Transcript Physical Networks

Internetworking
• An internetwork is typically comprised of many physical
networks over which data travels
• There are many different types of physical networks:
– Ethernet
– FDDI
– ATM
• Goals of internetworking:
– Make all the diverse network technologies function as a
coordinated whole
– Hide the details of the underlying network hardware
– Provide universal communication services
Review: Ethernet
• A popular packet-switched LAN technology
invented at Xerox PARC in the 1970’s
Properties of an Ethernet
• 10 – 100 Mbps
• Broadcast bus
• Best-effort delivery
• Distributed access control (CSMA/CD)
Distributed Access Control
• Carrier Sense Multiple Access (CSMA)
• If two transceivers both sense that the
network is idle and begin transmission
simultaneously a collision occurs
• Collision Detection (CD)
• Recovery
Ethernet Addressing
• Each machine connected to the network has
a unique 48-bit number (it’s address)
• To send a packet a computer puts the
destination address in the destination field
• That address can be:
– A unicast address
– A broadcast address
– A multicast address
Ethernet Frame Format
• Variable length (no shorter than 64 octets and no
larger than 1518 octets)
• Fields:
–
–
–
–
–
–
Preamble
Destination address
Source address
Frame type
Data
Cyclic Redundancy Check (CRC)
An Ethernet Frame
Extending an Ethernet
• Repeaters - hardware devices used to relay
electrical signals from one cable to another
• Advantages
• Disadvantages
Extending an Ethernet (cont)
• Bridges - receives valid frames and
retransmits it on another another cable
• Advantages
• Disadvantages
Access Control Alternatives
• Ethernet - distributed
• Advantages
• Disadvantages
• Can you think of other alternatives?
Fiber Distributed Data
Interconnect (FDDI)
• Popular LAN technology
• Higher bandwidth (100 Mbps) than Ethernet
• Uses optical fiber
• Implements a token ring
FDDI (cont)
FDDI’s Self-Healing Capability
• Two rings
• Counter rotating
• Uses the backup ring to bypass a failed
computer or link
FDDI Self-Healing (cont)
Asynchronous Transfer Mode
(ATM)
• A high-speed connection-oriented network
technology
• Operates from 100 Mbps to >1 Gbps
• Expensive
Recall: How Internetworking Works
Goals of Internetworking
• Make all the diverse network technologies
function as a coordinated whole
• Hide the details of the underlying network
hardware
• Provide universal communication services
Application-Level Interconnection
• One application-level program per machine
cooperating/interoperating with other
programs
• Example?
• Advantages
• Disadvantages
Network-Level Interconnection
• Have a set of communication conventions
• The O.S. for each computer provides
communication services for all application
programs
• Example?
• Advantages
• Disadvantages
Internetworking
• Takes the network-level interconnection
approach
• Functions as a virtual packet-switched
network
• Attempts to build a unified, cooperative
interconnection of networks that supports a
universal communication service
Properties of the Internet
• Hides underlying internet architecture from
the user
• Does not mandate a network
interconnection topology
• Establishes network independence in the
user interface
Network Interconnection
• Internet router
– A small computer that physically attaches to
two networks
– Passes packets from one network to the other
– As the Internet’s topology becomes more
complex, routers need to know about that
topology
– More on routers later…
Routing Decisions
• Can be complex
• Want to keep routing hardware simple and
cheap
• So routers only use the destination network
(not the destination machine) to make
routing decisions
• Routing table is proportional to the # of
networks in the internet (not the # of hosts)
All Networks Are Equal
• Any communication system capable of
transferring packets counts as a single
network independent of:
–
–
–
–
its delay
throughput characteristics
packet size
scale
Big Advantages of
Internetworking
• Users perceive a single, virtual network
through which all machines can be reached
• Users do not need to know the underlying
details of the network
• Application-level programs do not need to
know the underlying details of the network
Big Responsibilities of
Internetworking
• Cooperation
– Communication conventions
– Each participating network agrees to handle
traffic to/from other hosts in return for their
handling of its traffic
Unanswered Questions
• What is the addressing scheme in an
internet?
• What do packets look like?
• How is routing done?
• What happens when routers/hosts
malfunction?
• What happens when routers/hosts get
overloaded?