data Communications LAN Systems. ALOHA Packet Radio When station has frame, it sends Station listens (for max round trip time)plus small increment If.

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Presentation transcript:

Data Communications LAN Systems

ALOHA Packet Radio When station has frame, it sends Station listens (for max round trip time)plus small increment If ACK, fine. If not, retransmit If no ACK after repeated transmissions, give up Frame check sequence (as in HDLC) If frame OK and address matches receiver, send ACK Frame may be damaged by noise or by another station transmitting at the same time (collision) Any overlap of frames causes collision Max utilization 18%

Slotted ALOHA Time in uniform slots equal to frame transmission time Need central clock (or other sync mechanism) Transmission begins at slot boundary Frames either miss or overlap totally Max utilization 37%

CSMA Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Wait reasonable time (round trip plus ACK contention) No ACK then retransmit Max utilization depends on propagation time (medium length) and frame length Longer frame and shorter propagation gives better utilization

If Busy? If medium is idle, transmit If busy, listen for idle then transmit immediately If two stations are waiting, collision (1-persistent algorithm) With non-persistent algorithm, if medium is busy, go away and come back later P-persistent algorithm – if medium busy, wait; when idle, transmit with probability p (backoff with probability 1-p); p = 0.1 or 0.01

CSMA/CD With CSMA, collision occupies medium for duration of transmission Stations listen while transmitting If medium idle, transmit If busy, listen for idle, then transmit If collision detected, jam then cease transmission After jam, wait random time then start again Binary exponential back off

Collision Detection On baseband bus, collision produces much higher signal voltage than signal Collision detected if cable signal greater than single station signal Signal attenuated over distance For twisted pair (star-wired bus) activity on more than one port is collision

IEEE Frame Format

Ethernet (CSMA/CD) Carrier Sense Multiple Access with Collision Detection Xerox created Ethernet back in the 70s IEEE became the standard and Ethernet are not exactly the same

10Mbps Specification (Ethernet) 10Base510Base210Base-T10Base-FP Medium:CoaxialCoaxialUTP850nm fiber Signaling:BasebandBasebandBasebandManchester ManchesterManchesterManchesterOn/Off Topology:BusBusStar-wired busStar-wrd bus Nodes:10030<10033

100Mbps (Fast Ethernet) 100Base-TX 100Base-FX100Base-T4 2 pair, Cat 5 UTP 2 optical fiber4 pair, cat 3,4,5 4B5B, MLT-3 4B5B, NRZI8B6T, NRZ Also known as IEEE 802.3u. 100Base-TX most popular, uses same RJ-45 connectors and pin configuration as 10BaseT.

100Mbps (Fast Ethernet) EIA/TIA recommends 90m between end-points (to allow for 10m in jumper cables) with 100Base- TX. 100Base-FX can have 412m segments using 62.5/125 micron fiber.

1000 Mbps (Gigabit) Ethernet IEEE 802.3z standard for fiber, 802.3ab for twisted pair Data transfers over cables at 1 billion bps In 1998, prices per port for 10Mbps - $212; 100Mbps - $432, 1000Mbps - $2,200. In 2003, prices per port for 10Mbps - $12; 100Mbps - $35; 1000Mbps - $100.

Gigabit Ethernet - Potential Gbps throughput across network backbone between switches Gbps throughput between backbone switches and file servers Improved workstation and client/server application performance Gbps throughput across the client end of a small business network

Gigabit Ethernet – Potential Drawbacks Different patch cable formats may cause some interconnection difficulties Network devices that process TCP/IP packets in software as opposed to in chips may create bottleneck situations Gigabit Ethernet not supported everywhere yet Need Cat 5e or better NICs need to be PCI-X or 64-bit bus architecture

Gigabit Ethernet Configuration

Gigabit Ethernet - Physical 1000Base-SX Short wavelength, multimode fiber 1000Base-LX Long wavelength, Multi or single mode fiber 1000Base-CX Copper jumpers <25m, shielded twisted pair 1000Base-T 4 pairs, cat 5/5e UTP Signaling - 8B/10B

Gigabit Ethernet – Other Terms Backpressure – a flow-control technique that avoids frame loss by impeding external traffic from sending frames to congested interfaces GBIC (gigabit interface converter) – type of hot- swappable, standards-based transceiver used with switches. It converts electrical signals to optical signals and vice versa, and is usually used for connections on or to the backbone. VLAN – Virtual LAN – logical grouping of nodes regardless of their physical location

Gigabit Ethernet – Other Terms GVRP (Generic VLAN Registration Protocol) – A specification defining parameters that switches use to exchange information for registering VLANs on a Spanning-Tree network and to facilitate direct communication With GVRP, the switch can exchange VLAN configuration information with other GVRP switches, prune unnecessary broadcast and unknown unicast traffic, and dynamically create and manage VLANs on switches connected through 802.1Q trunk ports.

