Enterprise network

• 8.1:Introduction• 8.2:LANs• 8.3:Ethernet / IEEE802.3• 8.4:Token ring• 8.5:Bridges

8.1:Introduction

• PSTN

• ISDN

• PBX (private branch exchange)

8.2:LANs

• LANs are used to interconnect distributed communities of end systems

• To ensure the transmission bandwidth is shared fairly between all of the attached stations, a number of different medium access control (MAC) methods are used. These include (CSMA/CD) and Token ring

8.3:Ethernet / IEEE802.3

• Ethernet networks – and the more recent derivative IEEE802.3 – are used extensively in technical and office environment

• CSMA/CD– All the stations are attached directly to the same cable/bus ,it

is said to operate in a multiple access mode– The bus operates in the broadcast mode which means that

every frames transmitted is received by all the other stations that are attached to the bus

– Because of the broadcast mode ,this will result in the contents of the two frames being corrupted and a collision is said to have occurred

CSMA/CD Protocol

• Carrier Sense before transmission• Carrier Sense while transmission• Collision: Two or more stations transmitting

simultaneously• Backoff: Random delay after collision• Deference: Defers transmission if channel is

sensed busy• Collision Window (Slot time): Round-trip

propagation delay time plus some carrier sense time. In IEEE 802.3, this value is defined to be 51.2 us.

CSMA/CD Collision Handling

• Collision Signal is generated by Physical layer.• Jam signal (collision enforcement): To make sure that

all stations involved in the collision will detect collision. A pattern of 32 bits.

• Collision backoff and retransmission method (Truncated Binary Exponential Backoff Algorithm, BEBA):– n : number of collisions experienced (n <= 16)– k : Min (n,10) -- Truncation– r : Random delay time (unit: slot time) between 0 <= r

< 2k

CSMA/CD worse-case collision detection

Hub configuration principles

IEEE 802.3 Frame Format

• The 802.3 packet is very similar to the Ethernet packet. It contains the following information:

• Preamble. The preamble field consists of seven bytes of alternating ones and zeros. After synchronization is established, the preamble is used to locate the first bit of the packet. The preamble is generated by the LAN interface card.

• Start Frame Delimiter (SFD). The SFD is the 8-bit sequence 10101011 that is the same as the eighth byte of the Ethernet preamble. Together the 802.3 preamble and the SFD are identical to the Ethernet preamble.

• Destination Address. The 802.3 protocol gives the manufacturer the option of implementing either 16 or 48 bit addresses.. The destination address specifies the station or stations to which a packet should be sent. Each station examines this field to determine whether or not it should accept the packet.

• Source Address. The source address field is a 48-bit (6 byte) field that contains the unique address of the station that is transmitting the packet.

• Length Field. The 2-byte length field is equal to the number of bytes in the LLC field plus the number of bytes in the pad field. If the LLC is less than 46 bytes, then the size of the pad field is 46 minus the size of the LLC. The LLC plus pad must be a minimum of 46 bytes, but no greater than 1500 bytes.

• LLC Field. The LLC field contains the 802.2 packet that becomes part of the 802.3 packet.

• Pad Field. The LLC and pad fields must be between 46 and 1500 bytes in length. If the data is not a minimum of 43 bytes, the field is padded with undefined characters or groups of bytes. The pad is automatically stripped off by the LAN interface card.

• CRC Field. The Cyclic Redundancy Check (CRC) field is a 32-bit error checking field. The CRC is generated based on the destination address, source address, type and data fields.

8.4:Token ring

• All the stations are connected together by a set of unidirectional links in the form of a ring and all frame transmissions between any of the stations take place over it by circulating the frame around the ring

• Only one frame transfer can be in progress over the ring at a time

• Fig 8.5

Token ring network operation

Token ring Frame Format

• Starting Delimiter  consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. J and K are code violations.

• Access Control  this byte field consists of the following bits from most significant to least significant bit order: P,P,P,T,M,R,R,R. The P bits are priority bits, T is the token bit which when set specifies that this is a token frame, M is the monitor bit which is set by the Active Monitor (AM) station when it sees this frame, and R bits are reserved bits.

• Frame Control  a one byte field that contains bits describing the data portion of the frame contents .Indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.

• Destination address  a six byte field used to specify the destination(s) physical address .

• Source address  Contains physical address of sending station . It is six byte field that is either the local assigned address (LAA) or universally assigned address (UAA) of the sending station adapter.

• Data  a variable length field of 0 or more bytes, the maximum allowable size depending on ring speed containing MAC management data or upper layer information. Maximum length of 4500 bytes

• Frame Check Sequence  a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver.

• Ending Delimiter  The counterpart to the starting delimiter, this field marks the end of the frame and consists of the following bits from most significant to least significant: J,K,1,J,K,1,I,E. I is the intermediate frame bit and E is the error bit.

8.5:Bridges

• There are two types of bridges , the one are used with Ethernet LANs, knows as transparent bridges , and the others with token ring LANs, known as source routing bridges.

Bridge vs Repeater

8.5.1:Transparent bridges

• With a transparent bridge, as with a repeater,the presence of one (or more) bridges in a route between two communicating stations is transparent to the two stations . All routing decisions are made exclusively by the bridge(s)

• Fig 8.12• A bridge maintains a forwarding database• Bridge learning

– Forwarding database to be created in advanced

Transparent bridges(cont.)

8.5.2:Source routing bridges

• The major difference between a LAN base on source routing bridges and one base on spanning tree bridges is that with the latter the bridges collectively perform the routing operation in a way that is transparent to the end stations. Conversely, with source routing , the end stations perform the routing function.

• Fig 8.15

Token ring Frame Format

Example

8.6:FDDI• FDDI is an optical fiber-based ring network that

supports a bit rate of 100 Mbps . It can used for the interconnection of segments spread over a wider geographical area than a single building, such as a university campus or manufacturing plant.

• Fig 8.18• Use two counter-rotating rings to enhance

reliability:primary ring and secondary ring• Two type of station: DAS and SAS • Fig 8.19

Physical interface

FDDI Frame Format

8.7:High-speed LANs

• 8.7.1:Fast Ethernet

• 8.7.2:Switched Fast Ethernet

• 8.7.3:Gigabit Ethernet

8.7.1:Fast Ethernet

• Fast Ethernet was to use the same shared, half-duplex transmission mode as Ethernet but to obtain a*10 increase in operational bit rate over 10BaseT while at the same time retaining the same wiring systems , MAC method , and frame format.

• The major technological hurdle to overcome with Fast Ethernet was how to achieve a bit rate of 100Mbps over 100m of UTP cable.

• Fig 8.26

Collision detection

• Fig 8.28

• Detect a collision by detecting a signal on pair 2 while it is transmitting and , the hub detects a collision by the presence of a signal on pair 1

8.7.2:Switched Fast Ethernet

• In order to allow multiple access/transfers to be in progress concurrently, two developments have been made:– Switch hub architecture– Duplex working over the circuits that connect

the stations to the hub.

• Fig 8.29

8.8:LAN protocol

8.8.1:Physical layer

8.8.2:MAC sublayer

8.8.3:LLC sublayer

8.8.4:Network layer

• IPX– connectionless

• TCP/IP

8.9:Multicast LAN interconnect technologies

• 8.9.1:Intersite gateways

• 8.9.2:ISDN switched connection

• 8.9.3:Frame relay

• 8.9.4:High bit rate leased lines

8.9.1:Intersite gateways

8.9.2:ISDN switched connection

8.9.3:Frame relay

8.9.4:High bit rate leased lines

Summary