9/29/03

Bridge

• It is used to connect two Ethernet segments
• A bridge will forward a packet only if source and destination are in different segments
• To do this, the bridge builds a table of (computer, Ethernet segment) pairs. It listens in promiscuous mode and uses the sender’s address in each packet to add to the list
• At the boot time, the list is empty
• New entries are added to this list for each packet sent
• Each entry will be timed out and removed after sometime if a computer doesn’t send new packets

It is possible to extend a network using a bridge and an optical fiber:

Using a bridge to extend a network using a satellite link:

Two bridges are necessary in this case to prevent forwarding of unnecessary packets through the satellite link and keeping local traffic local.

It is possible to introduce cycles when using bridges:

1.      A broadcast packet could circulate forever. To solve this problem, the bridges may decide not to forward packets in some cases.

2.      At boot time, the bridges talk to each other and build a spanning tree, i.e., a graph with no cycles. The bridges use this spanning tree to decide if they should forward packets or not.

Ethernet Switches

• Physically, it looks like a hub
• The difference between a hub and a switch is that a switch will allow simultaneous communication of two different non-overlapping pairs of computers.
• Internally, the switch has a one-to-one connection to each port
• Each one-to-one connection uses it’s own bridge

• Each bridge accepts only one computer in the segment it connects
• The maximum total throughput considering all the connections in a switch is higher than a hub

HUB                                 SWITCH

      A -> B     5Mbps                     A -> B   10Mbps

      C -> D     5Mbps                     C -> D   10Mbps

               ----------                          --------

      Total      10Mbps                             20Mbps

• A switch gives more security since sniffing packets is limited

Long-distance and local loop digital connection technologies

• Used for WANs
• Large geographic distance
• Generally, these connections are leased from the telephone company
• Point-to-point technology
• Copper, optic fiber, satellite channels, microwave links
• Not part of conventional telephone system
• Customers may choose analog or digital connections
  • Analog – needs a modem at each end
  • Digital – needs a DSU/CSU device
    • DSU (Data Service Unit) – translates the digital data format from the format in the computer network to the format in the telephone system
    • CSU (Channel Service Unit)
      • Diagnostics
      • Balances number of 0’s and 1’s to prevent excessive current in the wire

10/1/03

Long distance and local loop digital connection technologies

• Point-to-point connection

• Generally, provided by telephone companies or internet providers

Digital Circuit Technology

• Developed by phone companies
• Designed for voice systems
• Analog audio from user’s phone is converted to digital format
• Several calls are put together in the same packet and transmitted long distances across telephone networks

• Digital format is converted back into analog signal (audio)
• The telephone standard for digital voice is called “pulse code modulation”.

Telephone Standards for digital circuits

• Specified by telephone industry in each country
• Differ around the world
• Known by the two-character standard name
• The name determines the circuit capacity

Name                                Voice Calls                             Bit Rate (Mbps)

                                                        1                                            0.064

T1                                                  24                                            1.544

T2                                                  96                                            6.312

T3                                                672                                          44.736

E1                                                  30                                            2.048

E2                                                120                                            8.448

E3                                                480                                          34.368  

·        Most common is T1 and T3 (28 * T1)

·        Also available – fractional T1 (56 Kbps circuit)

·        It is also possible to combine 2 or more circuits to produce an intermediate capacity circuit. This device is called “inverse multiplexer”.

·        The inverse multiplexer alternated data between circuits every other byte of every other bit

High capacity digital circuits

·        Also available from phone companies

·        Use optical fibers

·        Standard is called “STS” – Synchronous Transport Signals

·        The standard name starts with the “OC” letter which stands for Optical carrier

Name                          Voice Calls                             Bit Rate (Mbps)

OC-1                             810                                          51.84

OC-3                           2430                                        155.52

·        OC-3 is most popular

Local loop

• It is the standard connection between home / business to the central phone office

• Services –
  • Analog Service: POTS (Plain Old Telephone System) uses 0 – 3 kHz
  • Digital Service:
    • ISDN – Integrated Services Digital Network
      • Voice and Data
    • DSL – Digital Subscriber Line
      • New technology
      • It uses channels in frequencies higher than 3kHz
      • It uses multiple channels simultaneously to increase the throughput
    • ADSL – Asymmetric Digital Subscriber Line
      • Popular DSL variant
      • Runs on conventional phone lines
      • Higher throughput downstream
        • Downstream – 6.4Mbps
        • Upstream – 640 Kbps
      • You can use ADSL voice channel simultaneously
         

10/3/03

Cable Modem

·        Send / receive data over cable TV

·        Uses FDM (frequency division multiplexing) using frequency different than the ones used by the TV channels, digital TV, etc.

·        There are different frequency for upstream and downstream data

·        The throughput downstream is 10 times more than upstream

·        Typical throughput of cable modem:

o       10 Mbps downstream

o       1 Mbps upstream

·        a group of subscriber’s in the neighborhood share the bandwidth

WANs

·        span large geographic distances

·        to connect 2 or more LANs, you use a router

Qs. What is the difference between a router and a bridge?

• A bridge extends a LAN. It connects to segments of the network but the segments are in the same network
  
• The segments connected to the bridge, mostly use the same network technology. For example, Ethernet.
• A router connects multiple LANs. The LANs may use different network technologies. For example, Ethernet or token ring.

• A bridge understands only Ethernet packets, whereas a router understands IP packets that may be on two different network technologies like Ethernet or token ring.