Gigabit Ethernet – Other Terms IEEE 802.1p – A standard that provides quality of service in Ethernet networks. It relies on packet tags and allows switches to transmit packets in order of priority. These tags are in the 802.1q header. Possible priorities are 7 – network control; 6 – voice; 5 – video; 0 – best effort

Gigabit Ethernet – Other Terms IEEE 802.1q – A standard that defines Ethernet frame tags that carry VLAN identifiers The tag is inserted into the CSMA/CD frame between the SA and Ethertype/Length fields Tag is 4 bytes – first 2 bytes is Tag Protocol Identifier. Next 3 bits is Priority field (802.1p). Next bit is Canonical Format Indicator (used in Token Ring and FDDI). Final 12 bits are VLAN ID.

Gigabit Ethernet – Other Terms IEEE 802.3x – A standard that defines Ethernet frame start and stop requests and timers on full- duplex links; flow control dialog Port mirroring – A transmission method in which frames transmitted and received on one port can be duplicated on another port for diagnostic purposes Port trunking – A link aggregation technique that creates a single, high-speed logical link from several lower-speed physical links

Token Ring (802.5) MAC protocol Small frame (token) circulates when idle Station waits for token Changes one bit in token to make it SOF for data frame Append rest of data frame Frame makes round trip and is absorbed by transmitting station Station then inserts new token when transmission has finished and leading edge of returning frame arrives Under light loads, some inefficiency Under heavy loads, round robin

Token Ring Operation

Token Ring MAC Frame

Priority Scheme

802.5 Physical Layer Data Rate Medium UTP,STP,Fiber Signaling Differential Manchester Max Frame Access ControlTP or DTRTP or DTRDTR Note: 1Gbit in development (yeah, right)

FDDI 100Mbps LAN and MAN applications Token Ring on steroids

FDDI MAC Frame Format

FDDI MAC Protocol Same as except: Station seizes token by aborting token transmission Once token captured, one or more data frames transmitted New token released as soon as transmission finished (early token release in 802.5)

FDDI Operation

FDDI Physical Layer MediumOptical FiberTwisted Pair Data rate Signaling4B/5B/NRZIMLT-3 Max repeaters Between repeaters2km100m

Fiber Channel - Background I/O channel Direct point to point or multipoint comms link Hardware based High Speed Very short distance User data moved from source buffer to destination buffer Network connection Interconnected access points Software based protocol Flow control, error detection &recovery End systems connections

Fiber Channel Best of both technologies Channel oriented Data type qualifiers for routing frame payload Link level constructs associated with I/O ops Protocol interface specifications to support existing I/O architectures e.g. SCSI Network oriented Full multiplexing between multiple destinations Peer to peer connectivity Internetworking to other connection technologies

Fiber Channel Elements End systems - Nodes Switched elements - the network or fabric Communication across point to point links

Fiber Channel Network

Fiber Channel Protocol Architecture (1) FC-0 Physical Media Optical fiber for long distance coaxial cable for high speed short distance STP for lower speed short distance FC-1 Transmission Protocol 8B/10B signal encoding FC-2 Framing Protocol Topologies Framing formats Flow and error control Sequences and exchanges (logical grouping of frames)

FC-3 Common Services Including multicasting FC-4 Mapping Mapping of channel and network services onto fiber channel e.g. IEEE 802, ATM, IP, SCSI Fiber Channel Protocol Architecture (2)

If 100 workstations are connected to a CSMA/CD LAN with a 0.01 persistence algorithm, how many stations might try to transmit at the same time? In what type of scenario would we use CSMA over CSMA/CD? What is the Pad field (in CSMA/CD) used for? Why does a token ring station take its own data off the ring? Why is the ED field in token ring after the CRC? Review Questions