Chapter 2 Direct Link Networks
Network Technologies • Point-to-Point Links • Carrier Sense Multiple Access ( CSMA) – (for example the Ethernet) • Token Rings – (for example IEEE 802.5 and FDDI ) • Wireless– (for which 802.11 is the emerging standard)
Problems Connecting computers is a first step. There are additional problems to solve before they can exchange packets: • Encoding bits into the transmission medium • Framing the bits so they can be understood • Error detection • Reliable delivery, in spite of occasional errors • Media access control
Hardware Building Blocks • Networks are constructed from nodesand links • Nodes are general purpose computers such as workstations, multiprocessors or PCs as well as special purpose switches, routers. • Memory – finite – must be managed • Network Adapter (NIC) and its device driver • Links implemented on physical media, such as twisted pair, coaxial cable, optical fiber
Nodes Example workstation architecture
Links • Physical media are used to propagate signals as electromagnetic waves, traveling at the speed of light. • Properties of EM waves: • Frequency- or oscillations, measured in hertz • Wavelength – distance between adjacent maxima and minima, measured in meters
Electromagnetic Waves • Wavelength = speed / frequency • Voice grade phone lines carry waves ranging from 300 Hz to 3300 Hz • Voice-grade example: 300Hz in copper wire • Wavelength = Speed in Copper/ Frequency = 2/3 x 3 x 108 /300 = 667 x 103 meters
Links • A link is a physical medium carrying signals in the form of electromagnetic waves. • Binary data is encoded in the signal. • Lower layer is concerned with modulation, varying the frequency, amplitude or phase of the signal • Upper layer is concerned with encoding the data
Link Attributes • Another link attribute is how many bit streams can be encoded on it, at a given time. • One bit stream- connected nodes share access • Point-to-point – often two bit streams at once • Full duplex - two directions – simultaneously • Half duplex – one direction at a time • Simplex – one direction
Cables • Type of cable depends on technology • Coaxial – ( thick and thin) – within buildings • Category 5 ( CAT 5) – twisted pair, thicker gauge than telephone wire • Fiber –plastic or most often glass, more expensive, but used to connect buildings, and transmits light instead of electrical waves.
Leased Lines • To connect nodes on opposite sides of the country, or at great distances, you must lease a dedicated line from the telephone company. • DS1, DS3, T1, and T3 are relatively old technologies, defined for copper • STS-N links are for optical fiber (Synchronous Transport Signal), also called OC-N for Optical Carrier • Originally designed for voice, today can carry data, voice and video
Last-Mile links • Leased lines range in price from $1000/month to “don’t ask” • Last mile links span the last mile from the network service provider to the home or office. • Conventional modem- POTS (plain old telephone service) • ISDN – (Integrated Services Digital Network) – uses CODEC ( coder/decoder) to encode analog to digital signal • xDSL (Digital Subscriber Line) • Cable modem- uses cable television (CATV) infrastructure, available to 95% of US households
xDSL • Collection of technologies, able to transmit data at high speeds over standard twisted pair lines • ASDL ( Asymmetric Digital Subscriber Line)- different speeds in different directions (upstream and downstream) – called local loop • VDSL- (Very high rate Digital Subscriber Line)- runs over shorter distances – “fiber to neighborhood”
1.554 ─ 8.448 Mbps 16 ─ 640 Kbps Central Subscriber office premises Local loop ADSL downstream upstream ADSL connects the subscriber to the central office via the local loop.
─ 55.2 Mbps VDSL at 12.96 STS- N Neighborhood optical Central Subscriber network unit office over fiber ─ 4500 feet of copper premises over 1000 VDSL VDSL connects the subscriber to the optical network that reaches the neighborhood.
Shannon’s Theorem • Shannon’s theorem gives an upper bound to the capacity of a link, in terms of bits per second. C = B log2 (1+S/N) where C is channel capacity, B is Bandwidth, S is signal power, N is noise and S/N is the signal to noise ratio expressed in decibels, related as: dB= 10 x log10 (S/N)
Shannon’s TheoremExample • dB ratio pf 30 dB • S/N = 1000 • Bandwidth = 3000Hz C = B x log2 ( 1+S/N) C = 3000 x log2 (1001) C = 30 Kbps = roughly the limit of a 28.8 modem How are 56 Kbps modems possible? See p. 76
CATV • A subset of CATV channels are made available for transmitting digital data • A single CATV channel has a bandwidth of 6 MHz • Like ADSL, CATV is asymmetric with downstream rates much greater than upstream • 40 Mbps downstream ( 100 Mbps max) • 20 Mbps upstream ( roughly half as much) • Unlike DSL, bandwidth is shared among all subscribers in a neighborhood.
Network Adaptor Signals travel between signaling components. Bits flow between adaptors. Network interface cards are called NICs.
Network Adaptors • Nearly all the functions in this chapter are implemented in the network adaptor (NIC): framing, error detection and the media access protocol. The exceptions are the point-to-point automatic repeat-request schemes(ARQ), which are implemented at the lowest level protocol running on the host.
Interrupts • The host only pays attention to the network device when the adaptor interrupts the host, (for example, when a frame has been transmitted or one arrives). • A procedure is invoked by the operating system, and an interrupt handler is invoked to take the appropriate action. • While servicing this interrupt, the OS disables other interrupts.
