Ethernet
(continued)
- Interface network hardware checks the destination address of each packet. This prevents CPU overhead.
Packets can be sent to:
- Single destination (unicast)
- All stations on a network (broadcast)
- A subset of the stations on the network (multicast)
Broadcast on the Ethernet
- The broadcast destination address is all 1’s.
- The sender places the broadcast packet with all 1’s destination in the Ethernet
- Sender receives a copy of the packet, along with all computers on the network.
Multicast on the Ethernet
- Half the Ethernet addresses are reserved for multicast
- The network interface card can be configured by software to accept a specified multicast address.
- The NIC accepts
o Unicast, broadcast, and multicast
o All processed by hardware
Ethernet promiscuous mode
- Special mode in Ethernet card
- NIC always passes all packets to the OS
- The is used for special programs that do diagnostics in the network
o Snoop – Solaris
o Tcpdump – public
- Special privileges (root) are necessary to set the Ethernet interface into promiscuous mode.
Ethernet Type
- An integer field (2 bytes) that tells the recipient the type of data being carried.
- Ethernet type values
o 0x0800 – IP
o 0x806 – ARP
o 0x8137, 0x8138 – Novell
Network Interface Hardware
- Receives a copy of each frame
- Examines destination address and either discards or accepts packet
- Pass packet to Ethernet driver
Ether Driver Software
- Examines the Ethernet type
- Passes the frame to the corresponding protocol layer
Second Generation of Ethernet
- Used a thinner coaxial (TV cable)
- Formal name 10Base2
- Called thinnet
Terminator
- Prevents reflection of signal
Modern Ethernet Wiring
- Uses a hub which simulates a shared coaxial cable using electronics
- Also called twisted pair Ethernet
- Inexpensive
- Topology of twisted pair Ethernet
o Physically in a star
o Logically is a bus
o Called a star shaped bus
Fast Ethernet
- 100 Mbps
- Called 100BaseT
- Ethernet cards may work in dual mode (10/100 bps)
Gigabit Ethernet
- 1000 Mbps
- Slightly more expensive
- Second most popular topology
- Bits flow in one direction
- A special short message called “token” is passed around the computers
- Only the machine that has the token is allowed to transmit (This controls access to the network)
- Guarantees fair access
- A computer that is going to transmit wait for token
- The computer removed token and sends message
- The message circulates until it reaches destination and beyond
- Sender also receives its own message
- Sender introduces the token again
- When there is no data to transmit from any station, the token circulates freely
- If the token for some extraordinary reason disappears (noise, network failure), the stations timeout and will negotiate to introduce a new token
- IBM token ring (4Mbs, 16Mbps)
- FDDI (Fiber distributed data interconnect, 100Mbps)
- ProNet-10 (10Mbps)
- Uses optic fiber
- High reliability
- Immune to interference
CDDI (Copper distributed data interconnect)
- FDDI over copper
- Same format, same data rate
- Less noise immune
- Stations attach to hub
- Same frame as FDDI
- “Star shaped ring”
- Uses two rings instead of one (dual attached)
o Outer ring used for data
o Inner used for error recovery
- Automatic failure recovery (self healing network)
- The token in the second ring rotates in the opposite direction (counter rotation)
- A station adjacent to a failure will loopback
- Easy detection of:
o Broken rings
o Hardware failures
o Interference
- Broken ring disables entire ring, except in FDDI
- Difficult to add new computers
- ATM (asynchronous transfer mode)
- Designed by telephone companies
- Accommodates voice, video, and data
- Building block is an ATM switch
- Each station connects to switch
- ATM switches may be interconnected
- Two fibers required
o One to switch
o One from the switch
- Full duplex
- High data rates (155Mbps)
- Fixed size packets (Important for voice) – 53 octets
- Fixed Size Packets
o Called cells
o Important for voice
o Cell Size = 53 octets (48B Data, 5 Header)
- Connects two Ethernets
- Copies blindly data from one segment to the other
- Analogue device, it amplified signals from one segment before sending them to the other segment
- Also propagates noise and collisions
- Operates in both directions simultaneously
- Analogue device
- Connects multiple computers using Ethernet
- A hub simulates a shared Ethernet cable using electronics
- Does not understand packets
- Inexpensive
- Hubs may be interconnected in a daisy chain
- Also connects two LAN segments
- Smarter than a repeater
- It forwards frames, but does not forward noise and collisions
- Bridges understand frames
- Only forwards packets destined to the other segment