Direct Memory Access vs. Programmed I/O • There are two ways to transfer the bytes from the frame between the adaptor and host memory: • Direct Memory Access (DMA)- the NIC directly reads/writes to the host’s memory without CPU involvement, using a pair of buffer descriptor lists. • Programmed I/O (PIO)- network adaptor (NIC) copies message into its own buffer, until CPU can copy it into the host memory.
Memory Bottleneck • Host memory is often a limiting factor in network performance. • I/O bus speed corresponds to its peak bandwidth (bus width x clock speed). • Real limitation is the size of the data block being transferred ( See p. 145) • Memory/CPU bandwidth is same as bandwidth of I/O bus. • Must be aware of limits memory puts on network
Wireless Links • AMPS- Advance Mobile Phone System- standard for cellular phones • PCS- Personal communication Services – digital cellular services in US and Canada • GSM- Global System for Mobile Communication in the rest of the world. • They use a system of towers to transmit signals and are moving toward ringing the globe with satellites.
Local Wireless Links • Radio and infrared portions of the spectrum can be used over short distances. • Technology- limited to in-building environments • Radio bands at 5.2 GHz and 17 GHz are allocated to HIPPERLAN in Europe and 2.4 GHz for use with the IEEE 802.11 standard, which supports data rates up to 54 Mbps. • Bluetooth – radio, operates in the 2.45 GHz band • Used for all devices, printers, PDAs, phones • Networks of these devices are called piconets
Bit Rates and Baud Rates • Rate at which the signal changes is called thebaud rate. • When one bit is transmitted on a signal, the bit rate and baud rate may be equal. • Often multiple bits are encoded onto a signal, where for example with 4 bits per signal, the baud rate may be 4 times the bit rate
Encoding • First step in turning nodes and links into usable building blocks is to understand how to connect them so that bits can be transmitted. • Next encode binary data that the source want to send into signals that the links can carry and then decode the data back into the corresponding data at the receiving end. • The high and low signals correspond to 2 different voltages on a copper based system or 2 different power levels on an optical link.
NRZ Encoding • NRZ – non-return to zero, maps the data value 1 to the high signal and 0 to the low signal • A sequence of several consecutive 1’s means that the signal stays high for a prolonged period of time. • Two fundamental problems; • Baseline wander –makes it difficult to detect a significant change in the signal • Clock recovery needs frequent changes from high to low to be enabled • Sender and receiver clock must be precisely synchronized.
NRZI Encoding • NRZI – non-return to zero inverted, addresses the previous problem, by having the sender make a transition from the current signal to encode a 1 and stay at current signal to encode a 0. ( Solves the problem of consecutive 1’s, but not 0’s)
Manchester Encoding • Merges the clock with the signal by transmitting the exclusive–OR of the NRZ encoded data. • Results in 0 being encoded as a low-to-high transition and 1 encoded as a high-to-low transition. Because both 0s and 1 result in a transition, the clock can be recovered at the receiver. • Problem: doubles the rate at which transitions are made on the link, which gives receiver half the time to detect them.
4B/5B Encoding • Attempts to address the inefficiency of Manchester encoding. • It inserts extra bits into the bit stream to break up long sequences of 0s and 1s: • Every 4 bits of data are encoded in a 5 bit code (See table 4B/5B encoding on p. 79)
Packets and Frames ·Packetis ``generic'' term that refers to a small block of data. ·Each hardware technology uses a different packet format. ·Frame or hardware frame denotes a packet of a specific format used on a specific hardware technology.
Framing • Blocks of data (frames), not bit streams, are exchanged between nodes. • The network adapter (NIC) enables the nodes to exchange frames. • Recognizing what set of bits constitutes a frame, and where the frame begins and ends, is the challenge faced by the network adapter.
Frame Format ·Need to define a standard format for data to indicate the beginning and end of the frame ·Header and trailer used to ``frame'' the data (SOH and EOT) ·Can choose two unused data values for framing for example, if data is limited to printable ASCII characters, you can use ·``start of header'' (soh) ·``end of text'' (eot)
Frame Format • Framing in Practice ·Incurs extra overhead - soh and eot take time to transmit, but carry no data ·Accommodates transmission problems: ·Missing eot indicates sending computer crashed ·Missing soh indicates receiving computer missed beginning of message ·Bad frame is discarded
Framing • Suppose A wishes to transmit a frame to B • It tells adapter to transmit a frame from the node’s memory • A sequence of bits is sent over the link • The adapter on B then collects the sequence of bits arriving on the link and deposits them in B’s memory.
Framing Bits flow between adaptors, frames between hosts
Framing • There are several approaches to the framing problem: • Byte-Oriented Protocol (PPP) • Sentinel Approach (frame start and end) • Byte counting • Bit –Oriented Approach (HDLC) • Clock-based framing (SONET)
Byte-Oriented protocols • One of the oldest approaches to framing is to view each frame as a collection of bytes (characters) rather than bits. • BISYNC (Binary Synchronous Communication) protocol is a byte-oriented approach developed by IBM in 1960’s • DDCMP ( Digital Data communication Message Protocol) was used in Digital Equipment’s DECNET. • These are examples of the sentinel approach and the byte counting approach.
Sentinel Approach • A packet is a sequence of labeled fields. • Above each field is a number indicating the number of bits in the field. • Packets are transmitted beginning with the leftmost field. The beginning of the frame is the SYN (synchronization) character. • Data is contained between sentinel characters – STX (start of text) and ETX (end of text). • The header begins with a SOH (start of header) field. • It ends with a CRC (cyclic redundancy check) field.